Ventilator

Architectural Frameworks to Leverage Ventilation Protocols into Software

Quick and Easy Guide to Initial Ventilator Settings and Rapid Diagnostics

Magical VC Setting FiO2 100-PEEP 5-RR 15-TV 400: In Volume Control (VC) mode, initial settings such as FiO2 at 100%, PEEP at 5 cm H2O, a respiratory rate of 15 breaths per minute, and a tidal volume of 400 mL provide a foundational approach for the management of mechanically ventilated patients. These parameters serve as starting points and must be finely adjusted based on the patient's specific needs and responses, with ongoing monitoring to ensure optimal outcomes.

MV without Referring to PBW: While minute ventilation (MV) calculations ideally utilize predicted body weight (PBW) for accuracy, emergency clinical scenarios might necessitate quick estimations without immediate access to detailed PBW tables. Under such circumstances, a rough MV can be quickly calculated based on the patient's weight: approximately 5 liters per minute for a 50 kg patient, 6 liters for a 60 kg patient, and 7 liters for a 70 kg patient. These preliminary estimates are vital for initial ventilator setup, requiring subsequent adjustments as more information becomes available or as the patient's condition evolves.

Lung Condition Without C_dyn but Quickly Using Plateau Pressure: Dynamic compliance (C_dyn), a detailed measure of lung condition, typically considers values greater than 50 mL/cm H2O as normal, while lower values suggest potential ARDS, necessitating further diagnostics like chest X-rays. However, measuring C_dyn involves time-consuming inspiratory and expiratory holds. A quicker clinical alternative is to observe the plateau pressure. If the plateau pressure, based on an accurate MV computation per PBW, remains below 30 cm H2O, it suggests that the lungs are likely not severely compromised, indicating that the current ventilatory support is within safe limits and avoiding undue lung stress. If the difference between the peak pressure and plateau pressure is less than 5 cm H2O, it may indicate that the lung condition is within an acceptable range.

PEEP Setting Adjustments Based on Chest X-ray Findings: A clear chest X-ray typically warrants a PEEP of 5 cm H2O, while mild, moderate, and severe infiltrations might necessitate settings of 10, 15, and 20 cm H2O, respectively. However, maintaining PEEP below 12 cm H2O is generally advisable to avoid decreasing cardiac output and blood pressure:

FiO2	100	90	80	70	60	50	40	30
PEEP	12	11	10	9	8	7	6	5

Monitoring with Trend Feature - Airway Resistance Associated with Peak Pressure and Lung Condition with Plateau Pressure: When airway resistance increases, it is typically observed as a rise in peak pressure in the ventilator, while plateau pressure remains stable, particularly noticeable in Volume Control (VC) mode. Absence of an upper limit for peak pressure can cause significant increases. Conditions like pneumonia can escalate both peak and plateau pressures, necessitating rigorous monitoring. Utilizing the "Trend" feature of ventilators is crucial as it helps monitor these pressures over 24 hours, facilitating informed clinical decisions. If peak pressure rises while plateau pressure remains stable, this suggests increased airway resistance. Conversely, an increase in both peak and plateau pressures, or just plateau pressure, can indicate deteriorating lung compliance, potentially requiring further assessments such as a chest X-ray.

Use of Ventilator Loop Features: In ventilator management, the loop feature is an invaluable tool for comparing the current respiratory status against specific points in the past, helping identify changes and trends in a patient's condition. Modern ventilators, such as Servo ventilators, often feature a button labeled "Freeze," "Reference," or "Hold" that allows clinicians to compare the current loop with those from previous time points. By pressing this button, you can capture the current graph and overlay it with a graph from a previous time, facilitating a visual comparison of respiratory mechanics. This is particularly useful for evaluating changes when symptoms like sputum obstruction are present.

The Lower Inflection Point (LIP) on the pressure-volume loop provides crucial insights into the patient's respiratory mechanics. If the LIP shifts slightly to the right, it often correlates with airway obstruction, such as the presence of sputum, indicating that higher pressure is needed to overcome the resistance and begin inflating the lungs. Conversely, if the LIP remains the same or appears swollen (wider), it may suggest intrinsic lung issues, such as reduced lung compliance seen in conditions like ARDS or fibrosis, where the lung tissue is stiffer and harder to inflate. Increased airway resistance typically causes a rightward shift of the LIP without necessarily causing it to swell. Understanding these differences helps clinicians make informed decisions about ventilator management and patient care.

Note: It is crucial to customize all ventilator settings to the individual needs of the patient, considering their specific medical conditions, responsiveness to initial settings, and changes in their clinical status. This guide provides a foundational framework and should be utilized in conjunction with the latest clinical guidelines and under the supervision of experienced clinicians.

Lung Protection Strategies

Effective lung protection strategies are essential in mechanical ventilation, particularly when managing conditions such as Acute Respiratory Distress Syndrome (ARDS) and Chronic Obstructive Pulmonary Disease (COPD).

Plateau Pressure Management: It is crucial to keep the peak inspiratory pressure (PIP) below 40 cm H2O to mitigate the risk of barotrauma and volutrauma. For ARDS patients, ensuring the plateau pressure remains at or below 30 cm H2O is imperative. The driving pressure — defined as the difference between plateau pressure and Positive End-Expiratory Pressure (PEEP), also known as 'Pressure above PEEP' — should not exceed 15 cm H2O to prevent lung injury. This approach, supported by findings from Marcelo B.P. Amato et al. (NEJM, 2015), demonstrates that maintaining the driving pressure below 15 cm H2O significantly enhances survival outcomes by minimizing lung stress and reducing the risk of ventilator-induced lung injury (VILI).

Considerations for Managing Airflow Obstruction in COPD and Complicated Cases: In conditions like COPD where severe airflow obstruction is prevalent, it may be necessary to maintain a peak inspiratory pressure above 50 cm H2O to ensure adequate ventilation, especially since the plateau pressure is likely to stay below 30 cm H2O. This helps prevent hypoventilation and maintains sufficient oxygen delivery. However, special caution is necessary when COPD is complicated by conditions like new onset pneumonia, where unrestricted peak pressures can significantly increase plateau pressures, elevating the risk of lung injury. It is essential to meticulously monitor and adjust ventilator settings to keep the plateau pressure within safe limits.

Tidal Volume Adjustment: To prevent lung overdistension and minimize the risk of VILI, a lower tidal volume of 4 to 6 mL/kg of predicted body weight (PBW) is recommended for ARDS patients. Patients without ARDS can typically tolerate a standard tidal volume ranging from 6 to 8 mL/kg of PBW. (Generally, a lung condition is considered stable if the plateau pressure remains below 30 cm H2O, even when a tidal volume of 400 mL is used. This stability indicates that the lungs are not being overdistended and that the mechanical ventilation is within safe limits to support the patient's respiratory needs without causing additional harm.)

Ventilator Modes and Required Variable Settings

The table below provides a structured overview of common ventilator modes, arranged from those relying primarily on machine-driven parameters to those granting greater patient autonomy. Each mode lists the primary control variable, mandatory settings (with typical ranges where applicable), and additional adjustable parameters. The ranges and values are considered guidelines and may vary depending on clinical judgment and individual patient needs.

ST, SV Modes

Written on December 12th, 2024

Ventilator Parameter Settings for Patient Transfer

SIMV-PCV-PS is a ventilator mode that integrates Synchronized Intermittent Mandatory Ventilation (SIMV), Pressure Control Ventilation (PCV), and Pressure Support (PS). This configuration ensures that the ventilator delivers a set number of mandatory breaths at a predetermined pressure as defined by PCV. These breaths are synchronized with any spontaneous efforts by the patient, maintaining necessary ventilation support even in the absence of patient-initiated breathing.

In this mode, if the patient initiates a spontaneous breath, the ventilator shifts to Pressure Support (PS) mode. PS applies a consistent pressure during these spontaneous breaths, facilitating easier breathing by reducing the effort needed to inhale. If no spontaneous efforts occur, the ventilator continues with mandatory PCV breaths, providing uninterrupted support. Therefore, PS is only active during patient-initiated breathing; otherwise, ventilation defaults to the controlled settings of PCV.

• Vsens (Volume Sensitivity): Activates support when a spontaneous breath exceeds a preset volume threshold.
• Esens (Expiratory Sensitivity): Ends inspiratory support to allow natural exhalation based on the patient's breath.
• Ti (Inspiratory Time): Specifies the duration of pressure application during each ventilator-assisted breath.
• Pi (Inspiratory Pressure): Sets the pressure during the inspiratory phase to achieve the target breath volume in PCV mode.
• Psupp (Pressure Support): Provides additional pressure during spontaneous breaths to ease inhalation.
• Rate (Breathing Rate): Ensures a minimum set number of breaths per minute to meet ventilatory needs.
• PEEP (Positive End-Expiratory Pressure): Keeps the lungs slightly inflated at the end of exhalation to prevent collapse and enhance oxygenation.

• Rise Time: Adjusts the speed at which the set inspiratory pressure is reached during a breath in PS mode, enhancing patient comfort and synchrony with the ventilator.

Control Mode

Operational Differences and Patient Outcomes between VC and PC Ventilation

Practically speaking, a significant advantage of Volume Control (VC) ventilation is its ability to allow a single clinician to manage multiple patients simultaneously, especially in scenarios that require interventions like sputum suction, which can decrease minute ventilation (MV). In such situations, VC automatically increases the peak pressure to maintain the predetermined tidal volume (TV), giving clinicians additional flexibility and time to manage other tasks. Conversely, Pressure Control (PC) ventilation maintains a fixed pressure, which does not automatically adjust in response to changes in patient conditions, such as diminished TV due to airway obstruction. This characteristic necessitates a more immediate and direct response from clinical staff to ensure patient safety and effective ventilation.

Pressure Control (PC) ventilation offers several significant benefits due to its decelerating flow characteristics. Firstly, it is highly recommended for neonates because it reduces the risk of complications. Secondly, PC ventilation typically results in lower peak inspiratory pressures (PIP), which can minimize the risk of lung injury. The decelerating flow pattern in PC ventilation results in lower PIP because it better accommodates the natural compliance and resistance of the lungs. In contrast, the constant flow pattern in Volume Control (VC) ventilation can lead to higher PIP due to the inability to adjust to varying airway resistance dynamically. Additionally, an increase in mean airway pressure (MAP) can reduce dead space and enhance oxygenation. Furthermore, PC improves patient-ventilator synchrony and reduces the work of breathing (WOB), thereby decreasing the likelihood of the patient 'fighting' the ventilator. This is particularly advantageous as PC allows for flexible adjustment of the inspiration time, contributing to more natural breathing patterns that better meet the dynamic needs of the patient.

Plateau Pressure: In Volume Control (VC) mode, plateau pressure is not directly visible during regular ventilation cycles. It must be measured using an inspiratory pause, where airflow is briefly halted at the end of inspiration to allow pressures within the lung to equalize, giving a true measure of plateau pressure. Also, during the T-pause, which is the pause time during VC mode, the plateau pressure is estimated effectively due to the cessation of airflow, which allows for pressure equilibration across the pulmonary system, reflecting the pressure exerted by the ventilator against the lung compliance. In Pressure Control (PC) mode, since the ventilator delivers a preset pressure and maintains it throughout the inspiratory phase, the peak pressure is effectively the plateau pressure.

A Hybrid Approach of Volume Control and Pressure Modulation in PRVC

Pressure Regulated Volume Control (PRVC) integrates the precision of Volume Control (VC) with the protective features of Pressure Control (PC), forming a unique and dynamic approach to mechanical ventilation. In this mode, a preset tidal volume is targeted, much like in VC, but the delivery method adapts characteristics of PC by adjusting the inspiratory pressure automatically on a breath-to-breath basis. This adaptability ensures the set volume is delivered with the minimum necessary pressure, enhancing patient safety by reducing the risk of barotrauma. The key settings include not only the tidal volume but also an adjustable inspiratory pressure (capped at an upper limit to prevent lung injury), respiratory rate, inspiratory time, and positive end-expiratory pressure (PEEP), which aids in maintaining alveolar stability and improving oxygenation.

The operational flexibility of PRVC becomes particularly advantageous when managing patients with variable lung mechanics. If a drop in the delivered tidal volume is detected, the system initially applies a volume-type strategy, automatically adjusting to a pressure-type mode if the volume shortfall persists. This ensures the intended tidal volume is maintained even under changing physiological conditions. As the patient's lung mechanics stabilize and tidal volume recovers, the system responsively lowers the peak pressure. This dynamic adjustment not only optimizes gas exchange and lung protection but also minimizes the risk of ventilator-induced lung injuries, making PRVC an essential tool in modern respiratory care, particularly in scenarios where both volume consistency and pressure mitigation are critical. (Additionally, PRVC employs a decelerating flow waveform, similar to PC, enhancing gas exchange and matching ventilation closely to the patient's needs.)

Drawbacks: If the patient's airway resistance increases, the peak pressure cannot rise as rapidly or effectively as in PC mode due to the inherent limitations of PRVC. PRVC adjusts pressure gradually, both increasing and decreasing it slowly. This slower adjustment can lead to delays in reaching the necessary inspiratory pressure during sudden changes in the patient's airway resistance or compliance, making PRVC less responsive in acute situations where rapid pressure adjustments are necessary. Additionally, PRVC's dependence on accurate real-time respiratory data means that sudden changes in the patient's condition or data errors might prevent timely adjustments, potentially leading to inappropriate ventilatory support. Noteworthily, during procedures such as sputum suctioning, the removal of airway obstructions can suddenly reduce airway resistance. PRVC might respond by delivering a higher tidal volume than intended before recalibrating, risking overdistension of the lungs. This potential for increased tidal volume underscores the importance of careful monitoring during such procedures.

Dynamics of Pressure-Driven Ventilation

(A) Understanding PC and PS Mode

Ventilator pressure modes are categorized into controlled and support modes, each tailored to different patient needs. In controlled mode, the ventilator autonomously delivers breaths at preset times and volumes, which is essential for patients who are unconscious or unable to breathe voluntarily. This mode ensures that the patient receives adequate ventilation without relying on their respiratory effort, leading to generally seamless synchrony between the patient and the ventilator due to the absence of patient-initiated breathing efforts.

Conversely, support mode is designed to work in harmony with the patient's own respiratory efforts. It dynamically adjusts variables such as the inspiratory to expiratory (I:E) ratio, inspiratory time, tidal volume, and trigger sensitivity based on the patient's initiated breaths. This mode is particularly beneficial for conscious patients who are capable of voluntary breathing but still require assistance to maintain adequate ventilation. However, achieving synchrony in support mode can be challenging, as the ventilator must finely tune its responses to closely match the timing and intensity of the patient's spontaneous breaths.

Specific considerations arise within these modes, especially concerning Pressure Control (PC) and Pressure Support (PS) ventilation. In PC mode, patients with conscious and voluntary breathing may face synchrony challenges because the fixed pressure delivery might not align perfectly with their natural breathing patterns. This misalignment can lead to discomfort or inefficiency in ventilation support.

In contrast, PS mode often proves more suitable for conscious patients who can initiate breaths. This mode allows for a more natural interaction with the ventilator, which can significantly improve both synchrony and comfort by adapting the ventilation support more closely to the patient's actual respiratory needs. The inspiratory time in PS mode isn't set by the clinician but is instead determined by the patient's inspiratory effort and the cycle-off criteria, typically based on a percentage of peak inspiratory flow. This approach makes the inspiration time more dynamic and patient-driven, reflecting their current respiratory status and contributing to a more personalized ventilation strategy.

Overall, support mode is crucial for patients who are capable of participating in their own breathing yet require assistance to achieve or maintain adequate ventilation. This mode's flexibility helps accommodate individual respiratory patterns and reduces the work of breathing, making it a preferred option for patients in the process of recovering respiratory function.

(B) Adjusting Inspiratory Settings

Adjusting inspiratory settings for better outcomes in mechanical ventilation involves meticulous management of the dynamics between Pressure Control (PC) and Pressure Support (PS) ventilation modes. In PC ventilation, manual adjustment of the inspiratory time (Ti) is crucial to maintain patient-ventilator synchrony. Typically, longer Ti settings are well-tolerated during sleep, but adjustments may be necessary when a patient awakens and becomes more active. Changes in the breathing pattern, often observable on the pressure graph as an upward trend at the end of inspiration, indicate the patient's attempt to exhale while the ventilator continues to deliver air. Reducing Ti in these instances helps align the ventilator support with the patient's natural desire to breathe more shallowly or exhale, thereby reducing the risk of patient-ventilator asynchrony, often referred to as "fighting the ventilator."

In PS ventilation, the inspiratory cycle-off setting is critical. This setting determines when the ventilator ceases inspiratory pressure support and transitions to expiration, based on a predefined percentage of peak inspiratory flow. Typically, the inspiratory cycle-off is set to terminate inspiration at about 25% of the peak inspiratory flow. Adjustments to this setting are necessary when patients exhibit signs of discomfort or asynchrony, such as trying to exhale while still in the inspiratory phase. Decreasing this percentage to around 15% can allow the ventilator to switch to the expiratory phase sooner, thus reducing the risk of breath stacking and enhancing comfort. This adjustment better synchronizes the ventilator with the patient's breathing efforts, particularly if the patient completes their inhalation quickly or feels that the breaths are too long. Conversely, increasing the cycle-off percentage to around 50% prolongs the time the ventilator provides support during inhalation. This can be beneficial if the patient feels they are not getting enough air before the ventilator cycles off, as it allows more complete inhalation according to the patient's needs.

However, in PS mode, adjusting Ti is not an option since it is inherently designed to be patient-driven. The duration of inspiration is determined by the patient's inspiratory effort and the cycle-off setting, rather than a fixed time set by the clinician. This design emphasizes the supportive nature of PS ventilation, giving patients greater control over their breathing patterns, which is especially vital for those with varying levels of respiratory drive. This approach allows inspiration time to be more dynamic and patient-driven, reflecting their current respiratory needs and efforts, and enhances overall patient comfort and ventilator efficiency.

(C) Understanding and Managing Double Triggering in Mechanical Ventilation

Double triggering is a significant issue in mechanical ventilation, particularly in Pressure Support (PS) and Pressure Control (PC) modes, with a higher occurrence in PS. This phenomenon arises when the ventilator's sensitivity settings fail to align with the patient's actual respiratory demands, or if the inspiratory time set by the device does not synchronize with the patient's natural breathing pattern. Such misalignments can lead to a scenario where a patient initiates a second breath while the first is still being completed, potentially disrupting both patient comfort and the efficiency of the ventilation process.

In PS mode, the ventilator responds to the patient's breathing efforts, and if these efforts trigger the ventilator prematurely during the inspiratory cycle, double triggering may occur. This typically happens if the patient attempts to breathe in again before the machine has completed the cycle, which ideally should transition into expiration. In PC mode, although the ventilators are programmed with preset inspiratory times to mitigate this issue by strictly controlling the breathing cycles, mismatches with the patient's natural respiratory rhythm can still lead to synchronization problems and subsequent double triggering.

To manage and prevent double triggering effectively, it is crucial to ensure that the ventilator settings are meticulously adjusted to the patient's current respiratory status. This includes fine-tuning the sensitivity settings and inspiratory times to better match the patient's natural breathing patterns, thus minimizing the risk of premature initiation of a second breath.

(D) Adjusting Trigger Sensitivity for Improved Patient-Ventilator Synchrony

Effective management of ventilator settings, particularly trigger sensitivity, is essential, especially in the presence of leaks. The default setting for flow trigger sensitivity is typically +2, which detects changes in flow rate to initiate ventilation in response to patient-initiated respiratory efforts. To minimize the risk of auto-triggering due to a leak, it may be necessary to adjust this setting to -2 or lower. Settings in negative values use pressure triggers, which are more precise in detecting actual decreases in pressure, thus ensuring a more accurate response to genuine respiratory efforts and reducing false triggers.

For neonatal patients, who exhibit unique respiratory dynamics, a higher sensitivity setting of +5 may be required. This adjustment ensures that the ventilator can effectively detect the subtle respiratory efforts typical in neonates, which might not activate standard sensitivity settings.

(E) Challenges in ARDS Patients

In scenarios involving patients with Acute Respiratory Distress Syndrome (ARDS), the severely compromised lung function results in rapid exhalation phases, making Pressure Support Ventilation (PSV) unsuitable. PSV depends heavily on the patient's spontaneous breathing, which may be insufficient or unstable in ARDS cases. Consequently, Pressure Control Ventilation (PCV) is generally preferred as it provides better control over the breathing cycle with fixed pressure delivery, aiding in synchronizing ventilation with the patient's respiratory needs.

However, even in PCV, mismatches between the set inspiratory times and the patient's natural breathing rhythm can lead to double triggering — where the ventilator delivers two breaths in rapid succession without allowing for a full exhalation in between. Managing these cases may necessitate clinicians to adjust the trigger sensitivity to make the ventilator less responsive to minor respiratory efforts, increase the volume settings for more complete breaths, or use sedation to moderate the patient's spontaneous breathing efforts, thereby enhancing overall control over ventilation.

(F) Optimizing Pressure Rise Settings

In Pressure Control (PC) mode, rapid increases in pressure at the onset of inspiration can sometimes lead to disruptions in the ventilator's pressure graph, which may appear as abrupt changes or instabilities. Such occurrences can compromise the smooth delivery of ventilation and potentially affect patient comfort. To address this issue, it's essential to adjust the 'T insp rise' setting, which controls the rate at which pressure increases during the inspiratory phase.

Typically, if this setting is at zero (or off), the pressure rise is immediate and steep, leading to what might be perceived as an abrupt start to inspiration. Incrementally adjusting this setting by 5% or 10% allows for a more gradual and controlled pressure increase. This modification helps smooth out the initial pressure transition, thereby stabilizing the pressure graph at the beginning of inspiration. Such adjustments not only enhance the mechanical performance of the ventilator but also improve patient-ventilator synchrony and reduce potential stress on the patient's respiratory system.

(G) "PC above PEEP" and Driving Pressure: Insights from Marcelo B.P. Amato et al. (NEJM, 2015)

In Pressure Control (PC) mode, "PC above PEEP" refers to the pressure set above the baseline Positive End-Expiratory Pressure (PEEP) during the inspiratory phase, and this value is equivalent to the driving pressure. According to the study by Marcelo B.P. Amato et al. (NEJM, 2015), driving pressure (ΔP) is a critical factor associated with survival in patients with acute respiratory distress syndrome (ARDS). The study found that lower driving pressures, achieved by adjusting ventilator settings to avoid increases in peak inspiratory pressure (PIP) while optimizing PEEP, were strongly linked to improved survival rates.

Ventilator Asynchrony

Strategies for Managing Patient-Ventilator Asynchrony

Active Inspiration: Protecting lung integrity is critical, as highlighted by studies such as "Driving Pressure and Survival in the Acute Respiratory Distress Syndrome" by Marcelo B.P. Amato et al. (NEJM, 2015) and "Ventilation with Lower Tidal Volumes as Compared with Traditional Tidal Volumes for Acute Lung Injury and the Acute Respiratory Distress Syndrome" by The Acute Respiratory Distress Syndrome Network (NEJM, 2000). However, strict adherence to these guidelines might lead to insufficient breathing, potentially causing double triggering if patients inhale more than the set tidal volume. To mitigate this, clinicians may consider increasing the tidal volume, adjusting trigger sensitivity, or sedating the patient to decrease spontaneous efforts. Raising the trigger level can enhance ventilator response to patient-initiated breaths, while reducing it helps prevent double triggering. Modifications to ventilator alarm settings, such as the low PEEP alarm, are also often necessary. In Pressure Control (PC) mode, inspiratory time (Ti) is generally set between 0.8 to 1.25 seconds but may be shortened to minimize ventilator conflict. Unlike PC mode, Pressure Support (PS) mode does not allow direct control over Ti, thus incrementally adjusting the "End Inspiration" or "Esens" settings from 30% to 50% can assist in better managing breath termination timing.

Active Expiration: Challenges during expiration, particularly with high Positive End-Expiratory Pressure (PEEP) settings, increase work of breathing (WOB), potentially causing discomfort and leading to non-compliance with ventilation. Shortening the expiratory time in PC mode can effectively address these issues. In PC mode, adjusting the expiratory time primarily involves altering the inspiratory time (Ti) and the respiratory rate (RR), as these settings will indirectly affect how long the expiratory phase lasts. For example, decreasing the Ti from 1.2 seconds to 0.8 seconds and maintaining a consistent respiratory rate will naturally extend the expiratory phase. In Pressure Support (PS) mode, expiratory time adjustment differs since inspiratory time is typically patient-driven. Adjusting the "cycling off" criteria, for instance, setting it to terminate inspiration when the flow decreases to 25% of the peak flow rate, will typically shorten the inspiratory phase and potentially increase the expiratory time.

Managing Leaks: Leaks in Volume Control (VC) mode particularly pose challenges, as they can prevent the ventilator from delivering the set tidal volume accurately. This shortfall in delivered volume can destabilize vital signs and may prompt incorrect ventilator adjustments. "End Inspiration" settings may need to be adjusted upwards (e.g., from 30% to 50%), and "Trigger Pressure" should be set to a less sensitive level (ranging from -10 to -20). Regular checks, such as ensuring proper ET tube cuff pressurization and verifying ventilator integrity with a test lung, are essential for identifying equipment-related issues. If leaks persist, increasing minute ventilation (MV) may temporarily resolve issues until more definitive solutions like reintubation or manual bagging are executed.

Biased Flow: Biased flow maintains a continuous stream of air through the mechanical ventilation system, aiding in keeping airways open and detecting spontaneous breathing efforts. Typically set at 2 liters per minute, biased flow comprises a significant part of the total ventilation volume. For example, out of a total of 9 liters, 7 liters are ventilator-delivered, and 2 liters are biased flow. An active inhalation of 1.6 liters (or 80% of the biased flow) by a patient indicates effective inspiration. For smaller or weaker patients, increasing the biased flow (from 2L to 3L or 4L) ensures even minimal efforts are recognized, thus easing their breathing work. However, too high a setting may induce auto-triggering, where minor disturbances are mistakenly interpreted as breathing attempts. Adjusting sensitivity settings can help mitigate auto-triggering, ensuring the ventilator supports only genuine respiratory efforts.

Improving Patient-Ventilator Synchrony Through Neurally Adjusted Ventilatory Assist (NAVA)

Neurally Adjusted Ventilatory Assist (NAVA) represents a progressive form of mechanical ventilation that optimizes the synchronization between the ventilator and the patient by leveraging the electrical activity of the diaphragm (EAdi). This technology allows the ventilator to respond dynamically to the patient's breathing needs, ensuring that assistance is provided in direct correlation with their respiratory effort. This method not only enhances comfort but also aligns precisely with the patient's natural respiratory drive, crucial for those who can breathe spontaneously but require additional support due to conditions like respiratory failure or challenges in weaning from mechanical support.

The functionality of NAVA hinges on the continuous monitoring of Edi, which acts as the primary trigger for ventilatory assistance. When a patient initiates a breath, the resulting electrical activity of the diaphragm prompts the ventilator to deliver pressure proportionate to this signal, ceasing as the signal diminishes. This responsive system adapts to the patient's breathing needs in real-time, offering a level of support directly correlated to their respiratory muscle effort, thereby mitigating the risks of over- or under-ventilation and enhancing comfort.

The principle of NAVA lies in its ability to detect and utilize the diaphragm's electrical signals as the direct driver for ventilatory assistance. The ventilator adjusts its support based on real-time changes in EAdi, providing a tailored respiratory aid that prevents the common pitfalls of conventional ventilation, such as asynchrony or mismatched timing and intensity of support. This responsiveness to the patient's own respiratory signals makes NAVA particularly advantageous for patients with varying respiratory drive and those susceptible to discomfort with traditional mechanical ventilation methods. However, the efficacy of NAVA can be compromised in patients with conditions that impede the generation or detection of accurate EAdi signals, such as severe diaphragmatic dysfunction, certain neuromuscular disorders, or obstructive esophageal pathologies that prevent proper catheter placement.

NAVA is particularly beneficial in conditions like Acute Respiratory Distress Syndrome (ARDS) in adults and Respiratory Distress Syndrome (RDS) in neonates. It's also indicated in cases of pulmonary hypertension and scenarios requiring detailed assessment of respiratory activity, such as in central hypoventilation syndrome. However, its application is limited by conditions that affect the diaphragm's ability to generate reliable Edi signals, such as neuromuscular disorders, severe diaphragmatic dysfunction, or esophageal anomalies, which might impede the placement or function of the necessary esophageal catheter.

Setting up a NAVA system requires meticulous initial adjustments to ensure optimal patient support. These settings include establishing the NAVA level, which is calculated by the formula NAVA level x (Edi max - Edi min) + PEEP, and determining the trigger sensitivity. The chosen NAVA level, expressed in cmH₂OμV, should aim for a target Edi range of 5-20 μV, adjusted based on observed Edi max values to either enhance or reduce ventilatory support.

Clinically, NAVA has shown to significantly improve patient-ventilator synchrony, reducing the risks associated with mechanical ventilation such as ventilator-induced lung injury. By supporting the natural breathing pattern of the patient rather than overriding it, NAVA facilitates a more physiological form of respiratory support, which can lead to better outcomes in the weaning process and overall patient comfort. Its capability to adjust support based on the patient's immediate respiratory muscle output distinguishes it from other assisted modes like Pressure Support Ventilation (PSV), which may not accurately align support with patient effort.

The real-world application of NAVA, especially in complex clinical scenarios, underscores its significant benefits. Studies have documented improved outcomes in diverse patient populations, including adults with acute respiratory conditions and neonates experiencing respiratory distress (Beck et al., 2009; Breatnach et al., 2010). The ability to maintain effective ventilation despite challenges such as air leaks — common in both invasive and non-invasive forms of ventilation — further demonstrates NAVA's robustness (Beck et al., 2009). Its adaptability extends to non-invasive applications, where it can be used with nasal prongs or masks, allowing for a smoother transition from invasive to non-invasive support and potentially reducing the need for higher pressures typically required in conventional ventilation settings (Makker et al., 2020).

• Shah, S. D., & Anjum, F. (2024). Neurally Adjusted Ventilatory Assist (NAVA). In StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK572111/
Beck, J., Reilly, M., Grasselli, G., Mirabella, L., Slutsky, A. S., Dunn, M. S., & Sinderby, C. (2009). Patient-ventilator interaction during neurally adjusted ventilatory assist in low birth weight infants. Pediatric Research, 65(6), 663-668. https://doi.org/10.1203/PDR.0b013e31819e72ab
• Breatnach, C., Conlon, N. P., Stack, M., Healy, M., O'Hare, B. P. (2010). A prospective crossover comparison of neurally adjusted ventilatory assist and pressure-support ventilation in a pediatric and neonatal intensive care unit population. Pediatric Critical Care Medicine, 11(1), 7-11. https://doi.org/10.1097/PCC.0b013e3181b0630f
• Makker, K., Cortez, J., Jha, K., Shah, S., Nandula, P., Lowrie, D., Smotherman, C., Gautam, S., & Hudak, M. L. (2020). Comparison of extubation success using noninvasive positive pressure ventilation (NIPPV) versus noninvasive neurally adjusted ventilatory assist (NI-NAVA). Journal of Perinatology, 40(8), 1202-1210. https://doi.org/10.1038/s41372-019-0578-4

Spontaneous Breathing and Ventilator Weaning

A Comparative Analysis of Ventilation Modes for Patients with Spontaneous Breathing

When considering the various modes of mechanical ventilation with spontaneous patient breathing — Spontaneous Timed (ST), BiPAP (Bilevel Positive Airway Pressure), BiVent/APRV (Airway Pressure Release Ventilation), Volume Support (VS), Pressure Support (PS), and CPAP (Continuous Positive Airway Pressure) — it's essential to understand both their operational characteristics and the freedom they afford to patients. Each mode requires specific parameters to be set, ensuring optimal ventilation and patient comfort during respiratory support:

• ST: EPAP/IPAP, RR, FiO2, and Ti
• BiPAP: EPAP/IPAP, RR, FiO2 (, and Ti as the maximum duration of inspiration allowed by the ventilator)
• PS: FiO2, PEEP, and PS above PEEP
• CPAP: FiO2, CPAP, and trigger sensitivity

In understanding these modes, it's crucial to note that PS mode with 'PS above PEEP' set at zero essentially functions as CPAP, offering continuous positive airway pressure without additional support during inspiration. This equivalence underscores the versatility of PS mode, adapting its function based on settings to meet patient needs. BiPAP, on the other hand, provides variable pressure support during both inhalation and exhalation, offering enhanced ventilation assistance compared to CPAP or PS alone. However, despite its efficacy, BiPAP may feel more restrictive to patients due to its dual-pressure levels. Conversely, CPAP, whether delivered independently or through PS mode, offers continuous support with less complexity, potentially affording patients a greater sense of freedom in their breathing.

When considering freedom to breathe, PS mode stands out as it allows patients to initiate breaths spontaneously and breathe at their own pace. This mode synchronizes with the patient's respiratory effort, providing additional support when needed, while still allowing for variability in breathing rates and patterns. ST mode, while offering partial ventilatory support, imposes fixed intervals for mandatory breaths, which may feel somewhat restrictive compared to the flexibility of PS mode. BiPAP, with its dual-pressure levels, offers tailored support during both inspiration and expiration but may feel more constraining due to its preset parameters. CPAP, whether provided independently or through PS mode, offers continuous positive airway pressure without additional support during inspiration, providing a stable environment for breathing but potentially lacking the adaptability of PS mode.

BiVent/APRV is notable for its innovative ventilation strategy that emphasizes spontaneous breathing across the entire respiratory cycle. This mode operates by maintaining a high baseline airway pressure to optimize oxygenation and alveolar recruitment, while periodically dropping to a lower pressure to allow for CO2 elimination without disrupting the patient's natural breathing attempts. The high pressure phase supports the alveoli continuously, which is critical for patients experiencing severe respiratory distress. In contrast, the brief, controlled lower pressure phase acts like a sigh, aiding in more effective CO2 removal. This mode's ability to allow spontaneous breathing throughout its cycle offers a significant improvement in patient comfort compared to more traditional, restrictive methods.

Volume Support (VS), on the other hand, provides a blend of controlled and supportive ventilation, ideal for patients who are capable of initiating breaths but require assistance in achieving consistent tidal volumes due to varying respiratory muscle strength or lung compliance. Once a patient initiates a breath in VS mode, the ventilator adjusts the pressure dynamically to deliver a predetermined tidal volume. This adaptability ensures adequate ventilation while permitting the patient to control the timing and frequency of breaths, thereby reducing fatigue and promoting a more natural breathing pattern. Unlike modes such as Pressure Support (PS) or BiPAP, VS not only supports the breath initiated by the patient but also guarantees the delivery of a specific volume, adjusting pressure as needed on a breath-by-breath basis to maintain consistent minute ventilation.

Ventilation Settings and Their Role in Effective Weaning

Weaning patients from mechanical ventilation is a critical step in their recovery, marking the transition from artificial support back to natural breathing patterns. Typically, humans breathe using negative pressure, where the diaphragm creates a vacuum that draws air into the lungs. In contrast, mechanical ventilation employs positive pressure to push air into the lungs, which can lead to complications such as barotrauma from excessive air pressure and inadequate gas exchange due to low tidal volumes. To mitigate these issues, sedation often facilitates controlled mechanical ventilation with two primary modes: Volume Control (VC), where tidal volume is precisely regulated, and Pressure Control (PC), which adjusts "PC above PEEP" to ensure consistent lung inflation.

As patients stabilize, ventilator settings transition to support modes that allow more spontaneous breathing efforts. Pressure Support (PS) aids each breath initiated by the patient by maintaining "PS above PEEP," while Continuous Positive Airway Pressure (CPAP) maintains a constant pressure to keep the alveoli open, supporting natural lung function but not directly assisting the breathing effort. A notable challenge with support modes like PS is their inability to control the respiratory rate (RR), which can result in apnea if the patient's spontaneous efforts are insufficient.

Transitioning from Controlled Mandatory Ventilation (CMV) to Assisted/Control Mandatory Ventilation (ACMV) is significant, allowing the start of inspiration to be patient-initiated rather than ventilator-initiated, granting more autonomy in the breathing process. For patients requiring intermittent assistance, Synchronized Intermittent Mandatory Ventilation (SIMV) provides mandatory breaths as a backup, set by the SIMV rate, which defines the frequency of controlled breathing. Crucially, if there is an absence of spontaneous patient breathing, the system reverts from ACMV to CMV, ensuring continuous ventilation. However, between controlled breaths, PS can be activated to provide necessary support.

Moreover, transitioning from PS to CPAP involves shifting the control from "PS above PEEP" to a mode where only PEEP is provided, allowing patients under CPAP to utilize negative pressure for breathing. Significantly, if the inspiration strength in PS is set to zero, it effectively transitions the operation to CPAP, emphasizing passive support through maintained PEEP without additional pressure support. This phase is critical as it encourages patients to engage their respiratory muscles more actively, vital for successful weaning.

Assessing Readiness for Ventilator Weaning Using RSBI and P 0.1: To determine if a patient is ready for weaning from mechanical ventilation, two key measures are often evaluated: the Rapid Shallow Breathing Index (RSBI) and the P 0.1 or Occlusion Pressure. The RSBI is calculated by dividing the respiratory rate (RR) by the expiratory tidal volume (VTe) measured in liters. An RSBI value less than 105 breaths per minute per liter is considered indicative of a patient's readiness for weaning. This index, measured during a spontaneous breathing trial, helps assess the patient's workload of breathing, providing a quantitative basis for evaluating their ability to breathe independently.

Additionally, the P 0.1, which is the negative pressure exerted by the patient in the first 100 milliseconds of an occluded airway, is another crucial measure. A P 0.1 value between 1 to 2 cm H2O suggests a lower respiratory drive, which can indicate that the patient may be ready for weaning. This measure provides insight into the patient's respiratory effort and capacity to maintain adequate breathing without ventilatory support. Both the RSBI and P0.1 are essential in providing a comprehensive assessment of a patient's potential to successfully transition off mechanical ventilation.

Rapid Shallow Breathing Index (RSBI) Calculator

Predicts the success of weaning from mechanical ventilation. An RSBI ≤ 105 breaths/min/L is considered an indicator of readiness for weaning.

RSBI = f / VT

SIMV Rate and Breath Cycle Time Across Clinical Scenarios

I. Foundational terminology

• SIMV rate – number of SIMV intervals initiated each minute.
• Breath cycle time – programmed duration of one mandatory breath (inspiration + expiration).
• SIMV interval – the span between successive synchronization windows; mathematically \( \text{SIMV interval} = \dfrac{60\ \text{s}}{\text{SIMV rate}} \).
• Synchronization window – the first portion of each SIMV interval, lasting exactly one breath cycle time, during which a mandatory breath may be synchronized with patient effort.
• 90 % threshold – the safety mark at \(0.9 \times \text{breath cycle time}\); absence of effort by this point mandates a time‑triggered breath.
• Pressure support (PS) – flow‑cycled assistance given to spontaneous efforts that occur after the single mandatory breath within the current interval.

II. Delivery algorithm

1. The synchronization window opens at the onset of each SIMV interval and spans one breath cycle time.
2. If an effort arises before the 90 % threshold, a synchronized mandatory inflation is issued.
3. If no effort occurs by the threshold, a time‑triggered mandatory breath is delivered.
4. Only one mandatory breath is permitted per interval; subsequent efforts before the next interval receive PS.

III. Scenario analysis

1. SIMV rate 6 breaths·min^‑1; breath cycle time 4 s

   Computed parameters:

   • SIMV interval \( = \dfrac{60\ \text{s}}{6} = 10\ \text{s} \)
   • Synchronization window = 0–4 s
   • 90 % threshold \( = 0.9 \times 4\ \text{s} = 3.6\ \text{s} \)

   The remaining 6 s (4–10 s) constitute a free period for pressure‑supported breaths only.

   1. Scenario A — early spontaneous effort

      Elapsed time (s)	Action	Reason
      1.2	Synchronized mandatory breath	Effort detected well before the threshold.
      5.0	Pressure‑support breath	Mandatory breath already given; free‑period assistance only.

   2. Scenario B — late spontaneous effort (within the window)

      Elapsed time (s)	Action	Reason
      3.4	Synchronized mandatory breath	Effort detected at 85 % of window — still inside threshold.

   3. Scenario C — no spontaneous effort

      Elapsed time (s)	Action	Reason
      3.6	Time‑triggered mandatory breath	Threshold reached without effort.

   Clinical pearl: At low SIMV rates, the generous free period (6 s) facilitates respiratory‑muscle engagement yet mandates vigilant monitoring for hypoventilation.

2. SIMV rate 20 breaths·min^‑1; breath cycle time 3 s

   Computed parameters:

   • SIMV interval \( = \dfrac{60\ \text{s}}{20} = 3\ \text{s} \)
   • Synchronization window = 0–3 s (coextensive with the interval)
   • 90 % threshold \( = 0.9 \times 3\ \text{s} = 2.7\ \text{s} \)

   No residual free period exists; the ventilator begins a new interval immediately after 3 s.

   1. Scenario A — spontaneous effort present

      Elapsed time (s)	Action	Reason
      0.5	Synchronized mandatory breath	Effort detected early.

   2. Scenario B — no spontaneous effort

      Elapsed time (s)	Action	Reason
      2.7	Time‑triggered mandatory breath	Delivered at the threshold; interval ends at 3 s.

   Clinical pearl: High SIMV rates eliminate free periods, making the mode functionally similar to a fully synchronized controlled mode; inspiratory time and flow must be adjusted to avert insufficient tidal volumes.

IV. Physiological and operational rationale

• Minute‑ventilation assurance vs muscle preservation – one mandatory breath per interval guarantees baseline ventilation, while free periods (when present) enable diaphragm recruitment.
• Asynchrony mitigation – restricting synchronization to a defined window reduces double‑trigger and early‑cycle dyssynchrony.
• Progressive weaning – decreasing SIMV rate enlarges free periods, progressively shifting work of breathing to the patient.

V. Configuration strategy

1. Maintain the product constraint \( \text{SIMV rate} \times \text{breath cycle time} \le 60\ \text{s} \).
2. Target inspiratory time ≥ 1 s; high rates or very short breath cycles may necessitate higher peak flows.
3. Optimize trigger sensitivity to capture weak efforts without auto‑triggering.
4. Monitor waveforms for post‑mandatory fighting; adjust rise time, sedation, or synchronization settings accordingly.
5. Set vigilant apnea and minute‑ventilation alarms because no automatic backup exists outside the synchronization window.

VI. Pitfalls and safeguards

• Hidden hypoventilation at low rates if spontaneous drive fades  →  employ capnography and arterial blood‑gas checks.
• Inspiratory‑muscle fatigue when PS is inadequate  →  titrate PS to maintain diaphragmatic unloading.
• Dynamic hyperinflation at high rates  →  lengthen expiration or apply judicious external PEEP.

Written on June 17, 2025

Waveform-based Ventilation: Pressure, Flow, and Volume Insights

Interpreting ventilator PRESSURE-waveform anomalies

Typical findings and corrected explanations

1. Early upward convexity (“pressure bump”)

   A brief peak at the very start of inspiration indicates inspiratory pressure overshoot. The pattern appears when the set inspiratory flow or rise-time exceeds the capacity of the lung–airway system (low compliance, high resistance, or a very short rise-time).

   • Not an asynchrony by itself; considered a machine-related artifact.
   • Mitigation: lengthen rise-time or reduce peak inspiratory flow.

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   Effect of inspiratory rise-time on early pressure overshoot and patient comfort.
   (0) T_insp rise 5 % — factory default; balanced patient comfort.
   (1) T_insp rise 0 % — instantaneous rise; marked early-pressure overshoot (“fighting”).
   (2) T_insp rise 10 % — slower ramp; overshoot minimized, mean airway pressure (P_mean) and delivered tidal volume (V_T) fall slightly while synchrony improves.

   Inspiratory rise time is the interval from the onset of inspiration to the moment peak inspiratory pressure is reached (some ventilators use a flow-based definition). It is set as a fraction of the total respiratory cycle; with respiratory rate RR = 15 breaths·min^–1 (T_cycle = 4 s): \[ T_{\text{rise}} \;=\; f_{\text{rise}}\times T_{\text{cycle}} \;=\; f_{\text{rise}}\times \frac{60}{RR}. \] Thus a 10 % setting yields \(T_{\text{rise}} = 0.10 \times 4\text{ s} = 0.4\text{ s}\). Lengthening T_insp rise reduces overshoot and dyssynchrony at the cost of a modest drop in V_T and P_mean.

2. Mid-inspiratory upward spike

   A sudden transient rise after inspiration begins most commonly signifies a cough, hiccup, or transient glottic closure. Pressure climbs momentarily because the airway is briefly obstructed.

   • Falls under expiratory flow limitation / protective reflexes.
   • Not classified as “fighting.”

3. Late-inspiratory upward bulge

   Upward inflection just before inspiratory flow should cease identifies a premature expiratory effort — an example of expiratory (cycling) asynchrony or “early cycling / flow-termination mismatch.”

   • Recognised as patient–ventilator fighting because the patient attempts to end inspiration while the ventilator continues to deliver flow.

4. Mid-inspiratory downward deflection (“negative scoop”)

   A dip below the pressure baseline during inspiration represents a patient inspiratory effort stronger than delivered flow. The airway pressure falls because flow or tidal volume is inadequate.

   • Categorised as flow starvation / pressure-drop asynchrony.
   • Perceived as “fighting”; resolved by increasing peak flow or choosing a mode with demand-adapted flow.

5. Post-inspiratory double peak (“stacked breaths”)

   If spontaneous effort persists after the ventilator cycles off, a second mandatory breath may be triggered, producing two consecutive breaths. This phenomenon denotes double triggering (also called breath stacking).

   • Classic trigger asynchrony; a clear manifestation of “fighting.”

6. Progressive pressure decay during exhalation

   When measured PEEP drifts below the set PEEP and the expiratory pressure slope falls slowly toward zero, the usual cause is an air-leak (circuit, cuff, or chest drain).

   • Identified as a ventilator-circuit problem, not fighting.
   • Corrective action: locate and seal the leak or reinflate the tracheal-tube cuff.

7. Pre-trigger drop in PEEP before inspiration

   A noticeable fall below PEEP immediately before the inspiratory valve opens reveals a vigorous negative-pressure trigger effort. When excessive, the event is termed excessive negative-pressure trigger and may cause patient discomfort.

   • Still a trigger asynchrony; regarded as “fighting” when frequent or pronounced.

Are all of the above “patient-ventilator fighting” ?

The informal phrase “fighting the ventilator” broadly includes any patient–ventilator asynchrony that induces distress, ineffective ventilation, or abnormal work of breathing.

• True asynchronies (fighting) → items 1, 3, 4, 5, 7.
• Machine / circuit artifacts → items 1, 6.
• Protective / physiologic reflexes → item 2.

Accurate classification remains crucial: ventilator settings (flow, rise-time, cycling criterion) must be optimised, leaks eliminated, and reflex causes treated before every anomaly is labelled as “fighting.”

Troubleshooting checklist

1. Inspect the circuit for leaks, condensate, and cuff pressure.
2. Match inspiratory flow to demand; increase peak flow or prolong rise-time if negative scoops occur.
3. Adjust cycling criteria in pressure-support modes (e.g., raise the %-peak-flow threshold) to prevent premature expiratory effort.
4. Evaluate sedation and analgesia if coughs, double triggering, or vigorous efforts continue.
5. Consider proportional-assist or NAVA when conventional modes cannot achieve synchrony.

Written on June 7, 2025

Interpreting ventilator FLOW-waveform anomalies (Written June 7, 2025)

Typical findings and corrected explanations

1. Prolonged expiratory tail (“slow rise”)

   In a normal passive breath, expiratory flow decays exponentially to baseline within ≈1 s. Obstructive diseases (COPD, asthma) lengthen the time constant, producing a gently sloping tail that fails to reach zero before the next inspiration.

   • Represents expiratory flow limitation and air-trapping.
   • Predisposes to intrinsic PEEP (auto-PEEP) and CO₂ retention.

2. Incomplete exhalation before the next breath

   When inspiratory flow begins while expiratory flow is still positive, residual gas remains in the lungs.

   • Direct sign of auto-PEEP; flow never touches the zero line.
   • Total PEEP (measured during expiratory hold) exceeds the set PEEP by ≥2 cmH₂O.

3. Mid-expiratory inspiratory spike

   A brief reversal of flow halfway through exhalation reflects a premature spontaneous inspiratory effort that prematurely retriggers the ventilator.

   • Classified as trigger asynchrony (“fighting”).
   • Resolve by reducing trigger sensitivity or lengthening expiratory time.

4. Plateaued low-flow segment (“flow-plateau”)

   A long stretch of nearly constant, low expiratory flow followed by an abrupt drop suggests dynamic airway collapse or a pursed-lip–breathing effect.

   • Common in severe emphysema; titrate extrinsic PEEP to splint airways.

5. Saw-tooth oscillations on the expiratory curve

   Fine, regular undulations superimposed on the descending limb usually stem from secretions or condensate causing intermittent, partial obstruction.

   • Clear the airway or drain circuit water to restore a smooth trace.

How flow anomalies signal CO₂ retention

• Flow waveform: persistent expiratory flow at the onset of inspiration or mid-expiratory reversals implies trapped gas and increased dead-space ventilation.
• Minute ventilation (V̇_E): when V̇_E falls and PaCO₂ (or end-tidal CO₂) rises, alveolar ventilation is inadequate despite apparent tidal volumes.
• Total PEEP vs. set PEEP: an elevated measured PEEP during expiratory hold (> set PEEP) quantifies intrinsic PEEP and correlates with CO₂ retention severity.

How do patterns 1–4 actually differ?

• Why 1 ≠ 2

  Pattern 1 shows a gradual decay that merely suggests impending air-trapping; pattern 2 proves the trap has already occurred because inspiration interrupts active exhalation. Total-PEEP measurement confirms the difference.

• Why 3 ≠ 4

  Pattern 3 is defined by flow reversal—a spike crossing zero as the patient tries to inhale; pattern 4 never reverses but instead stalls at a low flow owing to airway collapse. The former is solved by trigger tuning, the latter by judicious extrinsic PEEP.

Troubleshooting checklist

1. Lengthen expiratory time (↓ RR or adjust I:E ratio) to allow complete exhalation.
2. Apply extrinsic PEEP judiciously to counteract dynamic airway collapse without aggravating hyperinflation.
3. Suction and humidify to eliminate saw-tooth patterns caused by secretions.
4. Optimise trigger settings (flow or pressure sensitivity, trigger delay) to prevent premature inspiratory spikes.
5. Monitor PaCO₂, end-tidal CO₂, and total PEEP; adjust ventilation strategy when values climb.

Written on June 7, 2025

Interpreting ventilator VOLUME-waveform anomalies

Normal symmetry: Inspired tidal volume (VT_i) matches exhaled tidal volume (VT_e) within ≈10 %. The volume-time trace returns to baseline before the next breath, forming uniform rectangles on the scalar display.

Typical findings and corrected explanations

1. VT_e < VT_i (“volume loss”)

   A persistent gap between inspired and expired volume indicates gas escaping the system.

   • Cuff or circuit leak is the usual culprit.
   • Waveform cue: descending limb slopes smoothly but ends below the zero-line target; baseline drifts downward over successive breaths.

2. Baseline not reached between breaths (“volume stacking”)

   The expiratory limb fails to return to zero before the next inspiration, causing the trace to “ride” progressively higher.

   • Represents incomplete exhalation / auto-PEEP.
   • Often accompanies obstructive flow patterns; VT_e may equal VT_i yet the baseline elevation betrays gas trapping.

3. Step-like double plateau (“breath stacking”)

   Two mandatory breaths delivered in rapid succession without an expiratory phase create a two-step inspiratory limb and a single, larger expiratory descent.

   • Caused by double triggering (trigger asynchrony).
   • Net VT_i nearly doubles; VT_e eventually matches, but oversize volumes risk barotrauma.

4. Abrupt volume collapse (“disconnection”)

   Both inspiratory and expiratory tracings plummet to zero suddenly and remain flat.

   • Signifies circuit disconnection or massive leak.
   • Immediate alarm and reconnection required.

5. Serrated descent (“secretions artifact”)

   Fine oscillations decorate the expiratory limb, mirroring saw-tooth flow patterns.

   • Attributed to airway secretions or water condensation.
   • Suctioning or circuit drainage resolves the serrations.

Key differentiation of common volume anomalies

Troubleshooting checklist

1. Confirm cuff integrity (pilot-balloon pressure, cuff leak test) for VT_e < VT_i.
2. Perform an expiratory hold to quantify total PEEP when baseline fails to return to zero.
3. Adjust trigger sensitivity or sedation level to stop breath stacking.
4. Reconnect or replace circuit immediately if a flat-line appears.
5. Suction airway and drain humidifier to remove serrated artifacts.

Written on June 7, 2025

Flow adaptation in volume-targeted ventilation: ACMV, CMV, and PRVC

I. Conceptual framework

Volume-controlled (VC) breaths deliver a preset tidal volume by using a square-wave inspiratory flow. Depending on whether the ventilator allows the patient to draw additional (“adaptive”) flow above the square baseline—or replaces the baseline with a decelerating pattern—the breath is classified as assist/control mandatory ventilation (ACMV), controlled mechanical ventilation (CMV), or pressure-regulated volume control (PRVC).

Flow adaptation denotes the ventilator’s ability to sense negative deflection in airway pressure or inspiratory effort and deliver extra flow above the programmed constant value, thereby meeting the patient’s instantaneous demand without altering the target tidal volume.

II. Mode-specific mechanics

1. ACMV — VC with flow adaptation

   When the patient initiates a stronger inspiratory effort during a square-wave VC breath, the ventilator permits a transient upward inflection in the flow waveform. This additional flow maintains patient comfort, shortens inspiratory muscle work, and prevents the pressure–time curve from dipping below baseline. Trigger sensitivity must remain high enough to capture spontaneous effort yet low enough to avoid auto-cycling.

2. CMV — VC without flow adaptation

   In CMV the flow waveform is rigidly constant; the ventilator ignores any further negative pressure generated by the patient after the first trigger. The square-wave remains flat, so dyssynchrony may occur if inspiratory demand exceeds the programmed flow. This mode is suitable for deeply sedated or pharmacologically paralysed patients in whom spontaneous effort is undesirable (e.g., during acute lung recruitment or intracranial pressure control).

3. PRVC — VC with decelerating flow

   PRVC targets a set tidal volume but controls pressure rather than flow. The ventilator measures the compliance on each breath, then computes the minimal inspiratory pressure needed to achieve the volume while imposing a decelerating flow pattern. Peak inspiratory pressure is automatically adjusted—typically within a ±3 cm H₂O window—to reach the volume goal with the lowest feasible pressure.

   When a ventilator lacks a dedicated PRVC mode, selecting a decelerating flow option under VC partially reproduces PRVC’s gentler pressure profile; however, pressure is no longer actively regulated and peak variations may grow if compliance changes abruptly.

III. Pressure-Regulated Volume Control (PRVC)

PRVC is a closed-loop “hybrid” mode that guarantees a set tidal volume (V_T) while automatically choosing the lowest possible inspiratory pressure on every breath. The ventilator delivers a decelerating flow waveform—similar to pressure control (PC)—which spreads the flow early and tapers it late, lowering peak inspiratory pressure (P_peak) compared with square-flow volume control (VC).

1. How the algorithm works
   • Starts with a test breath, measures dynamic compliance, then calculates the minimum pressure above PEEP needed to hit the target V_T.
   • On each subsequent breath it compares the measured exhaled V_T with the target and tweaks the driving pressure in small steps (≈ ±3 cm H₂O).
   • Regulation-pressure limit alarm. If V_T (e.g., 500 mL) still falls short, the ventilator keeps stepping the pressure up until either the target is met or the operator-set maximum pressure limit is reached—then an alarm (“regulation pressure limited”) announces that higher pressure is forbidden and V_T may drop.
2. Why PRVC behaves like VC and PC

   VC	PRVC	PC
   Volume target	Volume target with adaptive pressure	Pressure target
   Square flow	Decelerating flow → lower Ppeak	Decelerating flow
   Upper-pressure alarm terminates the breath early, often yielding a small VT	Controller raises pressure to avoid lost VT yet stays below the limit	Pressure is fixed; VT falls if compliance worsens

3. Operator-set variables
   • Target V_T (primary goal)
   • Respiratory rate (RR) — pairs with V_T to determine minute ventilation
   • Inspiratory time / I:E ratio
   • PEEP
   • Maximum pressure limit (e.g., 30–35 cm H₂O); set high enough to allow adaptation yet low enough to remain lung-protective
   • Trigger sensitivity & cycling criteria (synchrony)
4. Monitoring priorities
   • VC: watch delivered V_T; if the high-pressure limit aborts the breath, V_T drops.
   • PC: watch V_T; pressure is fixed, so stiff lungs yield smaller volumes.
   • PRVC: watch both:
     • P_peak — a gradual climb signals worsening compliance or resistance.
     • Exhaled V_T — a sudden fall means the pressure limit has been hit or spontaneous effort has waned.
5. Strengths
   • Decelerating flow and breath-by-breath pressure minimization lower P_peak and plateau pressure.
   • Maintains target V_T even when compliance changes, avoiding the low-volume alarms common in PC and the pressure spikes of square-flow VC.
6. Limitations & watch-outs
   • Hidden compliance deterioration: the ventilator quietly ratchets pressure up until it hits the limit; clinical decline may be missed unless P_peak is trended.
   • Slow to sudden load changes: stepwise 3 cm H₂O adjustments may lag rapid events (bronchospasm, tubing kink).
   • Spontaneous over-assistance: strong efforts raise V_T; the algorithm answers by lowering pressure, so the next passive breath may be under-supported.
   • Post-obstruction overshoot: after suctioning or secretion clearance, the previously high pressure can overshoot volume before the next recalibration—set tight volume alarms during airway procedures.
   • Sensor accuracy: leaks or water in the circuit can feed bad data to the controller, causing inappropriate pressure changes.

Clinical tip. Set the maximum pressure limit high enough (but safe) so PRVC can “do its job” in stiff lungs—commonly 5–7 cm H₂O above your plateau-pressure goal. Then trend both P_peak and exhaled V_T to catch rising pressures or falling volumes early.

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III. Pressure-regulated volume control (PRVC) - Old Version

PRVC is a closed-loop hybrid mode designed to deliver a clinician-selected tidal volume while capping airway pressure on every breath. The ventilator begins with a test breath, measures dynamic compliance, and then calculates the minimum inspiratory pressure required to achieve the target volume using a decelerating flow waveform. On subsequent breaths it compares the exhaled tidal volume against the set point and automatically adjusts the inspiratory pressure—typically in ±3 cm H₂O increments—until the volume target is met. Because flow decelerates, most of the breath is spent near the calculated peak pressure, yielding a pressure–time profile that resembles pressure control (PC), yet the controlling objective remains volume delivery, classifying PRVC as a volume-controlled strategy at its core.

The principal motivation for PRVC is to fuse the lung-protective advantages of PC (lower peak and plateau pressures, decelerating flow) with the metabolic certainty of a guaranteed tidal volume. This makes the mode attractive for patients whose compliance fluctuates rapidly—such as those with early acute respiratory distress syndrome, evolving pulmonary oedema, or post-operative atelectasis—where conventional VC risks pressure spikes and pure PC risks under-ventilation. However, PRVC can up-titrate inspiratory pressure to its safety ceiling if compliance deteriorates, potentially masking clinical decline; conversely, vigorous spontaneous effort may cause the algorithm to undershoot delivered volume.

Operator-controlled inputs are the target tidal volume, respiratory rate, inspiratory time (or I:E ratio), PEEP, and a maximum allowable pressure limit. The ventilator responds by modulating inspiratory pressure—and therefore peak flow—breath-by-breath, while maintaining the decelerating flow waveform typical of PC. The result is a mode that “looks and feels” like pressure control at the bedside, yet behaves as volume control under the hood, continuously trading small pressure adjustments for precise volume assurance.

• Operator-set variables

  • Target tidal volume (V_T) ― primary goal the algorithm strives to deliver.
  • Respiratory rate (RR) ― pairs with V_T to determine minute ventilation.
  • Inspiratory time or I:E ratio ― defines the duration available for pressure delivery and flow deceleration.
  • Positive end-expiratory pressure (PEEP) ― baseline pressure on which the inspiratory pressure is added.
  • Maximum pressure limit ― safety ceiling (often 30–35 cm H₂O); the ventilator will not exceed this value when auto-adjusting.
  • Trigger sensitivity & cycling criteria ― determine patient–ventilator synchrony but do not alter the adaptive algorithm.

• Why PRVC behaves like volume control

  • Guaranteed volume. Breath-by-breath adjustments continue until the measured exhaled V_T matches the set target.
  • Minute-ventilation predictability. Because V_T and RR are fixed inputs, minute ventilation remains stable despite compliance changes.
  • Alarms and data display. The ventilator still tracks “volume-delivered” errors similarly to conventional VC.

• Why PRVC mimics pressure control

  • Decelerating flow pattern. Flow peaks early and tapers, creating a pressure–time curve indistinguishable from PC.
  • Inspiratory pressure targeting. The machine sets and adjusts an inspiratory pressure above PEEP rather than a fixed flow.
  • Lower peak pressures. Auto-adjustment aims for the minimal pressure that achieves the target volume, often reducing PIP compared with square-flow VC.

• Limitations and watch-outs

  • Hidden compliance deterioration. The ventilator silently raises inspiratory pressure toward the safety ceiling as the lung stiffens. Because the rise is incremental, clinical decline may remain unnoticed until the preset limit is reached and tidal volume abruptly falls.
  • Sluggish response to sudden load changes. PRVC modifies inspiratory pressure in discrete steps (≈ ±3 cm H₂O per breath). When airway resistance or compliance shifts abruptly—such as during bronchospasm, kinking, or chest-wall splinting—the algorithm cannot elevate pressure as fast as pure pressure control (PC). The delayed rise can produce brief hypoventilation, elevated work of breathing, and alarm activation.
  • Spontaneous over-assistance. Vigorous patient effort inflates exhaled volume above the target. The controller reacts by lowering inspiratory pressure; when the effort subsides, the breath that follows may be under-supported, creating a pendulum of hypo- and hyper-ventilation.
  • Risk of inadvertent volutrauma after obstruction relief. During procedures such as tracheobronchial suction, the sudden removal of secretions markedly reduces airway resistance. The previously calculated pressure—now too high for the improved condition—can overshoot the target, delivering a larger tidal volume before the next breath permits recalibration. Close bedside monitoring and temporary volume alarms are advised whenever airways are manipulated.
  • Dependence on accurate real-time data. Sensor drift, water condensation, or circuit leaks may feed erroneous pressure–volume information into the controller, prompting inappropriate pressure adjustments that persist until the fault is detected.

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IV. Comparative summary

V. Practical bedside considerations

• Trigger and cycle settings. In ACMV, ensure sensitive inspiratory triggers and appropriate cycle criteria to harness adaptive flow without auto-triggering.
• Alarm limits. Because adaptive or decelerating patterns alter peak flow, flow and pressure alarms must be widened slightly to prevent nuisance alerts while retaining safety.
• Monitoring. Continuous evaluation of PIP, tidal variability, and patient effort (e.g., pressure-time product or diaphragm ultrasound) confirms that the chosen mode meets lung-protective and comfort goals.

VI. Key takeaways

• Flow adaptation distinguishes ACMV from conventional CMV by allowing demand-driven flow augmentation during square-wave volume breaths.
• PRVC replaces the square-wave with a decelerating flow, automatically titrating inspiratory pressure to deliver the set volume at the lowest feasible PIP.
• When dedicated PRVC is unavailable, selecting a decelerating-flow VC breath offers partial pressure relief but lacks breath-to-breath pressure regulation.
• Mode selection should weigh patient effort, lung mechanics, and the need for precise volume targeting against the potential for pressure-induced injury.

Written on June 7, 2025

Triggering mechanics in assisted ventilation: Pressure- vs Flow-Triggering

A mechanical breath begins when the ventilator detects patient effort through either a pressure trigger (drop below baseline circuit pressure) or a flow trigger (deflection of a continuous bias flow). Trigger sensitivity determines how much patient effort is required—less negative pressure or smaller flow reduction signifies a more sensitive trigger.

I. Pressure triggering

With pressure triggering the ventilator monitors circuit pressure relative to set PEEP. Inspiration is delivered when airway pressure falls by at least the trigger threshold \(\Delta P_{\text{trig}}\).

\[ P_{\text{aw}} \le P_{\text{EEP}} - |\Delta P_{\text{trig}}| \]

• Sensitivity scale. Typical adult thresholds range from −1 to −5 cm H₂O; a setting of −2 is relatively sensitive, whereas −10 is markedly insensitive and increases inspiratory workload.
• PEEP interaction. If PEEP = 7 cm H₂O and the trigger is −10, the patient must generate an airway pressure of −3 cm H₂O (7 − 10) to initiate a breath—often unattainable in respiratory fatigue.
• Clinical caveat. Excessive sensitivity (< −1 cm H₂O) may promote auto-triggering from cardiogenic oscillations or circuit bumping.

Figure. (1) With the trigger threshold set at −10 cm H₂O and PEEP at 7 cm H₂O, the patient-generated drop of −7 cm H₂O is insufficient to initiate a breath. (2) Reducing the threshold to −2 cm H₂O renders the ventilator sensitive enough to recognise the same effort and deliver inspiration.

II. Flow triggering

Flow triggering employs a constant bias flow—commonly 2 L·min⁻¹ in adults, 0.5 L·min⁻¹ in paediatrics—and recognises inspiration when a portion of that flow is diverted into the patient.

\[ F_{\text{trig}} = F_{\text{bias}}\!\left(1 - \frac{S}{10}\right) \]

where \(F_{\text{trig}}\) is the flow drop that activates the breath and \(S\) is the sensitivity setting (0–10).

• Higher numeric settings represent greater sensitivity (smaller flow drop).
• Over-sensitive values (e.g., \(S \ge 8\)) may auto-trigger during leak, patient repositioning, or water condensation movement, precipitating breath stacking.

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III. Comparative considerations

• Work of breathing. Flow triggering generally halves trigger effort compared with pressure triggering because the patient alters flow rather than generating extra negative pressure.
• Leak tolerance. Large leaks diminish net flow change, making flow triggering unreliable; pressure triggering may be preferred when unavoidable leaks exist (e.g., uncuffed tracheostomy, neonatal circuits).
• Noise immunity. Pressure triggering is less vulnerable to circuit vibration but more prone to missed efforts if set too insensitive.

IV. Practical adjustment algorithm

1. Start with flow triggering at \(S = 4\)–5 (≈ 40–50 % bias-flow drop) in adults; observe pressure–time curve for early dips indicating effort.
2. If missed efforts appear, increment sensitivity by 1 step and re-evaluate; if auto-triggering occurs, decrement by 1–2 steps or switch to pressure trigger.
3. For pressure triggering, begin at −2 cm H₂O; titrate toward −1 if effort remains high or toward −4 if auto-triggering persists.
4. Always reassess after position change, secretion clearance, or introduction of a circuit leak.

V. Key points

• Trigger sensitivity must balance patient comfort against inadvertent auto-cycles.
• Flow triggering reduces inspiratory workload but is leak- and motion-sensitive.
• Pressure triggering remains dependable when leaks are unavoidable, provided the threshold is not set excessively negative.

Written on June 7, 2025

T-pause and inspiratory time (Ti): timing mechanics in VC and PC

In volume-controlled (VC) ventilation, the clinician fixes the tidal volume and allows the ventilator to generate a square (constant) inspiratory flow to deliver that volume within the preset inspiratory time. Because the flow trace is flat, the entire inspiratory period is actively filling the lungs unless an explicit inspiratory pause (T-pause) is inserted at the end of inspiration. When a pause is added—commonly 10 % of the total cycle time—the active-flow window shortens while the delivered volume remains unchanged; the ventilator therefore compensates by increasing instantaneous flow, which in turn elevates peak inspiratory pressure (P_peak). Minute ventilation (Ė_V) under VC follows the direct proportionality Ė_V = V_T × RR, so any adjustment to respiratory rate (RR) or tidal volume (V_T) alters global carbon-dioxide clearance, whereas changes in pause fraction mainly influence alveolar pressure dynamics and airway stress.

Pressure-controlled (PC) ventilation, by contrast, targets a constant airway pressure rather than a fixed volume. The inspiratory flow automatically decelerates as the lungs fill, reaching near-zero toward the end of inspiration; this intrinsic taper means there is effectively no need for an additional T-pause. For a given compliance, PC can achieve the same tidal volume as VC while maintaining a lower P_peak, thereby reducing the risk of barotrauma in patients with stiff or injured lungs. Using the illustrative VC settings RR = 15 breaths·min^-1 (T_cycle = 4 s), I:E = 1:2 (ideal inspiratory window ≈ 1.33 s), and T-pause = 10 % (0.40 s), the active-flow time contracts to 0.93 s; reproducing the same volume in that shorter span demands a steeper flow ramp and raises P_peak. In PC mode the flow would simply taper earlier without an imposed pause, keeping airway pressures lower while preserving the delivered volume—provided that inspiratory time is long enough to reach the pressure target.

➤ In pressure-controlled ventilation there is no separate T-pause. Once the set driving pressure (P_control above PEEP) is reached, flow passively decelerates to near-zero, so the plateau pressure you observe is simply P_peak = PEEP + P_control. Any sudden change that increases airway resistance or decreases compliance—such as retained secretions—will cause the delivered tidal volume to fall at the same pressure target. For this reason, vigilant tidal-volume monitoring is essential in PC mode; a downward drift should prompt suctioning, bronchodilation, or adjustment of P_control / T_i to re-establish adequate ventilation.

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(1) T-pause 10 % (0.40 s, P_peak 29 cmH₂O); (2) T-pause 5 % (0.20 s, P_peak 27 cmH₂O); (3) T-pause 0 % (0 s, P_peak 26 cmH₂O). All examples use RR 15 breaths·min^–1 (cycle time 4 s) and the same tidal volume. As the inspiratory pause lengthens, the active-flow segment of inspiratory time (T_i) contracts, requiring a steeper flow ramp to deliver the preset volume and thus elevating peak inspiratory pressure. A moderate pause remains valuable for plateau-pressure measurement and alveolar recruitment despite the higher P_peak.

I. Timing building blocks

• Total cycle time \(T_{\text{cycle}} = 60 / RR\)
• Inspiratory phase partitions into an active-flow segment (\(T_{\mathrm{i,flow}}\)) and an optional T-pause.
• Inspiratory time \(T_{\mathrm i}\) (graphical width of the inspiratory flow trace) therefore equals \[ T_{\mathrm i}=T_{\mathrm{i,flow}}+T\text{-pause}. \]
• Expiratory time \(T_{\mathrm e}=T_{\text{cycle}}-T_{\mathrm i}\).

II. Computing Ti and T-pause in volume control (VC)

Given an operator-selected I:E ratio (\(I:E\)) and pause fraction \(P\) (decimal):

\[ \begin{aligned} T_{\mathrm i} &=T_{\text{cycle}}\times\frac{I}{I+E},\\ T\text{-pause} &=P\times T_{\text{cycle}},\\ T_{\mathrm{i,flow}} &=T_{\mathrm i}-T\text{-pause}. \end{aligned} \]

Numeric example

\(RR = 15\), \(I:E = 1:2\), \(P = 0.10\)

1. \(T_{\text{cycle}} = 4\ \text{s}\)
2. \(T_{\mathrm i} = 4\times\frac{1}{3}=1.33\ \text{s}\)
3. \(T\text{-pause} = 0.40\ \text{s}\)
4. \(T_{\mathrm{i,flow}} = 0.93\ \text{s}\)

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Inspiratory Time (Ti) Calculator

Computes total inspiratory time (Ti), the active-flow segment (Ti_flow), and the pause plateau from respiratory rate (RR), a single I:E ratio expressed as 1:E, and a user-defined T-pause percentage.

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III. Mode-dependent Ti behaviour

IV. Physiological implications of manipulating Ti and T-pause

• Gas distribution. Lengthening T-pause raises mean alveolar pressure, aiding alveolar recruitment and static compliance assessment.
• Peak pressure. In VC, a longer T-pause forces delivery of the same volume over a shorter \(T_{\mathrm{i,flow}}\), lifting \(P_{\text{peak}}\). In PC, \(P_{\text{peak}}\) stays locked, but V_T may fall if Ti is too short.
• CO₂ clearance. Effective minute ventilation obeys \(\dot V_{\mathrm E}=V_{\mathrm T}\times RR\); trimming Ti (especially \(T_{\mathrm{i,flow}}\)) without compensatory RR may elevate PaCO₂.
• Auto-PEEP risk. Prolonging Ti or T-pause at high RR shortens expiratory time, predisposing to air-trapping in obstructive physiology.

V. Bedside adjustment workflow

1. Choose baseline RR and I:E to assure \(T_{\mathrm e}\geq 2\tau\) (twice the patient’s expiratory time constant).
2. Introduce T-pause ≤10 % when accurate plateau pressure or alveolar recruitment is required.
3. After adding pause, re-check \(P_{\text{peak}}\) (VC) or delivered V_T (PC). If unsafe, either lower V_T, extend \(T_{\text{cycle}}\), or reduce P.
4. Monitor arterial gases; adjust RR if PaCO₂ rises due to reduced \(T_{\mathrm{i,flow}}\).

VI. Key takeaways ✏️

• Ti comprises an active-flow segment plus T-pause; only the active segment determines instantaneous flow and \(P_{\text{peak}}\).
• In VC, increasing T-pause elevates \(P_{\text{peak}}\) by squeezing flow into a shorter window; in PC, pressure stays fixed but volume may drop.
• Balanced timing preserves expiratory time, maintains minute ventilation, and avoids barotrauma or auto-PEEP.

Written on June 7, 2025

Written on June 7, 2025

PS and CPAP modes

Pressure-support (PS) and continuous positive airway pressure (CPAP) represent the least mandatory forms of mechanical assistance. These modes permit the patient to determine respiratory rate and—with appropriate settings—optimize comfort while maintaining adequate gas exchange. A safety net is provided by an apnea back-up algorithm that automatically delivers pressure-controlled (PC) breaths if spontaneous effort ceases.

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Apnea back-up ventilation

When no spontaneous trigger is detected within the pre-set apnea time (factory default 20 s but adjustable, e.g. 15 s in the alarm profile), the ventilator converts to a PC back-up pattern until patient effort resumes.

• Back-up pressure above PEEP (ΔP): clinician-selected driving pressure.
• Back-up respiratory rate (RR): typically 10–15 breaths·min^-1.
• Inspiratory:expiratory (I:E) ratio: often 1:2; ensures adequate expiratory time.
• Alarm behaviour: if the required ΔP to attain the target tidal volume approaches the operator-defined pressure limit, a “regulation pressure limited” alarm is annunciated, yet the ventilator strives to preserve ventilation without breaching the ceiling.

Even during back-up PC, the pressure limit should be set high enough to enable protective tidal volumes but low enough to avoid barotrauma.

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CPAP

Setting pressure support above PEEP to 0 cm H₂O converts the circuit to pure CPAP. The ventilator then provides only the selected PEEP and oxygen concentration, relying entirely on the patient’s inspiratory effort for tidal ventilation. Back-up PC remains available should apnea occur.

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Pressure-support (PS)

• Core adjustable variables

  1. Pressure support above PEEP (ΔP): primary determinant of tidal volume.
  2. PEEP: baseline pressure.
  3. FiO₂: oxygen fraction.
  4. T_insp rise: time from trigger to attainment of target pressure (comfort parameter).
  5. Trigger sensitivity: flow- or pressure-based threshold initiating inspiration.
  6. Inspiratory cycle-off (End inspiration): flow threshold (%) at which inspiration terminates and exhalation begins.

• Inspiratory cycle-off and timing mechanics

  PS lacks an I:E ratio; instead, inspiratory cycle-off governs inspiratory time (T_I) relative to total breath duration (T_tot). The ventilator switches to exhalation when inspiratory flow decays to a defined fraction of its peak.

  1. Default threshold: 30 % of peak inspiratory flow.
  2. Raising threshold (≥ 40 %): earlier cycling, shorter T_I, smaller tidal volume; useful when late-inspiratory “fighting” or double triggering is observed.
  3. Lowering threshold (< 30 %): prolonged T_I, larger tidal volume; may improve ventilation in restrictive lungs but risks air trapping if expiratory time becomes insufficient.

  Cycle-off setting	Effect on timing	Typical clinical cue
  70 %	Very early termination; shortest TI	Corrects late-inspiratory pressure spikes (patient attempts exhalation)
  30 % (default)	Balanced TI / Ttot	General use; adjust as compliance or demand changes
  1 %	Very late termination; longest TI	Extreme restrictive physiology when larger tidal volume is required

• Leakage and resistance considerations

  Circuit leaks (e.g., reduced cuff pressure) sustain elevated flow throughout inspiration, preventing decay to the cycle-off threshold and unintentionally prolonging T_I. When compensation raises the effective threshold above the set value, the cycle-off percentage should be increased to restore timely exhalation.

• Managing inspiratory “fighting”

  1. Increase the inspiratory cycle-off threshold (e.g., from 30 % to 40–50 %) to allow earlier cycling.
  2. Shorten T_insp rise when flow demand is high; lengthen it when overshoot occurs.
  3. Verify trigger sensitivity to prevent delayed inspiration.

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Inspiratory cycle-off effects in PS mode.
(1) Default cycle-off 30 % of peak flow.  (2) Cycle-off 70 % → earlier switch to exhalation.  (3) Cycle-off 1 % → prolonged inspiration, larger tidal volume.  (4) Cycle-off set to 30 % but cuff pressure falls; compensation elevates effective threshold (≈40 %), lengthening inspiration—raise the setting above 40 % to restore timing.  (5) Late-inspiratory “fighting” produces an end-inspiratory pressure spike; increasing cycle-off shortens T_I and resolves the dyssynchrony.

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Clinical summary

• PS and CPAP permit spontaneous breathing with adjustable pressure support and PEEP, while an apnea timer safeguards against hypoventilation via PC back-up.
• Inspiratory cycle-off replaces I:E ratio in PS; meticulous adjustment optimizes comfort, prevents air trapping, and avoids late-inspiratory dyssynchrony.
• Pressure limits in back-up PC and PS should balance lung protection with tidal-volume adequacy; both ΔP and delivered volumes require continuous monitoring.

Written on June 8, 2025

PRVC algorithmic software framework: breath-by-breath pressure regulation with decelerating flow

Pressure-regulated volume control (PRVC) is a hybrid mode of mechanical ventilation that combines the advantages of volume control (VC) and pressure control (PC). A target tidal volume (V_T) is delivered while the ventilator continuously adapts the inspiratory pressure on a breath-by-breath basis, using a decelerating (ramp-down) flow pattern. This strategy aims to minimize peak inspiratory pressure (PIP) and improve patient–ventilator synchrony.

Foundational principles

• Flow patterns during inspiration

  • Constant (square-wave) flow (typical of traditional VC): Delivers a fixed flow until the set V_T is reached. To achieve the same V_T as PC, VC generally requires a higher instantaneous PIP because gas must be driven into the lungs at a fixed rate irrespective of changing compliance or resistance. When an inspiratory pause time (T_pause) is added to VC, flow stops while the airway remains occluded, increasing both alveolar pressure and measured PIP even further.
  • Decelerating flow (used in PRVC and PC): Starts with a high peak flow and progressively tapers, distributing gas more evenly and lowering PIP for an identical V_T. The early, high-flow phase quickly meets patient demand, improving comfort and synchrony.

  

• Breath-to-breath pressure regulation algorithm

  PRVC employs an adaptive software loop:

  1. Initialization – “learning” breath: The ventilator often delivers a VC-like test breath at a modest flow to measure the patient’s compliance and airway resistance.
  2. Pressure estimate: Using the measured V_T and peak/plateau pressures, the microprocessor calculates the minimum inspiratory pressure (P_insp) projected to deliver the set V_T on the next breath.
  3. Delivery as a pressure-controlled breath: The subsequent breath is given in PC format at the computed P_insp with a decelerating flow waveform.
  4. Continuous adaptation: After each breath, the delivered V_T is compared with the target. If V_T is lower (or higher) than desired, P_insp is incremented or decremented—typically by 1–3 cmH₂O—on the following breath, constrained by an upper safety limit P_max.

  This closed-loop approach preserves volume assurance while always seeking the lowest effective pressure.

• Ventilatory mechanics: why PRVC lowers pressure yet raises oxygenation

  • The decelerating flow profile front-loads flow, so alveoli fill earlier and the remaining inspiratory time is spent at a lower flow and pressure. As a result, PIP is reduced by 3–10 cmH₂O compared with VC at the same V_T.
  • The prolonged end-inspiratory phase raises mean airway pressure (MAP) (the time-averaged pressure over the entire respiratory cycle), creating gentle alveolar recruitment that lowers physiologic dead space and enhances oxygenation without additional barotrauma risk.
  • Because flow decelerates naturally, patient-triggered efforts are less likely to fight a rigid flow profile, reducing the work of breathing (WOB) and the need for deep sedation or neuromuscular blockade.
  • Plateau pressure (P_plat)—the static alveolar pressure measured during an inspiratory hold—remains below the lung-protective threshold (≈ 30 cmH₂O) more readily than in VC where higher peaks are common, especially if an inspiratory pause is used.

Clinical advantages and caveats

• Key benefits

  1. Consistent tidal volume despite evolving lung mechanics.
  2. Lower PIP than VC, reducing barotrauma and volutrauma risk. The difference widens further when VC employs an inspiratory pause.
  3. Higher MAP (without a high PIP) improves oxygenation by recruiting atelectatic lung units and reducing ventilation–perfusion mismatch.
  4. Enhanced patient–ventilator synchrony lowers WOB and sedation requirements.
  5. In neonates, PRVC has been associated with fewer ventilator-related complications, including intraventricular hemorrhage.

• Important cautions

  • Rapid, large swings in resistance or compliance (e.g., severe bronchospasm or pulmonary edema) may temporarily outpace the algorithm, risking transient hypoventilation.
  • Significant circuit or airway leaks (uncuffed tubes, bronchopleural fistula) can lead the ventilator to raise P_insp inappropriately.
  • An overly low or overly high P_max undermines the safety of the algorithm, either limiting tidal volume delivery or eliminating the pressure-limiting advantage.

Written on June 19, 2025

Troubleshooting

Considerations When Using a Jet Nebulizer in Respiratory Therapy

When using a pneumatic jet nebulizer in respiratory therapy, it's crucial to consider its impact on mechanical ventilation parameters. (1) First, introducing a jet nebulizer can significantly increase the flow within the ventilatory circuit, which in turn may inadvertently raise the tidal volume (TV) delivered to the patient. This increase can lead to unexpectedly high lung volumes, risking volutrauma if not closely monitored and adjusted. (2) Additionally, this extra flow can cause a slight increase in airway pressure. While generally mild, this increase requires careful monitoring to prevent potential adverse effects on patients with compromised lung function or those at risk of barotrauma.

(3) Another consideration is the effect on the fraction of inspired oxygen (FiO2). The use of a jet nebulizer can dilute the oxygen concentration within the ventilatory circuit as air is utilized to nebulize the medication, resulting in a reduced FiO2. Adjusting the oxygen settings to compensate for this dilution is essential to ensure that the desired level of oxygenation is maintained. (4) Furthermore, the additional flow and pressure fluctuations introduced by the nebulizer can interfere with the ventilator's sensitivity settings, complicating the initiation of spontaneous breathing by the patient. This may increase the work of breathing and lead to discomfort or fatigue. To mitigate these effects, it may be necessary to adjust the trigger sensitivity settings on the ventilator to lower the threshold required for triggering, thereby reducing the respiratory effort needed from the patient.

Ventilator Troubleshooting Guide

High Airway Pressure (Paw) Alarm: This alarm can be triggered by increased airway resistance or decreased lung compliance. To reduce airway resistance, perform suctioning and administer nebulized treatments. If the issue is related to lung compliance, consider interventions such as inhaled nitric oxide or extracorporeal membrane oxygenation (ECMO) to enhance lung flexibility and functionality.

Decreased Oxygen Saturation: When decreased oxygen saturation is detected, indicated by alarms or monitoring systems, several critical steps must be taken to ensure optimal patient care. Oxygen levels are primarily monitored using SpO2 and arterial oxygen partial pressure (PaO2). Immediate actions include resolving any High Paw Alarm by suctioning and nebulization to decrease airway resistance. It is also vital to thoroughly check the ventilatory circuit for any disconnections or leaks, as these can significantly compromise the oxygen delivery system.

To assess the patient's oxygenation status more comprehensively, performing an arterial blood gas analysis (ABGA) is recommended to check PaO2 levels. Based on these results, adjustments to the fraction of inspired oxygen (FiO2) and positive end-expiratory pressure (PEEP) should be considered. Initially, FiO2 may be increased to deliver more oxygen, especially in acute settings. Concurrently, adjusting PEEP can help improve oxygenation and ensure the patient receives adequate oxygen while preventing potential lung damage.

A practical approach involves starting with higher FiO2 and PEEP settings and then gradually reducing them as the patient's condition stabilizes or improves. For example, beginning with an FiO2 of 100% paired with a PEEP of 12 cm H2O, and methodically lowering both to potentially an FiO2 of 30% and a PEEP of 5 cm H2O as the patient's oxygenation status allows. This systematic and intuitive adjustment process is aligned with the ARDSnet protocol guidelines, which provide extensive tables and further recommendations for managing FiO2 and PEEP in different clinical scenarios.

High Carbon Dioxide Levels (PaCO2): If this alarm activates, first assess whether increased airway resistance is contributing to inadequate CO2 removal. To enhance CO2 clearance, adjust the tidal volume (TV) upwards. Increasing the respiratory rate (RR) can also help by augmenting the overall ventilation. Another strategy is to modify the inspiratory to expiratory ratio (I:E Ratio), which can improve ventilation efficiency. However, care must be taken to ensure that these adjustments do not lead to air trapping, especially in patients with conditions like Chronic Obstructive Pulmonary Disease (COPD).

PEEP High Alarm Due to Filter Obstruction: A 'PEEP High Alarm' in mechanical ventilation typically signals a blockage in the expiratory phase, often due to a clogged filter that impedes the flow of expired air, leading to increased end-expiratory pressure. The crucial step in troubleshooting is to promptly remove the filter to check if it is the source of the obstruction. If removing the filter resolves the alarm and normal ventilation function is restored, this confirms the filter as the culprit. The immediate replacement of the obstructed filter is critical to maintain unimpeded airflow, ensuring the ventilation system operates efficiently and safely.

Management of Expiratory Filters: Expiratory filters are critical in managing airway resistance within mechanical ventilation systems. These filters usually last one to two days, though high-quality, genuine filters can remain effective for up to five days under ideal conditions. Obstructions in these filters are common and can be caused by various factors, including the administration of specific medications such as colistin and coagulants, or due to excessive nebulization. When filter issues are suspected, such as after administering colistin, it is advisable to perform routine checks and ensure timely maintenance or replacement of the filters to maintain optimal ventilation management.

Strategic Use of PEEP in Patients with V/Q Mismatch and Shunting: For patients exhibiting V/Q mismatch or shunting, often caused by lung infiltration as indicated by a chest X-ray, increasing the positive end-expiratory pressure (PEEP) can enhance their condition by improving oxygenation and reducing shunt effects. However, if the PEEP exceeds 10 cm H2O, it may lead to decreased venous return, potentially resulting in hemodynamic instability. In such scenarios, administering an inotrope may be necessary to help maintain or increase blood pressure.

Auto-PEEP Detection and Management (Version I): Auto-PEEP is a significant concern, particularly in patients receiving medications such as colistin or anticoagulants, which can increase the risk of this complication. Monitoring the end-expiratory flow, known as V̇ee on a Servo ventilator, is critical because an increase in V̇ee may indicate the presence of Auto-PEEP, necessitating prompt assessment and intervention to ensure optimal ventilator function and patient safety.

Effective management of Auto-PEEP includes adjusting the inspiratory to expiratory (I:E) ratio, for example, from 1:2 to 1:3. This adjustment extends the expiratory time relative to the inspiratory time, aiming to provide sufficient expiratory time — ideally more than three times the time constant (Tc). This strategy promotes complete exhalation, prevents lung overdistension, optimizes respiratory mechanics, and aids in preventing complications such as air trapping while ensuring efficient CO2 removal. Maintaining the inspiratory time (Ti) within 0.6 to 1.2 seconds, even when altering the I:E ratio, is essential to avoid negative impacts on the patient's breathing pattern.

In scenarios where there is a rapid increase in respiratory rate (RR), continuous monitoring of V̇ee through the "Trends" function on the ventilator is essential. This allows clinicians to observe changes over time and make informed decisions about ventilatory adjustments. Additionally, performing an arterial blood gas analysis (ABGA) is routine to check for CO2 retention and decreased O2 levels, further informing the adjustment process.

Further strategies to manage Auto-PEEP include ensuring clear airways and, if necessary, adjusting the respiratory rate (RR) and tidal volume (TV). Encouraging patients to take deep breaths can help decrease the buildup of intrinsic PEEP and facilitate better lung decompression. Additionally, adjunctive therapies such as nebulization, the administration of steroids, or bronchoscopy may be necessary to address underlying respiratory issues contributing to Auto-PEEP.

Air Trapping Indicators and Management (Version II): Air trapping, commonly seen in conditions like COPD, asthma, or airway edema, manifests through several indicators:

• Visual Inspection: The flow graph does not return to zero during expiration, indicating incomplete exhalation.
• V̇ee (End-expiratory Flow): A V̇ee greater than or equal to 1 suggests persistent air in the lungs post-expiration.
• Arterial Blood Gas Analysis (ABGA): This analysis is essential for checking CO2 levels, with a PaCO2 above 45 mmHg indicating CO2 retention.
• Lower Inflection Point (LIP) Changes in PC Mode: A rightward shift in LIP points to increased airway resistance, while a swollen or wider LIP indicates decreased lung compliance. Reassessing the LIP through sputum suction can confirm these conditions.
• Monitored PEEP Variations: If the monitored PEEP significantly exceeds the set value, it can lead to decreased cardiac output and unstable blood pressure, indicating severe air trapping. Sometimes, with air trapping, monitored PEEP might significantly exceed the set value (e.g., set at 5 but reads 19 or 20), leading to decreased cardiac output and unstable blood pressure.
• Total PEEP: When total PEEP readings exceed set PEEP, it suggests the presence of auto-PEEP, confirming air trapping (e.g., set PEEP at 5 but total reads 6).
• Re (Expiratory Resistance): Expiratory resistance, when measured during holds, that falls within the 30-40 range indicates severe CO2 retention and requires urgent intervention. This resistance ranges from normal (5-15) to critical levels (30-40), indicating severe CO2 retention and necessitating immediate attention. While Total PEEP primarily helps in assessing the degree of air trapping and managing auto-PEEP, Re (expiratory resistance) is crucial for diagnosing the extent and cause of airway obstruction and evaluating overall lung mechanics.

Management strategies for air trapping include increasing the expiratory time to allow more complete exhalation, frequent nebulization to reduce airway resistance, and ensuring the expiratory time is at least three times the time constant (Tc) to promote efficient gas exchange and prevent complications from incomplete exhalation.

Air Flow Reductions: In scenarios where there is inspiratory pressure but the airflow drops to zero, urgent assessment and management are crucial to identify and rectify underlying causes. This issue often points to problems with lung compliance or mechanical impediments in the ventilation system. Evaluating lung elasticity is essential, and it's important to ensure that inspiratory pressures are not excessively high, as this could exacerbate lung stiffness and compromise function.

In terms of mechanical causes, obstructions in the endotracheal tube (ET tube) should be checked. Such obstructions can be due to kinking, blockage within the tube, or from the patient biting the tube. Inspecting and, if necessary, replacing the ET tube is crucial. If the patient is biting the tube, using a bite block can prevent occlusion and ensure continuous airflow. Addressing these issues promptly is vital not only to restoring proper airflow but also to preventing potential lung damage from inadequate ventilation.

Additionally, understanding lung compliance through static and dynamic indices can provide insights into lung health and guide appropriate ventilatory support. Static Compliance (Cstat) is calculated from the formula VTe / Pplat - PEEP, where VTe is the exhaled tidal volume, Pplat is the plateau pressure after an inhalation pause, and PEEP is the positive end-expiratory pressure, with normal values ranging from 60 to 100 ml/cm H₂O. A value below this range indicates increased lung stiffness. Dynamic Compliance (Cdyn), calculated as VTe / EIP - PEEP, where EIP is the end-inspiratory pressure, typically ranges from 50 to 80 ml/cm H₂O. Values below 30 ml/cm H₂O suggest severe lung conditions such as Acute Respiratory Distress Syndrome (ARDS). Monitoring these indices helps in fine-tuning ventilatory settings to optimize patient care and outcomes.

Static and dynamic compliance insights into lung health

Static Compliance (C_stat)

Measures lung compliance without the influence of airway resistance.

Cstat = VT / (Pplat − PEEP)

Where P_plat is Plateau Pressure and P_EEP is Positive End-Expiratory Pressure.

Dynamic Compliance (C_dyn)

Dynamic compliance is measured during airflow and includes resistance.

Cdyn = VT / (Ppeak − PEEP)

Where P_peak is Peak Inspiratory Pressure and P_EEP is Positive End-Expiratory Pressure.

Managing ventilator alarms: Paw high and low expiratory minute volume

I. Airway-pressure high (P_aw high) alarm

An elevated airway-pressure alarm reflects either increased resistance (secretions, tube kinking, bronchospasm) or diminished compliance (pneumothorax, pulmonary oedema, acute respiratory distress syndrome). A tiered, protocol-driven response preserves lung integrity and forestalls secondary alarms.

1. Immediate safety check

• Briefly silence the alarm; keep pressure and flow waveforms visible.
• Verify that the upper pressure limit is appropriate (usual setting 35–40 cm H₂O); adjust upward only after reversible causes are excluded.

2. Rapid reversible causes

• Secretions ― coarse breath sounds or saw-tooth flow pattern: perform tracheobronchial suction.
• Endotracheal-tube problems ― inspect for kinking, biting, or endobronchial migration; reposition or exchange as required.
• Bronchospasm ― wheeze and prolonged expiration: administer a rapid-acting bronchodilator (e.g., albuterol 2.5 mg via in-line nebuliser) and reassess.
• Patient–ventilator asynchrony ― optimise trigger sensitivity or convert to pressure-controlled mode if demand flow is high.

3. Analgesia and sedation to reduce airway pressure

• Opioid bolus — fentanyl 1–2 µg·kg^-1 IV for acute coughing or bucking; follow with an infusion 1–3 µg·kg^-1·h^-1 if repeated events occur.
• Benzodiazepine — midazolam 0.05–0.1 mg·kg^-1 IV to temper agitation; titrate to a Richmond Agitation-Sedation Scale (RASS) target of −2 to −4.
• Propofol infusion — initiate at 5–20 µg·kg^-1·min^-1 when deep, rapidly titratable sedation is required; monitor haemodynamics closely.
• Continuously reassess airway pressure after each intervention to confirm a downward trend.

4. Compliance-related pathology

• Pneumothorax / hemothorax ― unequal excursion, sudden desaturation: confirm with ultrasound or chest radiography and insert a drain when indicated.
• Pulmonary oedema / ARDS ― bilateral infiltrates: escalate PEEP, consider recruitment manoeuvres or prone positioning.

5. Ventilator parameter optimisation

• Decrease tidal volume (V_T) by 1–2 mL·kg^-1 predicted body weight to reduce driving pressure.
• Extend inspiratory time or lower flow rate to blunt the square-wave peak, provided oxygenation remains adequate.
• Convert from volume control to pressure control to cap the peak inspiratory pressure (PIP).

PIP explained. Peak inspiratory pressure is the highest airway pressure recorded during inspiration.

• In pressure-controlled modes, PIP = PEEP + preset inspiratory pressure.
• In volume-controlled modes, PIP comprises PEEP plus resistive and elastic pressure components needed to deliver flow.

Maintaining PIP ≤ 30–32 cm H₂O mitigates barotrauma risk.

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II. Alarm cascade  P_aw high → low expiratory minute volume

Persistent high airway pressure may truncate tidal delivery or trigger premature cycling, precipitating a low expiratory minute-volume alarm.

1. Leak survey

• Verify cuff pressure (target 20–30 cm H₂O); reinflate if below range.
• Inspect humidifier, filter, and limb connections; re-seat loose fittings.
• Assess chest drains and pleural interfaces for large air leaks.

2. Airway patency reassessment

• Repeat suction to clear residual secretions displaced during earlier manoeuvres.
• Consider bronchoscopy if mucus plugs or clots are suspected.

III. Diagnostic cross-matrix

IV. Key points

• Resolve P_aw high promptly; unresolved pressure elevations frequently lead to low minute-volume alarms.
• Differentiate resistive issues (secretions, bronchospasm) from compliance loss (pneumothorax, ARDS) to guide targeted therapy.
• After leaks are excluded, titrate V_T, PIP, and RR incrementally while keeping plateau pressure ≤ 30 cm H₂O.
• Document alarm events and corrective steps; recurrent patterns often presage evolving pathology before overt decompensation.

Written on June 7, 2025

Clinical Case Study

Case1: Managing Dyspnea in an ALS Patient with ST Mode Ventilation

Managing a patient with ALS experiencing dyspnea necessitates a nuanced approach to both immediate and long-term respiratory support strategies. An effective initial strategy often includes adjusting the Inspiratory Positive Airway Pressure (IPAP) in Spontaneous/Timed (ST) mode, for example, increasing it from 13 to 15 cm H2O. This adjustment can enhance alveolar ventilation by increasing the tidal volume, potentially alleviating symptoms of dyspnea and reducing the patient's respiratory effort. Such strategic management is essential in addressing progressive neuromuscular conditions like ALS, where muscle weakness can intensify over time, escalating the patient's respiratory workload.

If the patient's dyspnea persists and there is an inclination to remove the ventilator, it might indicate a conflict during inspiration when the patient attempts to exhale. In such scenarios, it is crucial to consider adjusting or increasing the "Cycle" setting of the ST mode from the current 25% gradually upwards. This adjustment can help synchronize the ventilator more closely with the patient's natural breathing cycle, potentially reducing the sensation of fighting the ventilator and improving comfort.

Continuous assessment is crucial to determine if further adjustments or a switch in ventilation mode might offer additional benefits. In cases where adjustments to IPAP alone are insufficient for managing symptoms effectively, or when concerns arise regarding patient comfort and ventilator synchrony, exploring other modes such as BiPAP or CPAP may be warranted.

BiPAP is particularly beneficial due to its capability to independently control both IPAP and EPAP, allowing for customized support during both inhalation and exhalation phases. Unlike ST mode, which primarily focuses on maintaining a minimum number of breaths per minute with less flexibility, BiPAP facilitates dynamic adjustments that can more precisely respond to the patient's varying respiratory needs throughout the day or as their condition changes. This refined approach to respiratory support is especially advantageous in ALS, where patients may experience fluctuating respiratory efforts.

Furthermore, BiPAP can enhance patient-ventilator synchrony by independently and precisely adjusting IPAP and EPAP. This adjustment improves synchrony with the patient's natural breathing cycle, potentially reducing asynchrony and increasing comfort — critical factors for patients who can still initiate breaths but whose respiratory strength may vary. The versatility of BiPAP extends to periods of sleep or varying levels of respiratory muscle fatigue, making it an optimal choice for maintaining comfort and effective ventilation across different states of patient activity and rest.

Pressure Support (PS) mode offers another viable option for patients capable of initiating breaths independently but who require assistance to maintain adequate ventilation. PS mode supports the patient's breathing effort by providing a preset level of pressure support during inhalation, terminated based on the patient's inspiratory flow, thus facilitating a more natural breathing pattern. This mode significantly enhances patient-ventilator synchrony and comfort, particularly beneficial for conscious patients with fluctuating respiratory drive.

Conversely, Continuous Positive Airway Pressure (CPAP) maintains a constant pressure throughout the respiratory cycle. This mode might not suit ALS patients requiring active assistance with inhalation due to muscle weakness. CPAP is generally more appropriate for patients who can maintain adequate spontaneous breathing efforts and primarily need support in keeping their airways open.

Ultimately, the decision to adjust IPAP within ST mode or to transition to another mode such as BiPAP or PS should be driven by ongoing evaluations of the patient's respiratory status, including blood gas analyses and symptom assessment. Regular re-evaluation and close monitoring are essential to ensure that the ventilatory support continues to meet the evolving needs of the ALS patient, striving to achieve an optimal balance of adequate ventilation, patient comfort, and minimal respiratory effort. This comprehensive approach ensures that respiratory management is both effective and adaptable, providing tailored support that evolves with the patient's condition.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Case2: Tailoring BiPAP Settings for Managing Respiratory Distress with CO2 Retention

In managing respiratory distress in patients using BiPAP ventilation, specific adjustments are essential for those showing signs of hypoventilation and CO2 retention. Typically, these patients demonstrate reduced tidal volumes (Vt) of 150-250 mL, with arterial blood gas (ABGA) analyses indicative of mild respiratory acidosis, typically with a pH slightly below the normal range of 7.35-7.45, and elevated PCO2 slightly above the normal range of 35-45 mmHg. Bicarbonate levels may also be at the upper end of the normal range (22-28 mEq/L), indicating renal compensation. Additionally, an elevated PO2 well above the normal range for room air (75-100 mmHg) suggests the use of supplemental oxygen therapy, which is crucial for managing hypoxemia but requires careful monitoring to avoid complications associated with hyperoxia.

Rationale for Increasing IPAP: An increase in IPAP, for example from 13 to 15 cm H2O, is strategically implemented to enhance alveolar ventilation and improve CO2 clearance. This intervention is essential as it directly increases tidal volume, promoting deeper breaths that not only help in recruiting more alveoli for gas exchange but also effectively eliminate excess CO2.

Reasoning for Lowering Oxygen Supply: Even with stable SpO2 levels above 92%, reducing supplemental oxygen is crucial to prevent oxygen toxicity and hyperoxia, which could suppress respiratory drive — particularly harmful in conditions predisposing to CO2 retention. Adjusting oxygen delivery to maintain SpO2 just above 92% ensures sufficient oxygenation without compromising the patient's inherent respiratory efforts, thereby supporting natural respiratory mechanisms and enhancing overall respiratory function.

Necessity of Manual Adjustment of IPAP: Despite the dynamic capabilities of BiPAP machines to adjust IPAP and EPAP, manual intervention is often required. Automatic adjustments might not always perfectly align with rapidly changing or unique clinical situations due to limitations in preset algorithms. Manually increasing IPAP allows healthcare providers to fine-tune ventilatory settings based on real-time patient responses and specific ventilation goals, ensuring that the therapeutic approach is precisely tailored to achieve optimal outcomes. This level of customization is crucial for effectively managing acute respiratory distress and provides significant advantages over more rigid modes like ST, which are less adaptable to patient-specific needs.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Case3: Longitudinal Analysis of Ventilator Management in a Patient With Chronic Respiratory Failure

Home Ventilator Application Status
- Chronic CO2 retention and respiratory acidosis
- Compensatory metabolic alkalosis
  
- December 22, 2023: BiPAP initiated
  
- December 29, 2023: Due to diaphragmatic dysfunction, home ventilator initiated in SIMV-PS mode
     Tidal Volume (TV): 300–350 mL
     Inspiratory Pressure (Pi): 10–15 cmH2O
     Respiratory Rate (RR): 16 breaths/min
    
- February 2024: Home ventilator adjusted (SIMV mode)
     Pressure (Pr): 13 cmH2O
     PEEP: 5 cmH2O
     RR: 15 breaths/min
     O2: 1 L/min
    
- March 25, 2024: Changed to PSV mode
     Measured TV: ~300 mL
     Pressure Support (Pr): 12 cmH2O
     PEEP: 5 cmH2O
     RR: 15 → 14 breaths/min

- November 8, 2024: PSV mode continued
     Measured TV: ~300 mL
     Pr: 12 cmH2O
     PEEP: 5 cmH2O
     RR: 14 breaths/min

The patient has chronic CO2 retention and a compensatory metabolic alkalosis, indicating a long-term respiratory failure scenario. Initial noninvasive management (BiPAP) was followed by a transition to a home ventilator setup due to diaphragmatic dysfunction. Over time, the ventilator mode evolved from SIMV with pressure support to PSV, reflecting changes in clinical strategy and attempts to balance stable support with partial patient-driven ventilation. Ultimately, the patient has maintained stability on a PSV configuration without further reductions in pressure support.

Chronological Overview of Ventilator Management

1. December 22, 2023 (BiPAP Initiation)

   Documented Settings: BiPAP (Bilevel Positive Airway Pressure) initiated, but no exact pressures recorded.

   Typical Variables Required:

   • IPAP (Inspiratory Positive Airway Pressure)
   • EPAP (Expiratory Positive Airway Pressure)
   • FiO2 or O2 flow as needed

   Commentary: BiPAP provides two distinct pressure levels. At this stage, it was likely chosen to noninvasively support ventilation, reduce CO2 levels, and alleviate work of breathing. Precise IPAP and EPAP values would be necessary to fully reproduce the initial setup. The missing data here—such as exact pressure levels and O2—suggests incomplete information.

2. December 29, 2023 (Transition to SIMV-PS)

   Documented Settings: SIMV-PS mode: TV ~300–350 mL, Pi 10–15 cmH2O, RR 16 (PEEP and exact FiO2/O2 unknown)

   Typical Variables Required for SIMV-PS:

   • For Volume-Controlled SIMV: Set Tidal Volume, RR, FiO2/O2, PEEP
   • For Pressure-Controlled SIMV: Set Inspiratory Pressure, RR, FiO2/O2, PEEP, and inspiratory time
   • Pressure Support (PS) level for spontaneous breaths above PEEP

   Commentary: Transitioning to SIMV suggests a need for more structured ventilatory support. Some parameters (PEEP, FiO2) are not documented here, making it hard to fully replicate the initial SIMV setup. The mention of Pi (Inspiratory Pressure) suggests a pressure-limited or pressure-supported component for spontaneous breaths.

3. February 2024 (Adjusted SIMV Settings)

   Documented Settings: SIMV: Pr 13 cmH2O, PEEP 5 cmH2O, RR 15, O2 1 L/min

   Typical Variables for SIMV-PS:

   • Pressure Support level (Pr)
   • PEEP
   • Mandatory RR
   • FiO2 or O2 flow

   Commentary: By February, the documentation is more complete: a defined pressure support level (Pr 13 cmH2O) and PEEP (5 cmH2O) are stated, along with RR and O2 flow. This reflects a refinement in the ventilation strategy. The patient is still receiving partial mandatory support (SIMV) along with pressure support for spontaneous breaths. Although O2 is given as 1 L/min, the exact FiO2 remains unreported; still, this information is more sufficient for reproduction compared to December 29, 2023.

4. March 25, 2024 (PSV Mode Initiation)

   Documented Settings: PSV mode: Measured TV ~300 mL, Pr 12 cmH2O, PEEP 5 cmH2O, RR from 15 to 14

   Typical Variables Required for PSV:

   • Pressure Support level (above PEEP)
   • PEEP
   • FiO2/O2 flow
   • The tidal volume and RR are patient-dependent (not “set” but “achieved”)

   Commentary: Switching to PSV reduces mandatory breaths, relying on the patient’s inspiratory effort. Pressure Support (12 cmH2O) and PEEP (5 cmH2O) remain key settings. The measured tidal volume (~300 mL) and recorded RR (14) show patient-driven ventilation within supported parameters. Additional variables (exact FiO2, alarm settings) are not specified, but enough information is present to approximate the scenario.

5. November 8, 2024 (Continued PSV Mode)

   Documented Settings: PSV mode: TV ~300 mL, Pr 12 cmH2O, PEEP 5 cmH2O, RR 14

   Commentary: No change in support parameters since March 25, 2024. This long-term stability suggests the patient’s condition is neither deteriorating nor significantly improving toward lower support. The ventilator settings remain consistent, likely indicating chronic stable respiratory failure management.

Comparing Noninvasive and Invasive Modes, Including CPAP

• BiPAP vs. PSV as Final Steps:
  • BiPAP (Noninvasive): Often considered for patients who do not require intubation, or as a step down from invasive ventilation to noninvasive support at home. It is suitable as a “final step” in noninvasive respiratory support for stable chronic patients.
  • PSV (Invasive): Commonly used as a final step in weaning from invasive mechanical ventilation. Once a patient can maintain adequate ventilation and oxygenation on low levels of PSV, clinicians may consider extubation.
• What About CPAP?
  • CPAP (Continuous Positive Airway Pressure): Provides a single constant pressure level. It improves oxygenation by preventing alveolar collapse but does not offer additional inspiratory support.
  • BiPAP: Offers two pressure levels—IPAP and EPAP. IPAP assists inspiration, while EPAP (similar to PEEP) keeps alveoli open. This dual-level support can augment ventilation and reduce CO2 more effectively than CPAP alone.
• Naming and Variables:
  • BiPAP: "Bi" stands for “Bilevel Positive Airway Pressure,” reflecting the provision of two distinct pressure levels—one for inhalation and one for exhalation.
  • CPAP: "C" stands for “Continuous Positive Airway Pressure,” signifying a single (continuous) level of pressure throughout breathing cycles.
• Required Variables:
  • CPAP: CPAP pressure level, FiO2
  • BiPAP: IPAP, EPAP, FiO2 (or O2 flow)

Weaning Criteria and Advanced Indices

Weaning from mechanical ventilation—whether invasive (via PSV reduction) or noninvasive (gradual reduction in IPAP/EPAP)—typically requires objective and subjective assessments. Common criteria and indices include:

• Respiratory Mechanics and Gas Exchange:
  • Adequate oxygenation (e.g., PaO2 ≥ 60 mmHg on FiO2 ≤ 0.4 and PEEP ≤ 5–8 cmH2O)
  • Stable ventilation with near-normal PaCO2 and pH
• Hemodynamic Stability:
  • No significant hypotension, shock states, or significant arrhythmias
• Spontaneous Breathing Indicators:
  • RSBI (Rapid Shallow Breathing Index):
    • RSBI = (Respiratory Rate) ÷ (Tidal Volume in liters)
    • A value <105 suggests a higher likelihood of successful weaning.
  • Pressure support requirements: Lowering pressure support to 8–10 cmH2O or below without respiratory distress is favorable.
• Other Considerations:
  • Adequate airway protection
  • Mental status allowing cooperation
  • Absence of excessive secretions or severe metabolic derangements

Assessing Future Weaning Potential

By November 8, 2024, the patient remains stable on PSV at Pr 12 cmH2O and PEEP 5 cmH2O, with a tidal volume around 300 mL. To assess further weaning potential, it would be advisable to:

1. Gradually reduce the Pressure Support and observe if the patient can maintain adequate ventilation and comfort at lower levels (e.g., PSV 10 cmH2O).
2. Check RSBI or other parameters under these lowered support conditions.
3. Monitor arterial blood gases, ensure stable hemodynamics, and assess secretion management.

No attempts since March 2024 have been documented to reduce support from PSV 12 cmH2O to a lower level or to shift to a simpler mode (e.g., CPAP only) as a step toward extubation or liberation from mechanical ventilation.

For future monitoring, it would be prudent to consider:

• Trying lower pressure support levels to see if the patient can maintain adequate ventilation and oxygenation.
• Evaluating RSBI and other indexes when spontaneously breathing at lower support levels.
• Assessing cough strength, secretion management, and patient comfort.
• Monitoring arterial blood gases and overall clinical stability during any trial of reduced support.

If the patient tolerates reduced support (e.g., PSV 10 cmH2O or even transitioning to CPAP or minimal support), it may indicate readiness to attempt complete weaning or prolonged noninvasive support only (e.g., BiPAP at home if considered appropriate).

Rapid Shallow Breathing Index (RSBI) Computation

The Rapid Shallow Breathing Index (RSBI) is a clinical parameter used to assess the likelihood of successful weaning from mechanical ventilation. It relates a patient’s respiratory rate (RR) to their tidal volume (Vt), providing insight into respiratory efficiency and effort.

\[ \text{RSBI} = \frac{\text{Respiratory Rate (RR)}}{\text{Tidal Volume (Vt in liters)}} \]

• RR (Respiratory Rate): Measured in breaths per minute.
• Vt (Tidal Volume): Measured in liters. If Vt is given in milliliters (mL), convert it to liters by dividing by 1000.

Choosing the Appropriate Tidal Volume for RSBI

• VTe (Exhaled Tidal Volume): Preferred for RSBI calculations, VTe reflects the actual volume of air the patient exhales. It is less influenced by system compliance or leaks, making it more reliable for evaluating the patient’s respiratory effort and efficiency.
• VTi (Inspiratory Tidal Volume): Can be used if VTe is not available, but it may be less accurate. VTi can be influenced by factors like gas compression or leaks, potentially distorting the true measure of the patient’s respiratory effort.

Why VTe is Preferred

1. Reflects Effective Ventilation: VTe directly measures the patient’s exhaled volume.
2. Reduces External Influences: VTe is less affected by system compliance or leaks, ensuring more reliable RSBI values.

Using VTi If Necessary

If you must use VTi:

• Clearly document that VTi was used instead of VTe.
• Interpret the RSBI values with caution, keeping in mind the potential inaccuracies due to ventilator-related factors.

Example Calculation

• Given:
  • RR = 15 breaths/min
  • Vt = 350 mL = 350 ÷ 1000 = 0.35 L

\[ \text{RSBI} = \frac{15}{0.35} \approx 42.9 \]

An RSBI of about 42.9 is well below the commonly accepted threshold of 105, suggesting that the patient may be ready to begin weaning from mechanical ventilation.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on Decmeber 11, 2024

Case4: An Explanation of SIMV Ventilator Settings: Understanding "IP" and "SP"

Mechanical ventilation modes such as Synchronized Intermittent Mandatory Ventilation (SIMV) allow both mandatory (ventilator-driven) and spontaneous (patient-driven) breaths. Clinicians often use pressure-based settings to control or support these breaths. However, handwritten and device-displayed ventilator settings can sometimes differ in terminology, causing confusion about parameters like IP (Inspiratory Pressure) and SP (Pressure Support). This document clarifies these terms and illustrates how they translate across different devices.

Handwritten Ventilator Settings from the Transfer Request Form

A frequently encountered example is a handwritten note reading:

SIMV | O2 2L | RR 16 | IP 8 | SP 8 | PEEP 5

1. SIMV
Synchronized Intermittent Mandatory Ventilation, a mode where a set number of breaths are delivered by the ventilator at a preset level, while allowing spontaneous breathing in between.

2. O2 2L
Represents an oxygen flow of 2 L/min. Depending on the ventilator model, this may correlate to a specific FiO₂ (Fraction of Inspired Oxygen).

3. RR 16
A mandatory breath rate of 16 breaths per minute.

4. IP 8 → "P control"
Often shorthand for Inspiratory Pressure (in pressure-controlled or assisted modes). In many SIMV modes, “IP” may be the pressure above PEEP delivered during each mandatory breath.

5. SP 8 → "PS"
Common shorthand for Pressure Support for spontaneous breaths. Helps reduce work of breathing by assisting patient-initiated breaths with an additional 8 cmH₂O of pressure.

6. PEEP 5
Positive End-Expiratory Pressure of 5 cmH₂O, preventing alveolar collapse at end-expiration and improving oxygenation.

In this handwritten example, both the mandatory breaths (IP 8) and spontaneous breaths (SP 8) receive a similar pressure level (8 cmH₂O above PEEP).

Confirming IP and SP on a ResMed Device

Later, the ventilator settings were checked on a ResMed device, which displayed:

In this scenario:

• IP (Inspiratory Pressure) in the handwritten note correlates with PC (Pressure Control) on the ResMed device.
• SP (Pressure Support) in the handwritten note is labeled simply as PS on the ResMed device.

Clarifying IP vs. PS

Inspiratory Pressure (IP) / Pressure Control (PC)
Refers to the preset pressure for mandatory breaths. In pressure-controlled SIMV, the ventilator ensures each mandatory breath reaches this target pressure above PEEP.

Pressure Support (PS)
Augments spontaneous breaths initiated by the patient. Eases the patient’s work of breathing by providing extra pressure, helping overcome airway resistance and ventilator circuit resistance.

Typical Ranges for Adult Ventilator Parameters

Ventilator settings vary based on patient condition, but the following ranges serve as general guidelines:

Clinical Implications

1. Balancing Mandatory and Spontaneous Support
   In P-SIMV, maintaining appropriate Inspiratory Pressure (PC/IP) for mandatory breaths and sufficient Pressure Support (PS) for spontaneous breaths is crucial for optimal ventilation and patient comfort.
2. Weaning Strategy
   During weaning, the mandatory breath rate (RR) may be decreased, and Pressure Support (PS) may be gradually reduced to encourage spontaneous breathing. Close monitoring of respiratory effort, arterial blood gases (ABGs), and vital signs guides this process.
3. Customization and Monitoring
   Since no two patients present identically, constant reevaluation of airway pressures, blood gases, and clinical feedback is essential. Adjustments to PEEP, IP/PC, PS, and RR must be made to balance oxygenation, CO₂ removal, and patient comfort.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on February 25, 2025

Case5: Ventilator Weaning in a Neurologically Impaired Patient

A patient with a prior stroke secondary to arrhythmia initially received anticoagulant therapy, followed by a hemorrhagic stroke, which required neurosurgical intervention. After an extended hospitalization (~140 days) with persistent deep drowsiness, the patient was transferred to the current facility on March 21, 2025 for continued neurological care, infection control, and ventilator weaning.

• High secretion burden (requiring frequent suctioning).
• Recurrent pneumonia; currently improved on meropenem.
• Vancomycin-resistant Enterococcus (VRE) necessitating isolation precautions.

Tracheostomy and Auto-Suction Line

The patient has a tracheostomy with a T-tube (size 7.0). An auto-suction line—a specialized, often closed, suction system designed to reduce circuit breaks and minimize infection risk—is available. However, it is not currently used because previous episodes of aspiration pneumonia coincided with mealtime and feeding, leading the care team to opt for manual suctioning and more controlled airway management techniques.

Current Ventilator Management

1. Ventilator Mode (ResMed PSIMV)

   The patient is managed using PSIMV (Pressure Synchronized Intermittent Mandatory Ventilation) with Pressure Support (PS) for spontaneous breaths. A Pressure Control (PC) backup ensures a minimum mandatory ventilation if the patient’s spontaneous rate or effort becomes insufficient.

   Weaning Approach: In this mode, the focus is on gradually reducing Pressure Support. For example, from 10 cmH₂O to 8 cmH₂O, and then to 5 cmH₂O, provided the patient maintains adequate ventilation and stable vital signs.

2. Ventilator Settings Table

   

   Parameter	Current Setting	Typical/Normal Range	Target / Rationale
   Ventilator Mode	PSIMV (ResMed)	N/A	Main mode for partial support; PC acts as backup
   Pressure Control (backup)	10 cmH₂O	8–20 cmH₂O (varies by lung condition)	Reduce if the patient consistently meets spontaneous ventilation
   Pressure Support (PS)	10 cmH₂O	5–15 cmH₂O	Key for weaning; decrement as tolerated
   PEEP	5 cmH₂O	5–10 cmH₂O	Maintain end-expiratory lung volume; can adjust for oxygenation
   Set Respiratory Rate	12 breaths/min	12–20 breaths/min (adult)	Ensures a minimal backup rate
   Measured Tidal Volume (Vti)	~360 mL (0.36 L)	~6–8 mL/kg PBW1 (≈400–600 mL)	Monitor for adequate alveolar ventilation
   Supplemental O₂	3 L/min (nasal cannula)	1–6 L/min	Aim for SpO₂ ≥ 92%

   ¹ PBW = Predicted Body Weight

Challenges in Weaning

1. Neurological Limitation
   • Deep drowsiness complicates spontaneous breathing trials (SBTs) and patient cooperation.
2. Infection Control
   • High risk of recurrent pneumonia; meropenem therapy shows improvement but requires continued vigilance.
   • VRE isolation and strict aseptic techniques needed during tracheostomy care.
3. Secretion Burden
   • Frequent manual suctioning is necessary, especially in the absence of auto-suction line usage.
4. Assessment and Monitoring
   • Rapid Shallow Breathing Index (RSBI) = Respiratory Rate / Tidal Volume (L). With RR ≈ 20 and Vti 0.36 L, RSBI ≈ 55.6—below the usual cutoff of 105, suggesting potential readiness for progressive weaning.

Weaning Strategy

1. Stepwise Reduction in Pressure Support

   Lower PS from 10 cmH₂O to 8 cmH₂O, and subsequently to 5 cmH₂O if tolerated. Evaluate respiratory rate, tidal volume, oxygen saturation, and overall work of breathing.

2. Monitoring Gas Exchange

   Arterial Blood Gas Analysis (ABGA) remains the gold standard for assessing PaCO₂, PaO₂, and pH during weaning trials. If ABGA is not immediately available, rely on:

   • SpO₂ (pulse oximetry) to monitor oxygenation.
   • Heart rate and blood pressure.
   • Capnography (end-tidal CO₂), if accessible, to approximate ventilation status.
3. Infection and Nutrition

   Continue the antibiotic regimen until clinical and radiological resolution of pneumonia. Optimize enteral feeding (600–900 mL/day) to maintain adequate nutrition without exacerbating respiratory effort or aspiration risk.

4. Family Communication

   The patient’s husband remains the primary caregiver. Ongoing discussions about the risks, benefits, and timeline of ventilator weaning are essential to maintain trust and realistic expectations.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on March 21, 2025

Case6: ACMV-PC Mode and Its Relationship to ResMed’s P(A)CV and P(A)C

A patient has been transferred with a ventilator set to ACMV-PC (Assist-Control Mandatory Ventilation in Pressure Control) mode. The transfer document lists specific settings—PEEP 6 cmH₂O, Pi 10 cmH₂O, Rate 14 breaths/min, Ti 1.1 seconds, and a High Trigger—which invite a detailed examination of each parameter and a broader discussion comparing ACMV-PC to ResMed’s P(A)CV and P(A)C modes. The following sections provide a structured overview of these settings, their typical clinical ranges, and the rationale behind assist-control ventilation.

1. Mode Overview: ACMV-PC vs. P(A)CV and P(A)C

1. ACMV-PC (Assist-Control Mandatory Ventilation – Pressure Control)

   • Guarantees a minimum number of mandatory breaths per minute at a preset pressure (Pi).
   • Allows the patient to trigger additional breaths; each triggered breath receives the same preset pressure.
   • The ventilator is not in total control: if a patient initiates a breath, it is fully supported (“assist”). If the patient does not initiate, the ventilator provides time-triggered mandatory breaths (“control”).

2. P(A)CV in ResMed Devices

   • Often referred to as Pressure (Assisted) Controlled Ventilation.
   • Involves setting PC (pressure), PEEP, RR, Ti, Rise Time, Safety Vt, and Trigger.
   • Conceptually similar to ACMV-PC because both modes deliver pressure-controlled breaths at a defined rate and fully support patient-initiated breaths.

3. P(A)C in ResMed Devices

   • Sometimes termed Pressure (Assisted) Control in a bi-level format.
   • Utilizes IPAP (Inspiratory Positive Airway Pressure) and EPAP (Expiratory Positive Airway Pressure) instead of a single target pressure plus PEEP.
   • Other adjustable features are similar (RR, Ti, Rise Time, Safety Vt, Trigger), but the presence of IPAP/EPAP is a key differentiator, especially in noninvasive ventilation contexts.

2. Parameters in ACMV-PC: Provided Settings and Typical Ranges

The table below aligns the provided settings from the transfer document with typical clinical ranges used in practice. While these values are generally appropriate for many adult patients, they must be adjusted based on individual factors such as lung compliance, oxygenation requirements, and the patient’s spontaneous breathing effort.

3. The Meaning of “Assist-Control”

• Assist Component: If the patient makes an inspiratory effort above the set rate, the ventilator provides a full pressure-controlled breath (identical to the mandatory setting).
• Control Component: A guaranteed number of breaths (e.g., RR = 14) is delivered if the patient does not initiate inspiration.
• This mode is not purely “controlled”; instead, it “assists” when the patient attempts to breathe. Hence, the term “assist-control” emphasizes that the patient has room to breathe spontaneously but still receives full support with each breath.

4. Detailed Examination of Ti and Trigger

1. Inspiratory Time (Ti)

   • Duration of Pressure Application: In pressure-control modes, the ventilator applies a set inspiratory pressure (Pi) for the entirety of Ti.
   • Impact on Gas Exchange: A longer Ti can potentially improve oxygenation by allowing more time for gas distribution but may risk air trapping if expiration is insufficient or the respiratory rate is high.
   • Advanced Settings: Some ventilators offer adaptive Ti features that can adjust cycle-off criteria based on patient flow or effort, though many standard modes utilize a fixed Ti.

2. Trigger Sensitivity

   • Definition: Governs how easily the ventilator detects and responds to a patient’s spontaneous effort, typically via a flow or pressure trigger.
   • Clinical Adjustments:
     • High Trigger Threshold: Prevents auto-triggering (common if the patient is hemodynamically unstable or if circuit vibrations occur), yet demands a stronger inspiratory effort.
     • Low Trigger Threshold: Increases sensitivity, making it easier for patients to trigger a breath but risking false triggers.
   • Effect on Patient-Ventilator Synchrony: Proper trigger settings are essential to match patient effort closely with ventilator-delivered breaths, promoting comfort and reducing work of breathing.

5. Clinical Perspective and Mode Selection

1. Comparing ACMV-PC to ResMed’s P(A)CV: Both deliver pressure-controlled breaths at a preset rate, with the capacity for patient-triggered support. The main difference often lies in ventilator-specific nomenclature and additional features (e.g., Safety Vt, Rise Time).
2. Comparing ACMV-PC to ResMed’s P(A)C: In P(A)C modes, separate inspiratory and expiratory pressures (IPAP/EPAP) are set, typically in bi-level ventilation or specific noninvasive applications.
3. Personalizing Settings: Optimal PEEP, Pi, Rate, Ti, and Trigger must be regularly evaluated using clinical signs, blood gas analyses, and pulmonary mechanics. Adjustments are made to maintain adequate oxygenation/ventilation and reduce the risk of volutrauma or barotrauma.
4. Safety Net: Some advanced modes include a “Safety Vt” or other alarms to ensure that tidal volume remains within acceptable limits, preventing underventilation or inadvertent excessive volumes.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on March 27, 2025

Case7: Management of a 90-year-old patient on mechanical ventilation using a ResMed device

A 90-year-old male patient was transferred with the following ventilator document entry:

Ventilator Transfer Document

• PC/AC → P(A)CV
• Frequency (set): 26 → RR
• Tidal Volume: 400 mL
• PEEP: 4 cmH₂O
• I:E Ratio: 1:1.9
• Inspiration Time: 0.8 seconds

↑ Settings ↑

---

↓ Measurements ↓

• VTi: 403 mL
• Minute Volume: 11.2 L/min
• Peak Pressure (Ppeak): 22.4 cmH₂O

The document includes both settings (e.g., pressure control, respiratory rate, PEEP) and monitored values (e.g., measured tidal volume, minute volume, peak pressure). The following outline explains each parameter, discusses the equivalent mode on a ResMed ventilator, and provides a table of typical normal ranges to guide clinical decision-making.

1. Ventilator mode: PC/AC

• Definition: “PC/AC” typically denotes Pressure Control in Assist/Control mode. In this mode, the ventilator delivers each breath at a preset inspiratory pressure while also supporting breaths initiated by the patient.
• Equivalent ResMed mode: On a ResMed ventilator, this is usually referred to as Pressure Control Ventilation (PCV) or another pressure-targeted mode with assist/control functionality. Although naming conventions may differ, the clinical approach remains consistent: maintain a desired pressure level while delivering a set minimum rate.

2. Clarification of “frequency”

• Frequency (set) 26: “Frequency” describes the respiratory rate in breaths per minute. A set rate of 26 indicates a relatively high ventilation frequency, which may be necessary to maintain adequate alveolar ventilation in this 90-year-old patient, depending on blood gas results and clinical status.

3. Settings vs. monitored values

The list includes both configuration settings and measurements:

• Settings: Pressure Control level, frequency (respiratory rate), tidal volume (if in volume-control or target-based in pressure-control), PEEP, I:E ratio, and inspiratory time.
• Measurements: VTi (delivered tidal volume), minute volume, and peak pressure (Ppeak).

4. Typical normal ranges for ventilator settings

The following table provides general reference ranges. Actual targets must be tailored to each patient’s clinical condition, including lung mechanics, gas exchange requirements, and comorbidities.

5. Patient’s parameters in context

• PC/AC: Each breath is delivered at a targeted pressure, ensuring a minimum set rate of ventilation.
• Frequency (26 breaths/min): A relatively high rate, likely to support sufficient CO₂ clearance in a 90-year-old with possible compromised lung function.
• Tidal Volume (400 mL): Near the lower end of the lung-protective strategy range, depending on the patient’s predicted body weight.
• PEEP (4 cmH₂O): Slightly below the typical 5 cmH₂O starting point; may be appropriate if the patient tolerates lower PEEP without negative effects on oxygenation.
• I:E Ratio (1:1.9): Allows roughly 0.8 seconds of inspiration and about 1.5 seconds of expiration. Adequate for most adult lungs if no obstructive pattern is present.
• VTi (403 mL): Measured tidal volume aligns well with the set tidal volume of 400 mL.
• Minute Volume (11.2 L/min): Slightly above standard reference, which may be necessary to meet increased metabolic or clearance demands in this elderly patient.
• Ppeak (22.4 cmH₂O): Within a safe range; continued monitoring is crucial to ensure there is no upward trend suggestive of increasing airway resistance or decreased compliance.

6. Application on a ResMed ventilator

When transferring these parameters to a ResMed ventilator, the following approach is recommended:

1. Mode Selection: Choose Pressure Control Ventilation (PCV) or a similar pressure-targeted, assist/control setting to replicate PC/AC.
2. Respiratory Rate: Enter 26 breaths/min as the base rate, adjusting to maintain appropriate PaCO₂ levels as indicated by arterial blood gases.
3. Inspiratory Pressure / Tidal Volume: In pressure control, fine-tune the inspiratory pressure to achieve the target tidal volume (~400 mL). Monitor the exhaled tidal volume (VTi) to ensure lung-protective ventilation.
4. PEEP: Set at 4 cmH₂O initially, then adjust according to oxygenation requirements, end-expiratory lung volume, and hemodynamic tolerance.
5. I:E Ratio and Inspiratory Time: Configure the inspiratory time of 0.8 seconds to achieve approximately 1:1.9. Reassess if changes in airway mechanics arise (e.g., obstructive pathologies).
6. Monitor Trends:
   • VTi: Verify effective tidal volume delivery.
   • Peak Pressure: Watch for elevations that might indicate increasing airway resistance or decreased compliance.
   • Minute Ventilation: Ensure adequate CO₂ clearance without causing excessive inflation pressures.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on March 27, 2025

Case8: A Patient with Hypoxic Encephalopathy, Myoclonic Seizures, and Ventilatory Considerations

This case details a patient diagnosed with hypoxic encephalopathy who presented with myoclonic seizures. Pharmacological management included valproate, levetiracetam, and clonazepam, alongside sedative agents. While overt seizure activity subsided, persistent spasticity and a stuporous mental status remained.

Although the patient could initiate spontaneous breathing, intermittent apnea necessitated prolonged ventilatory support. Consequently, a tracheostomy was performed to secure the airway, maintain adequate ventilation, and mitigate the risk of aspiration. The discussion below concentrates on the ventilator settings reported, the indications for mechanical ventilation, and the rationale behind choosing Pressure Support Ventilation (PSV) over other modes.

Patient Presentation

Diagnosis

• Hypoxic encephalopathy
• Myoclonic seizures (controlled with antiepileptic and sedative medications)
• Spasticity
• Stuporous mental status

Respiratory Status

• Intermittent apnea
• Some spontaneous respiratory drive

Airway Access

• Tracheostomy for prolonged mechanical ventilation and airway protection

Ventilatory Management

1. Reported Settings

   The ventilator settings are reported as:

   “PSV, Above PEEP 9, PEEP 7, FiO₂ 25%”

   The notation “Above PEEP 9, PEEP 7” indicates that the patient receives 9 cm H₂O of Pressure Support (PS) in addition to a Positive End-Expiratory Pressure (PEEP) of 7 cm H₂O. Hence, each inspiratory effort is assisted by 9 cm H₂O on top of the 7 cm H₂O baseline. The fraction of inspired oxygen (FiO₂) is set at 25%, suggesting a comparatively low supplemental oxygen requirement.

   Patient’s Ventilator Settings	Value	Approximate Range
   Ventilation Mode	PSV
   Pressure Support (Above PEEP)	9 cm H₂O	5–20 cm H₂O
   PEEP	7 cm H₂O	5–10 cm H₂O
   FiO₂	25%	21–100% (adjusted per ABG results)

2. Indications for Mechanical Ventilation

   Mechanical ventilation was mandated based on neurologic compromise and respiratory insufficiency. In general, mechanical ventilation is considered when specific clinical or numerical thresholds are met:

   Indication	Typical Criteria/Threshold
   Apnea or Impending Respiratory Arrest	Absent or severely depressed respiratory drive
   Hypoxemia	PaO₂ < 60 mmHg on FiO₂ ≥ 0.50
   Hypercapnia with Respiratory Acidosis	PaCO₂ > 50 mmHg and pH < 7.25
   Inability to Protect the Airway	Diminished consciousness, high risk of aspiration
   Ventilatory Muscle Fatigue & Elevated Work of Breathing	RR > 35 breaths/min, rapidly rising PaCO₂
   Neurological Factors	Severe stupor, inability to maintain consistent ventilatory effort (e.g., GCS < 8, repeated apnea episodes)

   In this patient:

   • Intermittent Apnea and Reduced Respiratory Drive necessitated mandatory ventilatory support.
   • Stupor and a high aspiration risk indicated the need for a secure airway (tracheostomy).
   • Hypoxic Encephalopathy and sedation further compromised reliable spontaneous ventilation.

3. Rationale for Selecting Pressure Support Ventilation (PSV)

   A variety of modes—such as SIMV (Synchronized Intermittent Mandatory Ventilation) PC + PS, PSV, and BiPAP (Bilevel Positive Airway Pressure)—were assessed. Each mode differs in its level of control, synchronization, patient comfort, and reliance on spontaneous effort.

   Parameter	SIMV PC + PS	PSV	BiPAP
   Level of Control	Mandatory PC breaths + partial support	Patient-driven, pressure-supported breaths	Two-level (inspiratory/expiratory) pressure
   Backup Ventilation	Yes (preset mandatory rate)	No guaranteed rate	Limited (usually noninvasive)
   Patient Effort Required	Moderate	High (patient determines RR & VT)	Moderate to high (depends on mask seal)
   Weaning Potential	Good (can gradually reduce mandatory rate)	Very good (encourages spontaneous effort)	Generally used for noninvasive or mild cases
   Suitability for Altered Mental Status	Moderate (requires patient triggering for additional breaths)	Good if patient can trigger consistently	Less optimal if severe alteration or tracheostomy needed
   Key Advantages	Guarantees a minimum ventilation	Excellent synchrony & comfort, easier weaning	Noninvasive option for less severe cases
   Key Limitations	Possible dyssynchrony if sedation is off	No mandatory minimum ventilation	Not suitable for high apnea risk or severe consciousness depression

   Why PSV for This Patient?

   • The patient retains some spontaneous respiratory drive but is prone to intermittent apnea.
   • PSV affords augmented support during each spontaneous inspiration, reducing the work of breathing and improving comfort.
   • A guaranteed rate (as in SIMV) is less essential if close monitoring is feasible, and the sedation plus intermittent apnea can be managed by adjusting the support level.
   • BiPAP, typically noninvasive, is unsuitable here due to the tracheostomy and high apnea risk, necessitating a more invasive approach.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on March 28, 2025

Case9: Ventilator asynchrony in a patient with stage IV cerebral cancer

A 47-year-old woman with metastatic cerebral carcinoma developed marked tachypnoea (43–48 breaths min⁻¹) and apparent ventilator asynchrony after an episode of regurgitation while receiving pressure-controlled ventilation. This report summarises the clinical findings, analyses the potential mechanisms of patient–ventilator dyssynchrony, and proposes a structured, evidence-based management algorithm. The discussion emphasises protective ventilation, airway protection, neurocritical-care goals, and the importance of waveform analysis.

Case presentation

The bar chart below illustrates the disparity between the programmed and observed respiratory rates, highlighting probable trigger asynchrony.

Problem list

1. Patient–ventilator asynchrony (high spontaneous RR vs mandatory RR).
2. Post-regurgitation aspiration risk with possible airway obstruction.
3. Uncertain gas exchange (ABG pending) in a neuro-oncology patient where normoxia and normocapnia are critical.
4. Mild hyperthermia and potential sepsis.
5. Stage IV cerebral cancer with high intracranial pressure (ICP) risk.

Pathophysiological analysis

1. Types of dyssynchrony suspected

   • Ineffective triggering: Occurs when patient’s inspiratory effort fails to meet the ventilator’s trigger threshold, so no assisted breath is delivered despite diaphragmatic contraction.
     • Waveform clues: Small negative deflection on the pressure–time curve without corresponding flow delivery; “missed” dips on the flow–time curve.
     • Contributing factors: Too high trigger sensitivity setting, presence of auto-PEEP, weak respiratory drive or muscle fatigue, inadequate sedation level.
     • Clinical impact: Increased work of breathing, patient fatigue, risk of respiratory muscle exhaustion.
     • Interventions: Lower trigger threshold (flow or pressure), reduce PEEP or intrinsic PEEP, optimise sedation to balance comfort and drive.
   • Double-triggering / breath-stacking: Occurs when a spontaneous effort outlasts the set inspiratory time, prompting a second ventilator cycle before full exhalation.
     • Waveform clues: Two consecutive pressure pulses with minimal expiratory phase between; truncated expiratory flow in the flow–time curve.
     • Contributing factors: Inspiratory time too short, high patient drive, mismatched cycling criteria, inadequate tidal volume delivery.
     • Clinical impact: Excessive tidal volumes, risk of volutrauma or barotrauma, elevated mean airway pressure.
     • Interventions: Lengthen inspiratory time or switch to pressure-controlled assist/control, adjust cycling sensitivity (flow or time), consider slight increase in tidal volume target.
   • Reverse triggering: A form of entrainment in which controlled ventilator breaths provoke a reflex diaphragmatic contraction, effectively triggering a patient effort after the mandatory breath.
     • Waveform clues: Regular, time-locked patient efforts following each ventilator-delivered breath; visible deflections after mandatory cycles.
     • Contributing factors: Deep sedation or neuromuscular blockade reducing spontaneous initiation, high respiratory drive.
     • Clinical impact: Breath stacking, increased minute ventilation without true spontaneous effort, potential for lung injury.
     • Interventions: Reduce sedation depth to restore spontaneous rhythm, adjust ventilator rate to avoid entrainment, consider spontaneous modes (e.g., PSV) or temporary neuromuscular blockade for refractory cases.

2. Contributing factors

   • Insufficient P_control (8 cmH₂O) leading to low delivered tidal volume (Vᴛ), stimulating respiratory drive.
   • Anxiety, pain, or inadequate sedation.
   • Aspiration-induced hypoxaemia and airway irritation.
   • Possible metabolic acidosis from sepsis or tumour lysis.

Immediate bedside assessment checklist

1. Airway reassessment
   • Confirm ETT position and cuff pressure.
   • Perform oropharyngeal and subglottic suctioning; consider bronchoscopy if large particulate matter is suspected.
2. Ventilator waveform review
   • Inspect flow-time and pressure-time curves for ineffective efforts or double-triggers.
   • Measure Vᴛ and peak / plateau pressures.
3. ABG + lactate within 15 min.
4. Chest imaging (portable radiograph) for aspiration pneumonitis / ARDS.
5. Neurological status and pupillary size; ensure head-of-bed elevated ≥ 30 °.

Stepwise management algorithm

Trigger sensitivity and inspiratory time guidance

1. Normal trigger sensitivity

   Trigger type	Recommended range	Typical setting
   Flow trigger	1–3 L/min	2 L/min
   Pressure trigger	–0.5 to –2 cmH₂O	–1 cmH₂O

2. Normal inspiratory time (Tᵢ)

   Ventilator mode	Absolute Tᵢ	I:E ratio
   Volume-controlled	0.8–1.2 s	1:2 to 1:3 (≈25–33%)
   Pressure-controlled	0.8–1.0 s	Ti% ≈ 30–35%

3. Adjustment recommendation

   Setting	Adjustment	Purpose
   Trigger sensitivity	Lower threshold (↑ sensitivity) Flow: 1 L/min Pressure: –1 cmH₂O	Ensures patient efforts reliably trigger breaths Reduces work of breathing Aligns spontaneous and mandatory rates
   Inspiratory time	Maintain within above ranges; adjust Ti% based on drive and mechanics	Prevents double-triggering Enables adequate tidal volume delivery

4. Key rationale

   • Work of breathing is decreased when even modest inspiratory efforts are supported.
   • Respiratory drive is dampened, helping to normalize spontaneous rate.
   • Muscle fatigue risk is minimized, preserving respiratory muscle strength.

   |<--- Insp. (30%) --->|<------ Exp. (70%) ------>|
   

   Note: Excessive sensitivity may lead to auto-triggering (e.g., from circuit noise or cardiac oscillations). Fine adjustments guided by waveform inspection are advised.

Discussion

Protective ventilation in brain-injured oncology patients requires a balance between minimising ventilator-induced lung injury and avoiding secondary cerebral insults. Hypoxaemia and hypercapnia both augment cerebral blood flow and raise ICP; conversely, excessive hyperventilation may cause cerebral vasoconstriction and ischaemia. The observed tachypnoea likely reflects inadequate tidal volume delivery and a strong central drive, compounded by agitation and airway irritation after regurgitation.

Pressure control of 8 cmH₂O typically yields Vᴛ < 4 ml kg⁻¹ in most adults, insufficient for CO₂ clearance. Gradual increments, combined with careful waveform analysis, improve synchrony without abrupt ICP shifts. Sedation using agents with minimal ICP impact (propofol, dexmedetomidine) helps dampen excessive respiratory drive. Neuromuscular blocking agents should remain a rescue strategy given deleterious effects on neurological assessment.

Empirical antibiotics are justified when aspiration is likely and temperature rises; however, fever alone (37.7 °C) warrants cultures before escalation. Early bronchoscopy clears obstructive debris, reducing ventilator demands.

This manuscript has been proof-read and formatted for clarity. Figures and tables are included to aid comprehension.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on May 1, 2025

Case10: Ventilator management in an elderly female with aspiration pneumonia

2025-05-08 intubation, sedation (midazolam + remifentanil) → plan tapering if ABGA okay
target tidal volume: 250 – 350
Vent PS mode

2025-05-09 if vital signs maintain well overnight, reduce sedation, check CT and decide extubation timing. Extubation done. If saturation maintained well over the weekend, plan ward transfer

II. Key concepts clarified

Sedation does not inevitably abolish spontaneous breathing

Continuous infusions of midazolam (benzodiazepine) and remifentanil (ultra-short-acting opioid) are titrated to a sedation goal (e.g., Richmond Agitation-Sedation Scale, RASS −2 to −3). At such depths many patients maintain a degree of respiratory drive, allowing patient-triggered breaths even while intubated.

Why PS mode can be used in a sedated, intubated patient

Pressure-Support Ventilation augments each patient-initiated inspiration with a preset pressure. If the patient’s respiratory drive is preserved (albeit dampened), PS mode:

• reduces work of breathing,
• avoids excessive mandatory ventilation, and
• facilitates earlier liberation from the ventilator.

“Target tidal volume” in PS mode

In PS mode clinicians do not prescribe an absolute V_T; instead, the delivered volume is the result of:

1. the set pressure-support level,
2. patient effort, and
3. respiratory mechanics (compliance & resistance).

The note “250–350 mL” therefore represents an anticipated safe range (≈ 6 mL kg⁻¹ predicted body weight) used to titrate support, not a ventilator-enforced target.

1. Common modes immediately after intubation

   Mode	Typical initial use	Set VT?	Spontaneous breaths allowed?
   Volume-Controlled (VC)-AC	Severe respiratory failure, deep sedation or paralysis	Yes	No (backup mandatory)
   Volume SIMV + PS	Transition phase; partial support	Yes (mandatory breaths)	Yes
   Pressure-Controlled (PC)-AC	Poor compliance (e.g., ARDS)	No (volume varies)	No (unless additional PS window)
   Pressure-Support (PS)	Spontaneous breathing with reduced work	No	Yes (all breaths)
   CPAP ± PS	Final weaning step before extubation	No	Yes

2. Sequence on 2025-05-08 → 2025-05-09

   ⚙️ Interpretation: PS mode on 5-08 indicates the sedation level permitted spontaneous efforts. The documented “sedation reduction” on 5-09 likely refers to further tapering to reach minimal or no sedation before extubation. The ventilator was probably left in PS (or switched to CPAP with minimal PS) as the patient awakened, because these modes seamlessly accommodate returning respiratory drive without risking excessive mandatory volumes.

III. Sedation dosing, preparation, and titration guidelines

Disclaimer ⚠️ The following dosage ranges and preparation methods are based on widely used ICU references (e.g., PADIS 2018, ESPEN 2022).

1. Preparation steps (illustrative)

   1. Midazolam (ampoule 5 mg / 5 mL): draw 50 mg into a 50 mL syringe and add NS up to 50 mL → 1 mg mL⁻¹. Label “Midazolam 1 mg mL⁻¹, protect from light”.
   2. Remifentanil (vial 2 mg powder): reconstitute with 2 mL NS (1 mg mL⁻¹), withdraw, then dilute into 50 mL NS → 40 µg mL⁻¹. Use dedicated line; no heparin admixture.

2. Infusion-rate calculation formulas

   Midazolam pump rate (mL h⁻¹) = (Target mg kg⁻¹ h⁻¹) × Patient kg ÷ Concentration (mg mL⁻¹)
   Remifentanil pump rate (mL h⁻¹) = (Target µg kg⁻¹ min⁻¹) × Patient kg × 60 ÷ Concentration (µg mL⁻¹)

3. Titration & safety monitoring

   • Assess RASS and respiratory pattern every 15 min during up-titration, then at least hourly.
   • Decrease infusion by 10–20 % if RR < 10 min⁻¹, PaCO₂ rises > 10 mmHg from baseline, or RASS ≤ −4.
   • Prepare reversal agents at bedside (flumazenil 0.2 mg IV × repeated bolus, naloxone 40 µg IV × titration).

4. Predicting loss of spontaneous breathing

   In elderly or frail adults, apnea becomes likely when any of the following thresholds are exceeded for > 10 min:

   • Midazolam > 0.1 mg kg⁻¹ h⁻¹ (≈ 6 mg h⁻¹ in 60 kg)
   • Remifentanil > 0.12 µg kg⁻¹ min⁻¹ (≈ 430 µg h⁻¹ in 60 kg)
   • RASS ≤ −4, end-tidal CO₂ > 45 mmHg, RR < 8 min⁻¹.

5. Weaning strategy

   1. Decrease remifentanil by 0.02 µg kg⁻¹ min⁻¹ every 15–30 min while observing spontaneous tidal volumes ≥ 6 mL kg⁻¹.
   2. When remifentanil ≤ 0.04 µg kg⁻¹ min⁻¹, begin midazolam taper (e.g., 20 % every 30 min).
   3. Target RASS −1 to 0 before switching to Pressure-Support ≤ 8 cmH₂O or CPAP for a spontaneous-breathing trial.

IV. Practical answers to the posed questions

• Q 1: Does intubation + sedation abolish respiration?
  ➔ Not necessarily. Adequate but non-paralyzing sedation can preserve spontaneous breathing, enabling PS mode.
• Q 2: Is a “target tidal volume” incompatible with PS mode?
  ➔ The ventilator itself does not target V_T in PS. Clinicians reference an expected range to titrate pressure support and assure lung-protective ventilation.
• Q 3: Which modes are commonly used immediately post-intubation?
  ➔ VC-AC or PC-AC when deep sedation/paralysis is required; SIMV or PS as the patient regains effort. Modes that set V_T include VC-AC and SIMV (mandatory breaths).
• Q 4: How is ventilator mode chosen during sedation weaning?
  ➔ PS (or CPAP ± minimal PS) is favored because it allows full patient control while providing limited assistance to overcome tube resistance.
Q 5: Can Pressure-Support mode be maintained while the patient is still sedated?
➔ Yes. Sedation may be titrated to a light-to-moderate level (e.g., RASS −2 to −3) that blunts anxiety and coughing yet preserves respiratory drive. In this range, the patient continues to initiate breaths, and PS mode supplies supportive pressure for each effort, reducing work of breathing without imposing mandatory breaths. Continuous monitoring of respiratory rate, tidal volume, and blood gases is essential to ensure spontaneous ventilation remains adequate.
Q 6: Why is the patient sedated in this situation?
➔ Sedation is employed to (1) alleviate anxiety, pain, and distress caused by an endotracheal tube; (2) improve patient-ventilator synchrony, thereby preventing barotrauma or volutrauma from erratic breathing; (3) reduce oxygen consumption and metabolic demand during acute respiratory failure; (4) prevent inadvertent extubation or removal of lines; and (5) facilitate diagnostic or therapeutic procedures (e.g., chest CT) without coughing or movement. In aspiration pneumonia, early controlled ventilation under light-to-moderate sedation stabilizes gas exchange while allowing careful titration toward spontaneous breathing for eventual extubation.

V. Take-home messages

• Deep sedation is not synonymous with apnea; monitor respiratory drive continuously.
• Pressure-Support mode is appropriate once spontaneous efforts are reliable, even while sedated.
• “Target” tidal volumes in spontaneous modes are clinical benchmarks, not machine settings.
• During weaning, gradual sedation lightening and maintenance of a spontaneous-breathing mode facilitate successful extubation.

VI. References (concise)

1. Fan E et al. Ventilatory management of acute respiratory distress syndrome. JAMA. 2018.
2. Sessler CN & Pedram S. Sedation during mechanical ventilation. Clin Chest Med. 2023.

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on May 21, 2025

Virtual Ventilator Test-Lung

Indicative mechanical parameters for normal and diseased lungs

1. Reference ranges (adults)

2. Suggested preset modules for a software lung simulator

3. Implementation notes

For dynamic simulations, compute the time constant (τ) in real time as τ = (Ri + Re / 2) × C _dyn ; three time constants (≈ 95 % equilibration) mark the end of filling or emptying. Incorporating separate elastic and resistive work components allows visualisation of disease-specific energy demands. Finally, ensure breath-to-breath variability for stochastic realism, particularly in asthma and ARDS presets.

Written on June 9, 2025

Ventilator Test-Lung Simulations and Hemodynamic Modeling

In developing the nGeneHemodynamicSimulation_VentilatorTestLung and its interaction with the Ventilator, the choice of integrating both the Strategy and Observer design patterns is driven by the need for dynamic behavior adjustment and robust state monitoring within our simulation system. These patterns are selected to enhance the flexibility, maintainability, and operational efficiency of our hemodynamic simulation platform.

Why Strategy and Observer Design Patterns Are Chosen:

1. Strategy Design Pattern for Dynamic Behavior Control:
• The use of the Strategy pattern allows the nGeneHemodynamicSimulation_VentilatorTestLung to adapt its behavior dynamically through interchangeable ventilatory modes such as Mode_PC (Pressure Control), Mode_PRVC (Pressure Regulated Volume Control), and Mode_BiPAP (Bilevel Positive Airway Pressure).
• Each mode encapsulates specific algorithms that adjust the ventilation strategy according to different patient scenarios. For instance, Mode_PC delivers constant pressure irrespective of lung characteristic changes, Mode_PRVC adjusts pressure based on lung compliance and airway resistance to meet a target tidal volume, and Mode_BiPAP aids both inhalation and exhalation, mimicking non-invasive ventilation.
• This capability enables the nGeneHemodynamicSimulation_VentilatorTestLung to tailor its operational approach to the specific requirements of each simulation, ensuring high fidelity and precision in simulation outcomes.
2. Observer Design Pattern for Real-Time Monitoring and Response:
• Implementing the Observer pattern ensures that the Ventilator, acting as an observer, is consistently informed about any significant changes or results within the nGeneHemodynamicSimulation_VentilatorTestLung. This includes monitoring patient condition changes, alerts when physiological limits are reached, or when a simulation cycle completes.
• The ability for the Ventilator to receive these updates allows it to respond appropriately—whether by adjusting simulation parameters, reacting to emergent simulation states, or recording data for further analysis.
• This continuous monitoring is crucial for maintaining the integrity of the simulation and for implementing responsive adjustments that enhance the realism and utility of the simulation.

Benefits of This Design Approach:

• Flexibility and Scalability: The system's architecture allows for easy updates and additions to the simulation strategies without disrupting existing functionality. New ventilatory modes can be developed and integrated as strategies, providing scalability as new simulation needs arise.
• Enhanced Maintainability: Each ventilatory mode is encapsulated as a separate strategy, making the system easier to maintain and modify. This modular design allows individual strategies to be updated independently based on new research or clinical guidelines without affecting other parts of the system.
• Robustness and Reliability: With the Observer pattern, the system ensures that all state changes are captured and handled efficiently. This real-time monitoring and responsiveness safeguard the system against failures and ensure that the simulation operates within safe and realistic parameters.

High Flow Nasal Cannula (HFNC)

Addressing the Limitations of Conventional Oxygen Delivery Systems and the Transition to Mechanical Ventilation

High Flow Nasal Cannula (HFNC) therapy is increasingly recognized in modern respiratory care for its ability to deliver heated and humidified medical gas at high flows through a nasal cannula. This advanced respiratory support technique offers significant improvements over traditional low-flow systems (LFNC), which typically provide only up to 6 liters per minute, achieving maximum FiO2 levels of around 0.37 to 0.45. Although systems such as the 8-10L oxygen reservoir mask can deliver FiO2 levels of up to 80% or more, HFNC is preferred for its capability to provide up to 100% humidified and heated oxygen at flow rates reaching 60 liters per minute. This feature ensures a stable fraction of inspired oxygen (FiO2), even amid changing patient demands or respiratory patterns, effectively addressing a key limitation of traditional oxygen therapy methods like wall oxygen with a reservoir mask, which cannot guarantee stable FiO2 during flow changes.

HFNC significantly enhances oxygenation and ventilation by delivering air at rates that exceed a patient's physiological capacity, effectively addressing the limitations posed by physiological dead space. Typically, dead space accounts for about one-third of the tidal volume, leading to CO2 accumulation and decreased availability of oxygen for diffusion. HFNC uses high flow rates to displace stagnant CO2 with fresh O2, increasing the partial pressure of oxygen (PAO2) and creating a more favorable oxygen diffusion gradient. This mechanism is particularly beneficial for enhancing CO2 clearance and alleviating symptoms of dyspnea, thereby increasing minute ventilation and effectively reducing dead space. These benefits are especially advantageous for patients with chronic obstructive pulmonary disease (COPD), as HFNC therapy eases their work of breathing and improves overall respiratory efficiency.

Additionally, HFNC reduces nasopharyngeal airway resistance by applying positive pressure that dilates the airways. According to the Hagen-Poiseuille law, airway resistance is inversely proportional to the fourth power of the airway radius (R = 8 n l / π r⁴), meaning that increasing the radius decreases resistance and enhances airflow. This physiological advantage is pivotal in managing respiratory conditions that involve compromised airway dynamics or ineffective air exchange, making HFNC an essential tool in respiratory care. The clinical benefits of HFNC are particularly evident in conditions like acute hypoxemic respiratory failure and COPD exacerbations, where the reduction in airway resistance can improve gas exchange and reduce the work of breathing. In acute settings, the positive airway pressure provided by HFNC helps maintain alveolar stability, enhancing oxygenation and comfort, which is crucial for patient outcomes. Studies such as the FLORALI trial highlight HFNC's role in reducing mortality and increasing ventilator-free days compared to conventional oxygen therapies. Additionally, HFNC's consistent oxygen delivery makes it highly effective for patients during critical pre and post-extubation periods, ensuring optimal respiratory support.

Moreover, the humidification and heating provided by HFNC reduce dryness in the nasal passages and mouth, minimizing gastrointestinal disturbances and the risk of atelectasis by maintaining mucociliary function and airway patency. HFNC's effectiveness extends to neonatal care, providing gentle and appropriate respiratory support for the delicate nature of neonatal airways. This is facilitated by adjustable flow rates from systems like the widely used Airvo, OmniOx and MC FLO HFNC devices, which offer settings ranging from 10 to 60 L/min for adults and 15 to 30 L/min for pediatrics, managed by controlling the flow meter connected to wall oxygen.

However, HFNC is not without potential drawbacks. It can lead to excessive CO2 reduction, potentially suppressing the respiratory drive if not carefully monitored. The therapy may also increase expiratory resistance, leading to an inadvertent continuous positive airway pressure (CPAP) effect that could elevate airway pressures and result in complications such as barotrauma, volutrauma, and pneumothorax. Additionally, the possibility of patients adjusting or removing the nasal cannula can create stability issues and compromise the delivery of precise FiO2. In scenarios of severe respiratory failure where higher levels of support are necessary, traditional mechanical ventilation may be required. This includes situations necessitating intubation and invasive ventilation to manage higher pressures safely and effectively, highlighting HFNC's limitations in more critical care scenarios.

• Sharma, S., et al. (2023). High-flow nasal cannula. In StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing.
• Frat, J.-P., et al. (2015). High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure. The New England Journal of Medicine, 372(23), 2185-2196.

Case1: HFNC in an elderly patient with advanced lung cancer

Clinical case study

 • An 80-year-old man with a history of descending-colon carcinoma (resected 2021, adjuvant chemotherapy completed)
 • Newly diagnosed non-small-cell lung cancer, adenocarcinoma, left-upper-lobe, cT₂N₃M_1b, stage IV
 • Patient declined further oncologic therapy and is receiving comfort-oriented care
 • Current oxygen delivery in the ward listed as “FiO₂ 30 %, Flow 40 L/min”, yet nursing record shows 30 L/min, suggesting mis-entry of flow rate

Indications for HFNC in this case

• Hypoxemic respiratory failure secondary to metastatic lung involvement
• Need to improve oxygenation while minimizing discomfort from tight-fitting masks
• Potential to flush anatomical dead space and reduce work of breathing
• Ability to match high inspiratory flow demands that exceed conventional mask capacity

Three Key HFNC settings

Adjustable elements and titration protocol

1. Begin at FiO₂ 0.30 and Flow 40 L · min^-1 😊
2. Increase FiO₂ in 0.05 increments if SpO₂ < 92 % for > 2 min
3. Keep flow ≥ 40 L · min^-1 to maintain dead-space washout; flow may be raised to 60 L · min^-1 if tachypnea persists
4. Reduce FiO₂ gradually when SpO₂ > 96 % to avoid oxygen toxicity

Advantages of HFNC over conventional oxygen masks

Physician HFNC order

Patient: __________________ (Room ___)
Date/Time: __________
Order by: Dr _____________________

1. Initiate heated, humidified high-flow nasal cannula oxygen therapy
   • Flow: 40 L · min-1
   • FiO₂: 0.30 (30 %)

2. Target SpO₂: 92 – 96 %
★ Apply HFNC: 40 L/min@FiO2 0.30; keep SpO2>= 92 %

3. Titration protocol
   • If SpO₂ < 92 % for > 2 min → increase FiO₂ by 0.05 (max 0.60)
   • If FiO₂ ≥ 0.60 and SpO₂ < 92 % or rising PaCO₂, notify physician
   • When SpO₂ > 96 % for > 10 min → decrease FiO₂ by 0.05, maintain flow ≥ 40 L · min-1
4. Monitoring
   • Continuous pulse oximetry
   • Record RR, HR, work of breathing hourly
   • ABG as clinically indicated

5. Nursing documentation
   • Chart actual flow & FiO₂ hourly and after each adjustment
   • Note patient comfort and adverse events

6. Weaning criteria
   • Consider step-down to nasal cannula ≤ 5 L · min-1 when stable for ≥ 4 h on FiO₂ ≤ 0.30 and flow 30 L · min-1

Guardian consent obtained for IRB-approved clinical research aimed at disseminating better clinical practices in hemodynamics.

Written on June 3, 2025

ResMED

ResMed P-SIMV & V-SIMV — with Maquet SERVO SIMV Integration

I. Conceptual foundation of synchronized intermittent mandatory ventilation (SIMV)

• Mixed-mode architecture

  1. Mandatory breaths Delivered at the programmed frequency; pressure- or volume-controlled by the ventilator.
  2. Spontaneous breaths Initiated (and, for pressure modes, cycled) by the patient between mandatory inflations; usually pressure-supported.

• Synchronization window

  1. ResMed implementation Any patient trigger occurring within ≤ 60 % of the set cycle time (or ≤ 10 s, whichever is shorter) is converted into the next mandatory breath.

  The ventilator continuously tracks elapsed time since the last mandatory inflation. If a patient effort occurs within ≤ 60 % of the predetermined cycle time (or ≤ 10 s, whichever is shorter), the machine “synchronizes” and converts that effort into the next mandatory breath. Beyond this window, any patient‑initiated effort is treated as a separate spontaneous breath.

  2. Maquet SERVO implementation The ventilator aligns a detected effort with the next mandatory inflation when it falls inside the “start breath” detection period; outside this period the effort becomes a spontaneous, pressure-supported breath. The user may individually set the trigger sensitivity and breath cycle time (see Section V).

II. Distinction between pressure-targeted and volume-targeted SIMV (ResMed benchmark)

1. Pressure-targeted SIMV (P-SIMV)

   1. Mandatory component Pressure-controlled; inspiratory pressure determined by P control.
   2. Spontaneous component Pressure-supported; each patient-initiated breath receives support set by PS.

2. Volume-targeted SIMV (V-SIMV)

   1. Mandatory component Volume-controlled; the ventilator guarantees the preset tidal volume, adjusting inspiratory pressure as required.
   2. Spontaneous component Identical to P-SIMV—pressure-supported breaths defined by the same PS setting.

III. Timing logic & SIMV-rate weaning strategy (ResMed)

Practical weaning pattern used on most adult units: once the patient meets readiness criteria, clinicians commonly lower the mandatory SIMV rate in 2-bpm steps (e.g. 14 → 12 → 10 → 8 → 6 → 4 → 2 bpm).
• Pacing: up to three reductions per day, spaced ≈ 3 h apart, or slower if gas-exchange, work-of-breathing, or hemodynamics wobble.
• Targets: maintain VT ≈ 6 mL·kg⁻¹ IBW and SpO₂ > 92 %; draw ABGs when clinical status changes.
• Transition: at ≤ 4 bpm (or when ≥ 80 % of breaths are spontaneous and pressure-supported) many centres switch to pure PS/CPAP and begin daily 30-min spontaneous-breathing trials.
• Abort criteria: RR > 35, SpO₂ < 90 %, pH < 7.30, HR > 140 bpm, or marked distress.

IV. Pressure strategy illustrated (ResMed example)

1. The first inflation every 6 s is a mandatory pressure‑controlled breath reaching 17 cmH₂O (PEEP + P control).
2. Any patient‑triggered effort between mandatory cycles receives a pressure‑supported boost to 15 cmH₂O (PEEP + PS).
3. If no spontaneous effort occurs, the ventilator waits the full 6 s, then delivers the next mandatory inflation, ensuring a minimum minute ventilation.

PEEP 10 cmH₂O
P control 7 cmH₂O → Mandatory peak 17 cmH₂O
PS 5 cmH₂O → Spontaneous peak 15 cmH₂O
RR 10 bpm (6 s period)

V. Key parameters unique to Maquet SERVO SIMV

• Adjustable variables

  1. SIMV rate 1–60 breaths min^-1.
  2. Breath-cycle time, \(T_{\text{cycle}}\) 0.5–15 s (infant) or 1–15 s (adult).
  3. Implicit operating rule To avoid overlap, ensure \( \displaystyle \text{SIMV}_{\text{rate}}\times T_{\text{cycle}}\le60\ \text{s} \). When the product approaches the 60 s upper limit, any further increase in the SIMV rate must be accompanied by a proportional shortening of \(T_{\text{cycle}}\).
  4. Control-mode selection for mandatory breaths PC, VC or PRVC may be chosen; spontaneous breaths are always pressure-supported.

• Breath-delivery scenarios on the SERVO platform

  1. No spontaneous effort detected The ventilator waits the scheduled interval, then delivers a fully controlled breath at the SIMV rate.
  2. Spontaneous effort detected Mandatory breaths continue at the SIMV rate, but any additional patient triggers receive pressure support; the total respiratory rate is therefore patient-determined.

VI. Clinical pearls

1. PEEP stability End-expiratory pressure is constant across mandatory and spontaneous breaths, promoting alveolar recruitment.
2. Work-sharing principle SIMV supplements—but does not replace—patient effort, encouraging respiratory-muscle conditioning during recovery.
3. Weaning metric The ratio of spontaneous to mandatory breaths over several minutes provides a rapid gauge of readiness to progress to pressure support or CPAP.
4. Parameter coupling on SERVO When high SIMV rates are needed, ensure that T_cycle is shortened accordingly; otherwise, alarms for insufficient time may occur.

VII. Safety considerations

Mandatory back-up breaths protect against periods of apnea. Excessive mandatory rates may blunt intrinsic drive, whereas rates set too low risk hypoventilation if spontaneous effort diminishes. Continuous monitoring of tidal volume, minute ventilation, end-tidal CO₂ and patient comfort is essential on both ResMed and SERVO platforms.

Written on June 9, 2025

ResMed Astral P(A)CV vs P(A)C: Circuit Differences and Mode Nomenclature

Quick take: Both P(A)CV and P(A)C are pressure-controlled assist-control modes that can be patient-triggered or time-triggered.
Their differences hinge on two main points:
  1) The circuit type in use (valve/double-limb vs leak/single-limb), which changes the mode name and pressure terminology on the interface.
  2) Whether the Safety Vt volume-guarantee feature is enabled. With Safety Vt active in either mode, the ventilator automatically adjusts inspiratory pressure (up to 2 cmH ₂ O per breath, within a user-set ceiling) to achieve the target tidal volume—functionally the same concept as PRVC (pressure-regulated volume control).

Circuit types on ResMed Astral ventilators

ResMed Astral ventilators support two distinct patient circuit configurations: a valve (double-limb) circuit and a leak (single-limb) circuit . The choice of circuit influences how the ventilator labels its modes and pressure settings, even though the underlying breath delivery mechanics remain the same. In essence, switching between a double-limb and single-limb circuit does not change how breaths are controlled; it simply changes the terminology and measurement capabilities.

• Valve (double-limb) circuit: This setup uses separate inspiratory and expiratory tubing limbs and incorporates an active exhalation valve. The ventilator measures flow in both directions, allowing precise monitoring of exhaled volumes. On the Astral interface, the inspiratory pressure setting is labeled as P control (the pressure above baseline) and the baseline pressure is labeled PEEP . The available pressure-control mode with this circuit is labeled P(A)CV . Double-limb circuits are typically used in ICU or invasive ventilation settings where accurate volume measurement and tighter control of exhalation are required.
• Leak (single-limb) circuit: This setup has only one limb for both inhalation and exhalation; exhaled gas escapes via an intentional leak port or a vented mask. There is no separate exhalation valve, so the ventilator must compensate for and estimate leaks. On the interface, the inspiratory pressure target is labeled IPAP (Inspiratory Positive Airway Pressure) and the baseline pressure is EPAP (Expiratory Positive Airway Pressure). The pressure-control mode available with this circuit is labeled P(A)C . Single-limb circuits are commonly used for noninvasive ventilation (e.g., via mask) in home care, transport, or chronic settings where simplicity and portability are priorities.

Notably, the ventilator’s mode menu is partitioned by circuit type: when a double-limb valve circuit is selected in the settings, modes like P(A)CV become available; when a single-limb leak circuit is selected, modes like P(A)C appear instead. The underlying control scheme is equivalent— an operator can think of IPAP on a leak circuit as being analogous to P control + PEEP on a valve circuit —but the device uses the appropriate terms and mode names to match conventional usage in each setting.

The differences between single and double-limb operation are partly hardware and partly software. A double-limb circuit requires a ventilator with two separate ports (inspiratory and expiratory) and an exhalation valve mechanism. For example, the ResMed Astral 150 model can accommodate a detachable double-limb circuit adapter (providing the second port and valve), whereas the Astral 100 model has only a single port and therefore cannot support a true double-limb configuration. If a ventilator lacks the physical second port or valve, it is limited to single-limb modes. Conversely, even on a device with dual ports, the firmware must be capable of a "leak circuit" mode (with appropriate leak compensation and trigger algorithms) to safely use a single-limb setup. When the circuit type is changed on a capable device, the ventilator software adjusts its mode options and alarm algorithms accordingly. In Astral devices, a circuit change prompt will initiate a “Learn Circuit” calibration to account for the new circuit’s resistance and compliance.

Identifying the circuit type: In clinical practice, a double-limb circuit is easily recognized by its two hoses joining at a Y-piece (with an exhalation valve module on the expiratory limb), while a single-limb circuit has only one hose (and typically a vented mask or leak port for exhalation). The Astral’s user interface provides cues as well: if the screen displays settings like “P control” and “PEEP” (and shows measured exhaled tidal volumes), a double-limb circuit is in use; if it displays “IPAP” and “EPAP” and omits exhaled volume readouts, a single-limb circuit is in use. The device’s setup menu also explicitly indicates which circuit type is active, and different alarm messages tend to appear for each (e.g., valve disconnect alarms with double-limb, or high leak warnings with single-limb).

Overview of valve vs leak circuits

Mode availability by circuit type

Because of these hardware differences, certain ventilation modes are available only with one circuit type or the other on the Astral (and similar ResMed devices). The table below outlines which modes can be selected with each circuit:

In summary, the valve circuit option (available on Astral 150) enables traditional ICU modes like SIMV and precise volume-controlled ventilation, whereas the leak circuit option (common to Astral 100/150 and home devices) provides a simpler set of modes tailored to noninvasive support (including unique modes like iVAPS). Importantly, all of these modes, whether in valve or leak configuration, share the same core ventilation principles—differences lie in the interface labels and certain advanced features rather than the fundamental breath delivery.

Pressure-control ventilation: P(A)CV vs P(A)C

P(A)CV (Pressure Assist-Control Ventilation) and P(A)C (Pressure Assist-Control) are essentially the same type of ventilatory mode delivered via different circuit setups. Both are pressure-targeted, time-cycled modes in which every breath is delivered to a set inspiratory pressure for a set inspiratory time. They can function as assist-control modes: breaths will be delivered at a minimum set rate, but the patient can trigger additional breaths above the set rate (each triggered breath still receives the full preset pressure and duration). If the patient makes no effort, the ventilator will deliver breaths at the set rate—hence these modes provide full ventilatory support.

When using a double-limb circuit, this mode is labeled P(A)CV on the screen, and the settings will include a P control level above PEEP. With a single-limb circuit, the same mode appears as P(A)C, and the equivalent settings are an IPAP level and an EPAP baseline. In practice, these represent the same pressure targets: for example, an IPAP of 18 cmH ₂ O and EPAP of 8 cmH ₂ O on a single-limb setup correspond to PEEP = 8 and P control = 10 on a double-limb (since 8 + 10 = 18 total cmH ₂ O of inspiratory pressure). The ventilator’s control system ensures that the delivered pressure waveform is the same; only the nomenclature differs. In fact, the Astral device will automatically convert stored settings between IPAP/EPAP and P control/PEEP if the circuit type setting is changed, so that the actual delivered pressures remain consistent.

Shared mechanics: In both P(A)CV and P(A)C modes, inspiration is strictly time-cycled (i.e. the inspiratory phase lasts for a preset inspiratory time on each breath). The patient can initiate (trigger) breaths in either mode if they generate sufficient inspiratory effort, but they cannot terminate the breath early—each breath will continue until the set inspiratory time has elapsed (unlike a purely spontaneous breath in Pressure Support where the patient can cycle to exhalation). The adjustable parameters are identical: respiratory rate, inspiratory time (or I:E ratio), rise time (pressure slope), and trigger/cycle sensitivity thresholds are all available and function the same way in both modes. Neither P(A)CV nor P(A)C provides additional pressure “support” beyond the set pressure target; they are full mandatory breaths, simply allowing the patient to initiate them as needed. This means P(A)C is not a partially spontaneous mode—it is as much a mandatory breath mode as P(A)CV, just presented in the context of a leak circuit.

Safety V _t (volume guarantee)

ResMed’s Astral ventilators offer an optional volume-guarantee feature called Safety V _t . This feature can be enabled in several modes (including P(A)CV, P(A)C, S/T, and PS) to ensure a minimum tidal volume is delivered. When Safety V _t is active, the ventilator monitors the exhaled (or delivered) tidal volume each breath and compares it to a target set by the clinician. If the volume falls below the target, the ventilator will automatically increase the inspiratory pressure on the next breath (up to a limit of +2 cmH ₂ O per breath) until the target volume is achieved or a maximum pressure limit is reached. Similarly, if volumes are exceeding the target, it can decrement the pressure in small steps. The clinician sets an upper pressure cap ( P control max on a valve circuit, or IPAP max on a leak circuit) to restrict how high the pressure may rise. This closed-loop algorithm effectively converts the pressure-control mode into a pressure-regulated volume control mode: the ventilator adjusts pressure as needed to hit the volume goal while otherwise delivering pressure-controlled breaths. In other ventilator brands, analogous modes go by names like PRVC (Pressure-Regulated Volume Control), Autoflow, or PC-VG (Pressure Control - Volume Guaranteed). With Safety V _t enabled, P(A)CV and P(A)C perform equivalently to those modes. The Astral display will typically indicate when Safety V _t is on (for instance, by showing a blue shield icon or appending “SV” to the mode label), signaling that a volume guarantee is in effect.

Terminology differences on the interface

Because of the circuit-dependent labeling, the same physical setting may be named differently between P(A)CV and P(A)C. The following table maps the key parameters and limits in a pressure-control breath between the two circuit types, and explains the naming rationale:

In practical terms, IPAP = PEEP + P control when comparing these modes, and EPAP = PEEP . All alarm settings and monitoring readouts are based on absolute pressure values (for example, peak inspiratory pressure), so they remain consistent regardless of whether one is looking at a valve or leak circuit display. Clinicians can translate orders between the two terminologies by simple addition or subtraction. For instance, an order to provide “IPAP 20 cmH ₂ O / EPAP 5 cmH ₂ O” on a leak-circuit device would be implemented as PEEP = 5 with P control = 15 on a double-limb ventilator. Conversely, a setting of PEEP 5 and P control 12 cmH ₂ O on a dual-limb ventilator corresponds to EPAP 5 and IPAP 17 on a leak-circuit machine. The Astral will perform these conversions automatically if the circuit type selection is changed, carrying over the intended pressures appropriately.

Note on mode names: The “(A)” in P(A)CV or P(A)C indicates that the mode is assist-capable. If patient triggering is turned off (or not detected), the Astral will simply display the mode as PCV or PC (dropping the “A”). Thus, P(A)C with no spontaneous triggering becomes a pure timed Pressure Control mode. Likewise, when the Safety V _t feature is active, the Astral may add a suffix like “/SV” (or show an icon) to denote the active volume guarantee. These nomenclature cues help the user quickly identify whether a patient’s breaths are assisted and whether a volume guarantee is in place.

Practical guidance for selecting modes and circuits

General tips

1. Match the mode to the circuit: Ensure the ventilator is configured for the correct circuit type (valve vs leak) and choose the corresponding pressure mode. If using a double-limb circuit, use P(A)CV (Pressure AC Ventilation); if using a single-limb leak circuit, use P(A)C. The ventilator will only display the appropriate modes for the selected circuit, but it is still critical to verify that the hardware and software settings align.
2. Verify pressure settings after circuit changes: When switching between circuit types, double-check that the inspiratory and expiratory pressure settings make sense in the new terminology. The device should convert values automatically (e.g., an inspiratory pressure of 15 above PEEP becomes an IPAP of 15+PEEP), but it is good practice to confirm the numbers to avoid any unintended deviation in support.
3. Consider using Safety V _t if volume delivery is a concern: In patients with changing lung mechanics or leaks (e.g., varying compliance, mask leaks, or disease progression), enabling the volume guarantee can provide more consistent ventilation. If Safety V _t is activated, be sure to set a reasonable IPAP max or P control max as a safety limit to prevent excessive pressures.
4. Perform a circuit calibration: After any change in circuit hardware (for example, switching from a leak adapter to a dual-limb adapter), run the ventilator’s “Learn Circuit” or calibration procedure. This ensures the ventilator properly compensates for the new circuit’s characteristics and can trigger and cycle accurately.
5. Optimize patient comfort and synchrony: Adjust trigger sensitivity and rise time settings as needed, regardless of circuit type. These parameters function the same way in P(A)CV and P(A)C modes. A sensitive trigger and appropriate rise time can help the patient feel more comfortable, especially when they are initiating breaths. Monitor the patient for signs of asynchrony and adjust accordingly.

Example clinical scenarios

The following are scenarios illustrating how one might choose between a leak vs valve circuit and utilize the corresponding mode:

1. ARDS in the ICU: A patient with acute respiratory distress syndrome on a protective ventilation strategy may be managed on an Astral 150 with a double-limb circuit. Using P(A)CV mode (pressure control with assist) allows precise control of tidal volumes (e.g., ensuring ≤6 mL/kg ideal body weight) and accurate monitoring of exhaled volume. Safety V _t can be enabled as a safeguard to maintain target volumes without exceeding a set pressure limit.
2. COPD exacerbation requiring NIV: A hypercapnic COPD patient in respiratory failure is placed on an Astral with a single-limb leak circuit and a full-face vented mask. P(A)C mode is selected to deliver bilevel pressure support with a backup rate (Spontaneous/Timed mode). If the patient’s ventilation needs are variable, Safety V _t can be turned on to automatically adjust IPAP and ensure adequate tidal volumes, potentially averting intubation.
3. Patient transport with portable ventilator: During an inter-facility transport, an Astral configured with a leak circuit is used for its simplicity and lighter weight. P(A)C mode provides necessary ventilatory support in a single-limb setup, which is quicker to set up and less cumbersome in tight transport environments. The trade-off is that exact exhaled volume measurement is not available, but clinical observation and other monitoring compensate during the short transport.
4. Weaning via tracheostomy: A chronically ventilator-dependent patient with a tracheostomy in a weaning process might be managed on an Astral 150 with a double-limb circuit. P(A)CV is used initially for full support and monitoring accuracy. As the patient improves and initiates more breaths, the team could consider switching to a spontaneous mode or simply keep P(A)CV and let the patient trigger most breaths. In this scenario, having a valve circuit enables precise tracking of the patient’s respiratory volumes and easier connection to devices like speaking valves or inline suction.

Clinical pearls

• Always align the hardware setup, ventilator circuit selection, and on-screen labels. A mismatch (for example, using a single-limb circuit but the ventilator set to “valve” mode) can lead to incorrect readings or alarms.
• Choose a valve (double-limb) circuit whenever accurate measurement of tidal volume and minute ventilation is crucial to patient management. Use a leak (single-limb) circuit for situations where portability or noninvasive interface is paramount and precise volume data is less critical.
• Safety V _t can greatly stabilize delivered volume in the face of changing patient conditions, but it depends on appropriate alarm limits. Set the maximum pressure carefully and monitor the patient’s peak pressures and volumes regularly to ensure safety.
• Refer to the ventilator’s clinical guide for detailed information on mode functionality, circuit setup, and any device-specific considerations before deployment. Manufacturer guidelines provide important details on safe operation and limitations for each mode and circuit.

Ventilation mode nomenclature across different manufacturers

Different ventilator manufacturers often use distinct names for modes that are functionally similar. Understanding the terminology differences is important when comparing protocols or translating settings from one machine to another. Below is a comparison of mode naming conventions among three systems: the Maquet Servo series, the ResMed Astral, and the MEK HFT700 (a non-invasive ventilator). The modes are organized from full mandatory support to spontaneous support to highlight their equivalence.

Notes: In the Astral column, parentheses around a letter (e.g., (A) or (S)) indicate that the letter will appear when the function is active. For example, Astral displays “(A)CV” when patient triggering is available; if the patient trigger is turned off, it would simply display “CV” for a volume-controlled mode or “PC” for a pressure-controlled mode. Similarly, “(S)T” indicates a spontaneous/timed mode; when the patient is breathing above the set rate, breaths are spontaneous (S), but if they do not, the machine delivers timed (T) backup breaths. Astral appends “SV” or “SVt” to modes like (S)T or PS to denote the Safety Volume guarantee is enabled on those modes (providing a volume-assured pressure support).

When converting ventilation orders or protocols between different ventilator brands, it is crucial to match the intended clinical strategy rather than relying on mode names alone. Pay attention to suffixes or special mode designations, as they often signify important features like patient-trigger availability or volume guarantees.

• Parentheses in mode names: On ResMed devices, modes are often displayed with parentheses (e.g., (A) or (S)) to indicate optional components. If the component is active (assist or spontaneous triggering), the letter is shown; if not, it is omitted on the interface.
• “SV” suffix: Modes labeled with “SV” or “SVt” include a Safety Volume guarantee. This indicates a closed-loop adjustment of pressure to maintain a target tidal volume (similar to PRVC behavior on other ventilators).
• Adaptive mode transitions: Some ICU ventilators (e.g., Maquet Servo’s Automode) can automatically shift a patient from controlled to supported breaths as they initiate effort. ResMed’s Astral does not have an Automode; instead, one would manually select a mode like (S)T for backup rate or add Safety V _t for volume assurance. Understanding these differences is important when developing weaning protocols across platforms.

Understanding Spontaneous vs Timed bi-level modes (S, S/T, T)

Many noninvasive ventilators classify their bi-level modes using the letters S, T, and S/T, which correspond to how breaths are triggered and cycled:

• S (Spontaneous): All breaths are initiated and cycled by the patient. The ventilator provides two pressure levels (IPAP and EPAP), but it will only pressurize to IPAP when it senses the patient’s inspiratory effort. There is no backup rate in pure S mode – if the patient does not breathe, the ventilator will not trigger a breath. This is essentially pressure support ventilation (bi-level PAP) without any mandatory breaths.
• T (Timed): Breaths cycle between IPAP and EPAP at a fixed frequency set by the clinician, regardless of any patient effort. In T mode, the ventilator delivers only mandatory breaths at the set rate and inspiratory time. This is equivalent to a pressure-controlled ventilation mode applied noninvasively (full control by the ventilator’s timing).
• S/T (Spontaneous/Timed): This mode combines aspects of S and T. It functions like spontaneous bi-level support when the patient is breathing, allowing the patient to trigger and cycle breaths. However, if the patient becomes apneic or drops below the set respiratory rate, the ventilator will intervene and deliver time-cycled pressure breaths at the prescribed rate. In other words, S/T provides pressure support with an automatic pressure-control backup. It is not the same as SIMV; all breaths in S/T are pressure-targeted (there are no volume-cycled mandatory breaths), and the backup breaths are synchronized only in the sense that they occur after a period of no triggers.
• CPAP (Continuous Positive Airway Pressure): Although not a bi-level mode, CPAP is often listed alongside these modes in ventilator interfaces. CPAP provides a single continuous pressure (equivalent to EPAP alone) throughout the respiratory cycle with no IPAP elevation. There are no machine-delivered breaths or pressure increases, so CPAP serves purely as a baseline pressure for spontaneous breathing (useful for improving oxygenation or treating obstructive sleep apnea).

In some devices, an additional designation is used for volume-assured pressure support. For example, one might see “S/T (AVAPS)” on a Philips ventilator or “S/V” on another device, indicating that while the mode is fundamentally Spontaneous/Timed, the ventilator will automatically adjust the IPAP level to maintain a target tidal volume (this is often termed Average Volume Assured Pressure Support). ResMed’s implementation of a volume-assured pressure support is the Safety V _t feature, which can be added to its S/T mode (displayed as “(S)T / SV”) or to pure PS mode (“PS/SV _t ”). Regardless of naming, these modes still follow the same principles: S mode breaths when the patient triggers, T mode backup if they do not, and an optional algorithm to tweak pressure for volume targets.

Written on July 10, 2025

ResMed Fictitious Setting Examples

• P(A)CV (Pressure (Assisted) Control Ventilation):

  

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• P(A)C (Pressure (Assisted) Control):

  

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• (A)CV (Volume (Assisted) Control Ventilation):

  

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(A)C Mode UNCONFIRMED

(A)C (Volume (Assisted) Control): Also known as Volume Control or Volume Assist/Control, this mode generally ensures a set tidal volume (V_T) is delivered with each breath, whether the patient triggers it (assist) or the ventilator initiates it (control). Similar to how P(A)C uses IPAP/EPAP and a mandatory rate, (A)C mode often provides a parallel set of parameters:

• Tidal Volume (V_T): Instead of pressure targets (IPAP/EPAP), you specify the exact volume delivered on each breath.
• Respiratory Rate (RR): A minimum back-up rate ensures the patient receives mandatory breaths if spontaneous efforts are inadequate.
• PEEP: Similar to EPAP in P(A)C, PEEP maintains a baseline positive pressure at end-expiration.
• Inspiratory Time (T_I) or I:E Ratio: Defines how long each inspiration lasts and balances inspiratory versus expiratory phases.
• Flow Pattern/Shape: Determines how airflow is delivered during inspiration (e.g., square or decelerating ramp).
• Peak Inspiratory Flow (PIF): Sets the maximum flow the ventilator can achieve while delivering the prescribed tidal volume.
• Trigger Sensitivity: Adjusts how easily the ventilator detects and assists a patient-initiated breath.

   Where P(A)C controls pressure and allows volume to vary, (A)C fixes volume and permits pressure to fluctuate as needed to reach the set tidal volume. This can provide more predictable minute ventilation but requires careful monitoring of peak pressures, particularly if the patient’s lung mechanics change.

Note: The exact naming conventions and parameter options may vary by device or manufacturer. While these settings typically apply to ResMed devices that offer an (A)C or Volume Control mode, be sure to verify with the official device manual or manufacturer guidelines to confirm the specific parameters available on a particular model.

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V-SIMV (Volume Synchronized Intermittent Mandatory Ventilation):



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P-SIMV (Pressure Synchronized Intermittent Mandatory Ventilation):



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ST, SV Mode:



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PS (Pressure Support):

OmniOx HFT700

Historical perspective

Modern non-invasive ventilation (NIV) progressed from early nasal CPAP in the 1980s to bi-level positive airway pressure (BiPAP) in the 1990s and to HFNC + NIV hybrids in the 2010s. The OmniOx HFT700 embodies this trajectory by integrating HFNC, CPAP, and bi-level modes in one turbine-driven, fully humidified platform.

The OmniOx HFT700 unifies high-flow nasal cannula (HFNC), CPAP, and advanced bi-level ventilation within a versatile platform. Mastery of key parameters such as IPAP (PIP), EPAP (PEEP), trigger modes, trigger sensitivity, Ti, and rise time allows clinicians to tailor therapy to patient needs, minimising asynchrony and improving clinical outcomes.

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HF

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Click to view photos taken after removing the protective film

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1. Key Variables

   • Total gas flow (L min^-1)
   • Fraction of inspired oxygen (FiO₂)

2. Typical Operating Ranges

   Variable	Common starting point	Usual clinical span
   Total flow	30 – 40 L min-1	30 – 60 L min-1 (up to 70 L min-1 when demand is very high)
   FiO2	0.40 – 0.60	0.21 – 1.00 (21 – 100 %)

3. Condition-Specific Initial Flow Targets

   Clinical scenario	Recommended starting flow (L min-1)
   General adult (no major respiratory comorbidity)	35 – 40
   COPD	25 – 35
   Cardiogenic pulmonary oedema	40 – 60
   Obesity or baseline dyspnoea	Up to 60
   Community-acquired pneumonia	30 – 60
   ARDS	40 – 70
   Acute asthma	30 – 50

   Rationale: Higher flows improve nasopharyngeal wash-out and generate flow-dependent PEEP (≈ 3–5 cm H₂O); however, COPD patients and those at risk of CO₂ retention may require initially lower velocities.

4. Stepwise Titration Workflow (“10-Minute Check-Cycle”)

   1. Start at a mid-flow (30 – 40 L min^-1) plus an FiO₂ sufficient to reach the target SpO₂.
   2. Re-assess after 10–15 min:
      • Respiratory rate (RR)
      • SpO₂ (or PaO₂)
      • Visible work of breathing / accessory-muscle use
      • Patient comfort or distress
   3. Adjust flow in 5 – 10 L min^-1 steps:
      • ↑ Flow if RR > 24 min^-1, persistent dyspnoea, or rising PaCO₂.
      • ↓ Flow if patient reports discomfort (nasal dryness, pressure), marked CO₂ retention, or leaks.
   4. Maintain / Wean once stable:
      • Gradually reduce FiO₂ to < 0.40 and flow to < 20–25 L min^-1.
      • Trial discontinuation if stability persists on these settings.

5. Flow Selection by Respiratory Severity

   Severity markers	Flow target (L min-1)	Why
   Mild hypoxaemia, RR < 22	30 – 35	Meets inspiratory demand with minimal noise/velocity.
   Moderate distress, RR 22 – 28	35 – 50	Enhances CO2 clearance; accommodates ↑ minute ventilation.
   Severe distress, RR > 28 or hypercapnia	50 – 60 (+)	Maximises PEEP-like effect and O2 delivery; consider escalation if no improvement.

6. Example Workflow & Weaning (COPD)

   Step	Setting	Clinical response
   Initiation	25 L min-1 + FiO2 0.35	SpO2 89 %, RR 26
   Adjustment	Increase flow to 30 L min-1	SpO2 ≥ 92 %, RR ≤ 24
   Stabilisation	Gradually ↓ flow by 5 L min-1 steps	Maintain target saturation & comfort
   Readiness for discontinuation	FiO2 < 0.40 and flow ≤ 20–25 L min-1	Consider switch to Venturi / low-flow O2

7. Escalation & Safety Triggers

   • FiO₂ requirement > 0.60 or flow > 60 L min^-1 for > 30 min without improvement
   • Worsening hypercapnia, decreased mental status, or haemodynamic instability
   • Inability to maintain SpO₂ target despite optimisation

   Escalate promptly to non-invasive ventilation or invasive airway management.

8. Practical Tips

   Tip	Explanation
   Humidification:	Always run a heated humidifier; dry gas quickly causes discomfort and mucosal injury.
   Cannula sizing:	Use the largest prong that fills ≤ 2/3 of the nares to minimise leak yet allow purge flow.
   Positioning:	Semi-recumbent ≥ 30° head-up improves diaphragmatic excursion and secretion clearance.
   Monitoring:	Track RR, SpO2, subjective dyspnoea, and transcutaneous/arterial CO2 where available.

9. Quick Reference Flow Chart

   START 30–40 L → Re-assess @10 min
           ↓
   RR >24 or ↑ work? ──► +5–10 L
   RR <18 and stable? ─► –5 L (or maintain)
   SpO₂ < target? ─────► ↑ FiO₂
   SpO₂ > target? ─────► ↓ FiO₂
   Stable on FiO₂ <0.40 & Flow <25? ─► Trial off HFNC
   

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Appendix – Extended Technical Clarifications

1. Respiratory-rate (RR) detection

   • Functional overview

     The RR detection option activates an internal algorithm that analyses subtle fluctuations in the delivered high-flow gas stream to identify each inspiratory–expiratory cycle. When set to On the ventilator:

     • Calculates the patient’s spontaneous respiratory rate in real time and displays the value in the monitoring area.
     • Enables alarms linked to abnormally high, low, or absent respiratory activity (for example, apnoea or tachypnoea alerts).
     • Stores the RR trend so that clinicians can review minute-to-minute changes and export the data to central monitoring systems.

     When Off, the screen shows “— — —” instead of a numeric RR, and every alarm that depends on RR is automatically suppressed.

   • Clinical rationale

     Continuous RR surveillance during high-flow nasal cannula (HFNC) therapy is recognised as a sensitive indicator of deteriorating respiratory mechanics or metabolic demand. Because high flows can mask accessory-muscle movement and muffle breath sounds, electronic detection:

     1. Offers an objective early-warning sign of respiratory fatigue or airway obstruction.
     2. Facilitates precise titration of flow, FiO₂, and escalation to CPAP / NIV when necessary.
     3. Supports quality metrics such as S/F-ratio trends, which rely on synchronised SpO₂, FiO₂, and RR data.

   • Operating principle

     The turbine maintains a constant baseline flow (1 – 60 L min^-1). Each inspiratory effort causes a transient fall in circuit pressure proportional to inspiratory demand, while expiration produces the opposite deflection. The micro-processor applies a leak-compensated, band-pass filter to these oscillations, then timestamps the mid-points between adjacent peaks to derive the breath-to-breath interval. The inverse of this interval is reported as RR (breaths min^-1). Artefacts caused by coughing, talking, or line occlusion are rejected, and calculation is paused if flow is intentionally interrupted for suction.

   • Benefits of enabling RR detection

     • Early intervention: Deviations from baseline RR often precede changes in oxygen saturation or blood-gas values, allowing intervention before clinical deterioration.
     • Alarm synergy: Integrated high- and low-RR alarms complement SpO₂, heart-rate, and apnoea alerts, creating a multilayered safety net.
     • Trend analytics: Continuous RR trending supports audit, research, and protocol optimisation for HFNC therapy.
     • Workflow efficiency: Automated RR measurement reduces the need for manual counting, freeing staff time for other tasks.
     • Regulatory compliance: Many institutional protocols require documented RR for every ventilated or oxygen-supplemented patient; enabling detection ensures this metric is captured.

   • Potential drawbacks and rationale for the default “Off” setting

     • Alarm fatigue: High leak, patient movement, or frequent mouth-breathing can trigger false-positive alarms, potentially desensitising staff to true events.
     • Signal reliability: RR estimation depends on stable flow and an adequate nasal-interface seal; inaccurate readings can mislead clinical judgment.
     • Increased processor load: Continuous signal processing marginally raises system power consumption and processor workload, which may shorten maintenance intervals.
     • Data-management burden: Additional trend data require storage, review, and integration with hospital information systems, which some facilities may not yet support.
     • Training requirements: Users unfamiliar with the algorithm may misinterpret artefacts as true respiratory patterns, leading to inappropriate adjustments.

     To minimise nuisance alarms where these limitations outweigh the benefits—or where alternative monitoring is already in place—the manufacturer ships the device with RR detection Off by default, leaving activation to the clinician’s discretion.

   • System response once RR is detected

     Feature	RR detection On	RR detection Off
     Numeric display (main screen)	0 – 300 bpm range	“— — —” placeholder
     High / low RR alarms	Enabled (user-set limits)	Disabled
     Apnoea timer	Counts down from last detected breath	Inactive
     Trend storage & export	RR trend plotted and logged	No RR trend recorded
     Remote-monitoring output (HL7 / Wi-Fi)	RR transmitted	Field blank

   • Configuration and default behaviour

     • Default state: RR detection is Off in factory settings to prevent unnecessary alarms in institutions that prefer conventional vital-sign monitoring or have limited staff training.
     • Setting range: Off or On — selectable on the Settings ▶ HF page.
     • Alarm limits: Lower and upper RR thresholds remain user-adjustable (typical adult defaults: 8 and 35 bpm). When RR detection is Off, these fields are greyed out.

   • Practical considerations

     • High leak (e.g., ill-fitting nasal prongs) or excessive patient movement can degrade RR signal quality and trigger “Respiration not detected” messages.
     • Deliberate mouth-breathing may reduce inspiratory pressure swings below the detection threshold; adjust cannula fit or raise flow if required.
     • When SpO₂ feedback (TSF) is enabled, RR detection should remain active so that both oxygenation and ventilation indicators are captured synchronously.
     • After toggling RR detection, verify alarm volumes and nurse-call connectivity to avoid inadvertent silencing of critical alerts.

   • Key take-away

     RR detection provides a low-burden, high-value safeguard against unnoticed respiratory compromise during HFNC therapy. Nevertheless, potential for false alarms, signal instability, and added data-management duties justify a conservative factory default of Off. When the clinical environment supports reliable signal acquisition and staff are trained to interpret the data, enabling the feature is strongly recommended to preserve full-featured monitoring and alarm functionality.

2. High-flow oxygen therapy: comparison between nasal cannula and T-piece

   High-flow oxygen therapy (HFOT) delivers heated, humidified, precisely titrated gas flows that meet or exceed the patient’s spontaneous inspiratory demand. Two principal interfaces are in current clinical use: the high-flow nasal cannula (HFNC) and the high-flow T-piece (HFT, sometimes termed high-flow tracheal oxygen when connected to an artificial airway). Their shared goal is optimized oxygenation with enhanced comfort, yet subtle technical and physiologic distinctions guide interface selection.

   1. Definitions

      1. High-flow nasal cannula (HFNC)

         A system that provides blended air–oxygen via wide-bore nasal prongs at 20 – 60 L min⁻¹ in adults, with independently titratable FiO₂ 0.21 – 1.00. Gas is actively warmed to 37 °C and fully saturated (≈ 100 % relative humidity).

      2. High-flow T-piece (HFT)

         A heated-humidified high-flow circuit that connects through a T-shaped adapter to an artificial airway (tracheostomy or endotracheal tube) or, less commonly, to a face mask. Typical adult settings employ 30 – 60 L min⁻¹ flow with the same FiO₂ range as HFNC. Flows below 10 L min⁻¹ constitute conventional (non-high-flow) T-piece trials.

   2. Physiologic effects

      • Dead-space washout and mild positive pressure: HFNC yields measurable pharyngeal PEEP (≈ 2 – 7 cm H₂O) and efficient nasopharyngeal dead-space clearance, whereas HFT produces lower pressures because of its fully open distal limb.
      • Humidification and mucociliary protection: Both interfaces supply 37 °C gas at full saturation, reducing secretion tenacity and limiting airway injury.
      • Work of breathing: HFNC demonstrates flow-dependent reductions in respiratory effort; HFT offers less unloading unless flow reaches or exceeds 40 L min⁻¹.

   3. Settings: comparative matrix

      Parameter	HFNC	High-flow T-piece	Clinical note
      Initial flow (adult)	20 – 35 L min−1, escalate to 60 L min−1	≥ 30 L min−1, up to 60 L min−1	Higher flows required in HFT to compensate for open-circuit leak.
      FiO2 range	0.21 – 1.00	0.21 – 1.00	Both use entrainment blenders or ventilator-integrated mixers.
      Gas conditioning	37 °C; ~100 % RH	37 °C; ~100 % RH	Essential for secretion management, especially with bypassed upper airway.
      Positive airway pressure	2 – 7 cm H2O (flow-dependent)	< 2 cm H2O	Lower with HFT because of the open expiratory limb.
      Dead-space washout	High (nasopharyngeal)	Moderate (tracheal only)	Upper-airway washout is lost once the airway is bypassed.

   4. Indications favouring transition to a high-flow T-piece

      • Presence of a tracheostomy that requires continuous heated humidification and high-flow washout.
      • Endotracheal-tube patients undergoing spontaneous breathing trials where high flow is preferred over conventional T-piece.
      • Nasal or maxillofacial surgery or trauma that precludes cannula placement.
      • Need for cough-assist or suction access through the artificial airway without interrupting oxygen flow.
      • Intolerance of nasal prongs despite optimization of size, humidification, and fixation.

   5. Practical considerations for switching from HFNC to HFT

      Careful flow escalation is advised: flows below 30 L min⁻¹ risk inadequate humidification and secretion crusting, whereas excessive flows may provoke discomfort if temperature control is lost.

      1. Assess interface suitability. Confirm tracheostomy patency and cuff status; select a dedicated tracheostomy connector.
      2. Set baseline parameters. Initiate 30 L min⁻¹ flow and titrate FiO₂ to maintain SpO₂ 92 – 96 % (88 – 92 % in chronic hypercapnia).
      3. Monitor physiologic response. Observe respiratory rate, accessory-muscle use, and end-tidal CO₂; adjust flow in 5 – 10 L min⁻¹ increments.
      4. Evaluate for weaning readiness. Employ ROX or modified physiologic indices when applicable; transition to a heat-moisture exchanger once stable.

CPAP

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1. Variables that require adjustment

   • Therapeutic pressure (fixed, or minimum / maximum when auto-titrating)
   • Fraction of inspired oxygen (FiO₂)

2. Typical adult ranges

   Variable	Common starting point	Usual clinical span
   Pressure	8 – 10 cm H2O	4 – 20 cm H2O (mean ≈ 9)
   FiO2	0.21 – 0.30	0.21 – 1.00 (21 – 100 %)

3. Clinical decision guide: pressure selection

   Patient profile	Suggested pressure (cm H2O)	Considerations
   Mild-to-moderate OSA, BMI < 30 kg m-2, AHI < 15	5 – 8	Often adequate; lower starting pressure improves comfort and adherence.
   Severe OSA (AHI > 30) or positional apnoeas	8 – 12	Higher baseline pressure reduces event frequency across sleep stages and positions.
   Morbid obesity, significant hypoxaemia, or requirement for high supplemental O2	12 – 16	Addresses elevated closing pressures of pharyngeal airway; confirm tolerance and leak control.
   Persistent residual events on auto-CPAP (95th percentile pressure > 15 cm H2O)	15 – 20	Fixed high-pressure trial or bi-level conversion may be needed; monitor for aerophagia.

4. Titration strategy

   Select pressure via in-lab titration or validated auto-titration, aiming to abolish apnoea–hypopnoea events and normalise oxygen saturation. Increase FiO₂ only if residual desaturation persists after optimal pressure is confirmed. Re-evaluate after significant weight change, surgical intervention, or continued residual events on download reports. Review adherence, residual index, and patient comfort at each follow-up to guide further optimisation.

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Appendix – Extended Technical Clarifications

1. CPAP mode – Pressure Assist (PA⁺ / PA^-)

   1. Functional description

      In CPAP mode the HFT700 delivers a constant baseline pressure (4 – 20 cmH₂O). Pressure Assist + (PA⁺) and Pressure Assist - (PA^-) are small, patient-triggered pressure pulses that momentarily augment or relieve the baseline to synchronise with the patient’s inspiratory and expiratory efforts, thereby improving comfort and reducing work of breathing.

      Parameter	Set-range	Typical use
      PA+	0 – 3 cmH2O above CPAP baseline	Inspiratory boost for weak inspiratory effort
      PA-	0 – 3 cmH2O below CPAP baseline	Early expiratory unloading to facilitate exhalation
      Trigger flow	3 – 20 L min-1	Flow deviation required to activate PA

   2. Relationship between baseline PEEP and PA adjustments

      Parameter	Set-range	Typical use
      PA+	0 – 3 cmH2O above CPAP baseline	Inspiratory boost for weak inspiratory effort
      PA-	0 – 3 cmH2O below CPAP baseline	Early expiratory unloading to facilitate exhalation
      Trigger flow	3 – 20 L min-1	Flow deviation required to activate PA

      The CPAP baseline (for example 5 cmH₂O) is delivered continuously to keep the alveoli open. Pressure-assist pulses are additive or subtractive transients that ride on top of this baseline for 200 – 400 ms:

      • PA⁺: The controller briefly increases flow, raising airway pressure by the programmed increment (e.g. baseline 5 → 8 cmH₂O). After the pulse the pressure returns to the set PEEP without manual intervention.
      • PA^-: The controller momentarily reduces flow or vents a bypass valve, dropping pressure below baseline (e.g. 5 → 3 cmH₂O) to ease expiration before resuming the continuous PEEP.

      The baseline therefore remains constant over the respiratory cycle, while the PA waveform provides phasic synchronisation that can be titrated (0 – 3 cmH₂O) without altering the underlying PEEP.

   3. Flow-trigger sensitivity

      The trigger value (3 – 20 L min^-1) represents the required deviation from the bias flow for the ventilator to recognise spontaneous effort:

      • Lower settings (e.g. 3 L min^-1) are more sensitive; small patient efforts initiate PA sooner but also increase the risk of auto-triggers from leaks or artefacts.
      • Higher settings (e.g. 20 L min^-1) are less sensitive; useful when large intentional leaks exist (e.g. naso-buccal masks) or when auto-triggers must be avoided.

   4. Indications

      • Spontaneously breathing patients on CPAP who exhibit inspiratory muscle fatigue yet do not require full bi-level NIV.
      • Situations where CO₂ wash-out is desirable without switching to bi-level ventilation.
      • Paediatric or neonatal patients needing minimal assist (start with 0.5 – 1 cmH₂O increment).

   5. Contra-indications / precautions

      • Unstable haemodynamics or untreated pneumothorax (risk of pressure spikes).
      • Severe air-leak interfaces; large leaks may prevent the flow trigger from identifying patient effort.
      • Very shallow breathing where trigger sensitivity cannot be safely lowered.

BiPAP

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I. Basic Variables

1. Variables that require adjustment

   • Expiratory positive airway pressure (EPAP)
   • Inspiratory positive airway pressure (IPAP)
   • Fraction of inspired oxygen (FiO₂)
   
   

   Abbreviation	Full term	Clinical meaning	Equivalent
   IPAP	Inspiratory Positive Airway Pressure	Peak pressure during inspiration in bi-level mode	PIP (Peak Inspiratory Pressure)
   EPAP	Expiratory Positive Airway Pressure	Baseline pressure during expiration	PEEP (Positive End-Expiratory Pressure)

   Sleep-medicine literature adopts IPAP/EPAP nomenclature, whereas critical-care texts often prefer PIP/PEEP; the underlying physiology is identical.

2. Typical adult ranges

   Variable	Common starting point	Usual clinical span
   EPAP	4 – 5 cm H2O	4 – 10 cm H2O
   IPAP	8 – 10 cm H2O	8 – 25 cm H2O
   FiO2	0.30 – 0.50	0.21 – 1.00 (21 – 100 %)

3. Clinical decision guide: EPAP & IPAP selection

   Clinical situation	EPAP (cm H2O)	IPAP (cm H2O)	Key objectives
   Mild obstructive sleep-apnoea events in hospital	4 – 6	8 – 12	Maintain upper-airway patency; modest pressure-support for ventilation.
   Hypercapnic COPD exacerbation (pH 7.30 – 7.35)	4 – 6	12 – 18	Improve alveolar ventilation, targeting pH normalisation and PaCO2 decline.
   Acute hypoxemic pneumonia (non-hypercapnic)	5 – 8	10 – 16	Recruit atelectatic alveoli, enhance oxygenation, and lower work of breathing while avoiding excessive tidal volumes.
   Obesity hypoventilation or morbid obesity (> 35 kg m-2)	6 – 10	16 – 22	Counteract high pleural pressures; larger pressure-support to achieve adequate tidal volumes.
   Severe restrictive neuromuscular disease	4 – 6	18 – 25	Generate tidal volumes 8 – 10 mL kg-1 predicted body weight; prevent atelectasis.

4. Titration strategy

   • EPAP — start at 4 – 5 cm H₂O; raise in 1 – 2 cm H₂O steps for unresolved obstructive events or persistent hypoxaemia.
   • IPAP — target a pressure-support (IPAP − EPAP) of 4 – 10 cm H₂O, increasing as needed to achieve tidal volume 5 – 8 mL kg^-1 ideal body weight and reduce PaCO₂.
   • FiO₂ — titrate to maintain the ordered saturation range; prefer EPAP adjustment before raising FiO₂ when obstruction is suspected.

   Close observation of mask fit, leaks, and patient–ventilator synchrony is essential.

5. Parameter	Lower setting	Higher setting	Clinical caveats
   EPAP (Expiratory Positive Airway Pressure)	Less upper-airway splinting  →  increased risk of airway collapse or atelectasis Reduced mean airway pressure  →  potential fall in arterial oxygenation Lower intrathoracic pressure  →  more favourable venous return and blood pressure	Greater alveolar recruitment & functional residual capacity  →  enhanced oxygenation Diminished work of breathing in obstructive events Possible reduction in preload  →  hypotension in volume-depleted states	Incremental increases of 1 – 2 cm H2O are preferred. Monitor blood pressure, oxygen saturation, and signs of auto-PEEP in obstructive lung disease.
   IPAP (Inspiratory Positive Airway Pressure)	Smaller pressure-support (IPAP − EPAP)  →  lower tidal volume and minute ventilation Potential for persistent hypercapnia in hypoventilatory disorders Improved comfort, less risk of barotrauma or gastric insufflation	Larger pressure-support  →  increased tidal volume, better CO2 clearance Greater unloading of respiratory muscles  →  reduced dyspnoea Heightened risk of mask leak, aerophagia, patient-ventilator asynchrony, and barotrauma	Increase IPAP in 2 cm H2O steps while observing tidal volume (target 5 – 8 mL kg-1) and PaCO2. Reassess for gastric distension, excessive leak, and patient tolerance.

II · Terminology and parameter definitions

1. Trigger modes

   The OmniOx HFT700 offers S , T , and S/T modes; ResMed platforms additionally provide ST/SV (adaptive servo-ventilation) and related algorithms. Comparative characteristics are summarised below.

   Mode	Cycling principle	Typical indications	Advantages	Limitations
   S (Spontaneous)	Patient effort triggers & terminates each breath	Intact respiratory drive (e.g., OSA, mild COPD)	Natural timing, high comfort	No backup — apnoea risk if hypoventilation occurs
   T (Timed)	Ventilator delivers fixed RR & Ti regardless of effort	Absent drive (neuromuscular disease, CNS depression)	Guaranteed minimum ventilation	Potential asynchrony; comfort trade-off
   S/T	Spontaneous preferred; timed backup if no trigger within preset interval	Variable or intermittent drive (obesity hypoventilation, overlap OSA–COPD)	Safety net with good comfort balance	Requires leak management to avoid auto-triggers
   ST/SV *	As S/T, but servo algorithm varies pressure support breath-by-breath to target recent average ventilation	Central sleep apnoea, Cheyne-Stokes respiration, complex sleep apnoea	Dynamic stabilisation of PaCO 2 ; mitigates periodic breathing	Not available on HFT700; contraindicated in chronic hypercapnia without close monitoring

   ^* ResMed labels the servo variant as ASV or Adapt SV .

   
   

   Spontaneous-only mode (S): In the spontaneous (S) mode on a bi-level device, every breath is initiated and terminated by the patient. The ventilator simply supports each inspiration with increased pressure (IPAP) and allows exhalation at the lower pressure (EPAP). There are no machine-triggered breaths at all. Consequently, in S mode, settings for a backup rate or a fixed Ti are absent because they are not needed—the patient’s respiratory centre is fully in control of timing. This mode is common for patients who have sufficient drive to breathe but need support (for example, many obstructive sleep apnoea patients on BiPAP or patients with obesity hypoventilation who are mostly breathing spontaneously).

   

   Timed mode (T): In a timed mode, the device delivers breaths entirely according to preset timing, ignoring patient effort. For instance, if set to 12 breaths per minute with a Ti of 1.2 s, the ventilator will cycle to IPAP for 1.2 s and back to EPAP for the remainder of each 5-second cycle, regardless of what the patient does. T mode is essentially like an assist/control mode on a ventilator where every breath is machine-cycled. This mode would be used for patients who cannot breathe adequately on their own at all (for example, someone with a high spinal cord injury or severe neuromuscular disease in acute care, or during certain procedural sedations). In home bi-level units, pure T mode is less common, but some devices do allow it for specific clinical indications.

   

   Spontaneous/Timed mode (S/T): The S/T mode is a blend of spontaneous and timed. As detailed earlier, it behaves like spontaneous mode as long as the patient is breathing above the backup rate, but it will initiate a breath if the patient becomes too slow or stops. This mode is very useful in chronic respiratory insufficiency conditions where the patient has a pattern of breathing on their own but is at risk of apnoea or hypoventilation. It provides peace of mind that the ventilator will “kick in” if needed. Most advanced bi-level machines intended for home ventilation (like for neuromuscular disease, obesity hypoventilation, etc.) offer S/T mode because it covers both needs in one setting.

   

   Servo Variant (SV), Adaptive and Volume-assured modes: Beyond the basic S, T, and S/T, manufacturers have developed modes like volume-assured pressure support (where the machine adjusts pressure to meet a target tidal volume) and adaptive servo-ventilation (the SV/ASV mode discussed above). These modes still fundamentally operate either by supporting spontaneous breaths or adding timed breaths, but they have additional control algorithms (for volume or ventilation targets). They typically still require the clinician to set a backup rate (except in pure CPAP or other purely passive therapies) and they operate with some form of Ti control when providing mandatory breaths.

   Choosing and using modes: If a device is running in pure S mode, the clinician will notice that settings for RR and Ti (and sometimes even rise time, depending on the device) might be hidden or disabled on the interface. This is because they are irrelevant unless the machine is going to trigger breaths. When switching into a mode that includes machine-triggered breaths (like S/T, T, or an adaptive mode with backup), the interface will require inputs for RR, Ti, and possibly other parameters because these become crucial. The selection of mode is guided by the patient’s condition: a patient who breathes reliably but needs pressure support for hypoventilation or obstruction might do well on S mode; a patient who sometimes hypoventilates or has central apnoeas might need S/T (or ASV if primarily central apnoea); a patient with almost no drive at all would require T mode or a full ventilator support mode. It is important to fine-tune the backup settings (RR, Ti, trigger sensitivity, etc.) in any mode that provides them, so that when the ventilator does step in, it does so in harmony with the patient’s needs rather than causing discomfort or asynchrony.

2. Trigger level (sensitivity)

   The trigger level denotes the patient effort required to initiate inspiration. Typical defaults are pressure –2 cmH ₂ O (range –1 to –5) or flow 2 L min ^-1 (range 1 – 5).

   1. Raising sensitivity (making the trigger easier, i.e. a smaller absolute value) eases triggering but risks auto-triggering from leaks or cardiogenic oscillations.
   2. Lowering sensitivity (making the trigger less sensitive) reduces false triggers but increases the work of breathing.

3. Ti (Inspiratory time)

   In bi-level ventilation, Ti has two roles:

   • Timed breaths: In pure timed mode or backup breaths during S/T, Ti fixes the inspiratory-phase duration—analogous to pressure-controlled (PC) ventilation.
   • Spontaneous breaths: Ti may act as Ti Max and Ti Min limits (as in ResMed’s TiControl™) to prevent premature cycling or breath-stacking.

   Although volume-controlled (VC) ventilation relies on a set Ti to shape flow delivery, bi-level devices use Ti to define how long pressure support is applied during any machine-initiated breath and to set guardrails around patient-initiated breath duration.

   Determination: For adults, a starting Ti of about 1.2 s—the factory default on the HFT700—is typical; shorten it (< 0.8 s) when there is a risk of air-trapping (e.g., in COPD), or lengthen it (1.2 – 1.5 s) in restrictive lung disease to improve oxygenation. Values outside 0.5 – 2.0 s are seldom required in adult practice.

4. Rise time

   Rise time is the interval from EPAP to IPAP after a breath is triggered.

   • A short rise time (≈ 50 – 100 ms) rapidly unloads intrinsic PEEP and benefits obstructive physiology.
   • A long rise time (≈ 200 – 300 ms) softens the pressure increase, improving comfort in restrictive or pediatric lungs.

   Even outside of volume-controlled modes, a mismatched rise time can provoke flow starvation (if too slow) or overshoot and discomfort (if too fast). Careful titration of rise time enhances patient-ventilator synchrony and comfort.

III · Practical adjustment pathways

Initial adult bi-level template — individualise thereafter

1. EPAP 5 cmH ₂ O (increase to alleviate upper-airway obstruction or counter intrinsic PEEP).
2. IPAP 10 cmH ₂ O (pressure support of 5 cmH ₂ O; titrate higher to achieve V _T ≈ 6 – 8 mL kg ^-1 ).
3. Backup RR 12 bpm (adjust if needed for chronic hypoventilation syndromes).
4. Ti 1.0 s (alter per patient’s respiratory mechanics; see § III-4).
5. Rise time ≈ 150 ms (tailor to patient comfort; see § III-5).
6. Trigger sensitivity: flow trigger 2 L min ^-1 or pressure trigger –2 cmH ₂ O; re-titrate after leak correction.

Continuous monitoring of leak, patient synchrony, delivered tidal volume, and blood gases is recommended after any setting change.

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Appendix – Extended Technical Clarifications

1. Spontaneous/Timed (ST) vs. Servo Ventilation (SV)

   1. Spontaneous/Timed (ST) mode

      Often labeled as S/T on bi-level devices, this is a hybrid ventilation mode that combines patient-driven breathing with a timed safety net. In ST mode, the ventilator allows the patient to initiate breaths spontaneously (triggering from EPAP to IPAP whenever the device senses an inspiratory effort). If the patient does not initiate a breath within a preset time (determined by the backup rate setting), the ventilator automatically delivers a machine-initiated breath to maintain a minimum breathing frequency.

      Mechanics: In ST mode, when a patient-triggered breath occurs, the device cycles to IPAP and back to EPAP based on the patient’s own inspiratory time or flow cycling criteria. If a patient effort is absent for longer than the interval set by the backup respiratory rate (for example, 5 seconds if the backup rate is 12 breaths per minute), the device will trigger a timed breath. That timed breath will be delivered at the set IPAP level and maintained for the duration of the set Ti (ensuring a full breath is given).

      Indications: ST mode is indicated for patients with an inconsistent or intermittent respiratory drive. Examples include conditions like obesity hypoventilation syndrome, overlap syndrome (combined COPD and obstructive sleep apnoea), or certain neuromuscular disorders where patients breathe on their own most of the time but are at risk of episodic hypoventilation or apnoea. The mode provides a safety net by guaranteeing a minimal respiratory rate.

      Advantages: ST mode offers the comfort and synchrony of spontaneous breathing with the assurance of backup ventilation. Patients breathe at their own rhythm when able, which feels natural, but if they slow down or pause, the machine ensures ventilation continues. This combination helps maintain adequate minute ventilation and prevents prolonged apnoeas without heavily compromising comfort.

      Limitations: One challenge with ST mode is the potential for auto-triggering. If there is a significant leak in the mask or circuit, or if the device’s sensitivity is set too high, the ventilator might falsely detect a breath and trigger IPAP when the patient hasn’t actually made an effort. Such false triggers can disrupt sleep and lead to over-ventilation. Proper mask fit and appropriate trigger sensitivity settings are important to avoid this issue. Additionally, ST mode still relies on the patient initiating breaths most of the time, so it may not fully support someone who has almost no spontaneous drive (in those cases, a pure Timed mode might be necessary).

   2. Servo Ventilation (SV)

      Servo ventilation, commonly known as Adaptive Servo-Ventilation (ASV), builds upon the S/T framework with an added layer of automation. In SV mode, the ventilator continuously monitors the patient’s recent ventilation (usually averaging the minute ventilation over a few minutes) and dynamically adjusts the level of pressure support on a breath-by-breath basis. The goal is to maintain a stable ventilation target; for example, the machine may target approximately 90% of the patient’s recent average ventilation. If the patient’s breathing decreases (as in a central apnoea or period of hypoventilation), the device increases support (raising IPAP) to deliver a larger breath. If the patient’s own breathing effort increases or they start hyperventilating, the device will reduce support to avoid over-ventilation.

      Indications: SV modes are especially beneficial in conditions like central sleep apnoea, Cheyne-Stokes respiration (a pattern often seen in heart failure patients, with waxing and waning breaths and central apnoeas), and complex sleep apnoea (where treating obstructive events with CPAP/BiPAP reveals underlying central apnoeas). In these scenarios, the issue is not just airway collapse but a dysregulation of breathing rhythm or drive. ASV can adapt to these changes by providing just enough support to smooth out the ventilation.

      Advantages: The primary advantage of servo ventilation is its ability to stabilise breathing automatically. By preventing excessive dips and rises in ventilation, it helps maintain a steadier PaCO ₂ (carbon dioxide level) and more consistent oxygenation. Patients with periodic breathing patterns often experience improved sleep quality because the machine can effectively counteract central apnoeas and the arousals that accompany them. The therapy is highly responsive: each breath’s support level is tuned based on the immediate need, which can be very effective for certain forms of sleep-disordered breathing that do not respond well to fixed-level support.

      Limitations: Not all ventilators offer SV/ASV modes (for instance, the OmniOx HFT700 does not include a servo-ventilation algorithm). Moreover, ASV must be used with caution in patients with certain conditions. For example, in patients with chronic hypercapnia (high CO ₂ ) such as those with chronic COPD and CO ₂ retention, aggressively maintaining ventilation could suppress their drive to breathe; such patients require careful monitoring and setting adjustments. Additionally, in some heart failure patients, early studies raised concerns about ASV potentially affecting mortality, so clinicians exercise caution and adhere to the latest guidelines when considering ASV for those individuals. Overall, SV is a sophisticated mode that should be titrated and monitored by experienced practitioners.

   In summary, ST mode provides patient-initiated breathing with a timed backup to prevent apnoea, while SV mode extends this concept by adaptively adjusting pressure support to stabilise the patient’s ventilation in real time, targeting a consistent respiratory output even in the face of central breathing irregularities.

2. Trigger Sensitivity Setting (Flow Trigger 3 – 20)

   • Flow-based detection: The HFT700 uses flow triggering rather than pressure triggering. This means it senses a patient’s inspiratory effort by detecting a drop in the continuous flow through the circuit, rather than a drop below a baseline pressure. As a result, the sensitivity is specified in terms of flow (litres per minute) and there are no negative pressure values in its trigger settings.
   • Numeric scale (3 to 20): The trigger level on this device is adjustable from 3 to 20, which corresponds to the flow drop (in L min ^-1 ) required to trigger an inspiratory support. For example, a setting of 7 means the ventilator will interpret a 7 L min ^-1 decrease in flow as a patient effort and will cycle to IPAP. In simpler terms, the patient must create a 7 L min ^-1 dip in flow by inhaling (or there must be an equivalent leak or pressure change) for the machine to deliver a breath.
   • Sensitivity vs. specificity: A lower trigger number (closer to 3) makes the ventilator easier to trigger. Even a small inspiratory effort by the patient (causing a small drop in flow) will bring the ventilator into inspiration. This is helpful for patients who are very weak and cannot generate a strong inhalation. However, if the setting is too low (too sensitive), the ventilator might self-trigger due to minor disturbances like leaks or even oscillations from the patient’s heartbeat (cardiogenic oscillations). Conversely, a higher trigger setting (toward 15 or 20) requires a larger deliberate effort (or a larger flow disturbance) to trigger a breath. This can prevent false triggers in a leaky system but may also make it difficult for a patient with weak effort to get the ventilator to cycle when they need it.
   • Clinical adjustment: The default value around 7 L min ^-1 represents a middle-ground sensitivity appropriate for many situations. Clinicians will adjust the trigger level based on patient factors and observed ventilator behaviour: if the patient is having difficulty triggering the machine (e.g., the clinician observes the patient trying to inhale but the ventilator isn’t sensing it), the trigger may be made more sensitive (lower value). If the ventilator is auto-triggering (delivering breaths the patient didn’t initiate, as evidenced by ventilator waveforms or by lack of patient effort corresponding to ventilator breaths), the trigger should be made less sensitive (higher value). The wide range (3 through 20) provides flexibility to accommodate a variety of leak conditions and patient effort levels, from very weak to very strong.

3. Expiratory Pressure Assist [PA(–)]

   • Feature description: PA(–) is an optional expiratory pressure assist setting on the HFT700. It can be set to Off or –1 . When it is Off , the ventilator simply transitions from IPAP down to the set EPAP at the end of inspiration normally. When set to –1 , the ventilator briefly drops the pressure 1 cmH ₂ O below the EPAP level at the moment of cycling from inspiration to expiration.
   • Physiological rationale: That tiny drop below EPAP (only in the first moment of exhalation) creates a slight vacuum effect in the circuit, which can hasten the exhalation flow. In essence, it “pulls” a little bit to initiate expiration. This can help signal the end of inspiration and encourage the patient to exhale, which is especially useful if the ventilator or patient was a bit slow to switch into the exhalation phase.
   • Use cases: In most patients, expiratory cycling is detected and handled well by the ventilator, so PA(–) can remain off. However, in conditions where there is significant airflow obstruction (such as COPD) or intrinsic PEEP (air-trapping), patients sometimes have difficulty getting each breath to cycle off and may begin inhaling again (breath-stacking) or not fully exhale before the next breath. In such cases, turning PA(–) to –1 may help by actively drawing out the end of the breath and breaking the cycle of air-trapping. It effectively helps to reduce dynamic hyperinflation by improving expiratory flow at transition.
   • Precautions: The magnitude of the drop is intentionally small (just 1 cmH ₂ O) to avoid causing any large negative pressure swings that might collapse airways. If used inappropriately (for example, if a patient has no issue with exhalation to begin with), the negative assist could potentially shorten the inspiratory phase too much or make the transition to exhalation feel abrupt, which might be uncomfortable or counterproductive. Therefore, it is usually enabled only after observing that a patient is having difficulty exhaling or when the flow tracing suggests late or ineffective cycling to EPAP. Clinicians should monitor the patient’s comfort and the ventilator waveforms after activating PA(–) to ensure it is providing benefit.

4. The Role of Inspiratory Time (Ti) in BiPAP

   1. Definition of Ti: Ti refers to the inspiratory time, i.e., the length of time the ventilator spends in the inspiratory phase (at IPAP) during a breath. In a BiPAP or bi-level ventilator, this setting primarily comes into play during machine-delivered breaths. In a spontaneous breath, the patient’s own effort usually determines inspiratory duration, but some devices allow setting minimum and maximum Ti limits to ensure breaths aren’t too short or too long.
   2. Why BiPAP needs a Ti setting: In modes that include machine-triggered breaths (like the timed backup in S/T mode), Ti must be set so the ventilator knows how long to maintain IPAP when it initiates a breath. Essentially, whenever the ventilator, not the patient, is in charge of cycling, Ti defines the “on” time of the breath. Without a Ti setting, the device wouldn’t know whether to give a very short puff of air or a prolonged inspiration for a backup breath. Furthermore, Ti limits, even in spontaneous breathing, serve as safety bounds: a Ti Max can stop a breath that drags on too long (preventing an unintended long breath if the machine fails to cycle off), and a Ti Min can ensure a breath isn’t cut off too early due to a transient drop in flow.
   3. Comparison to other ventilation modes: In traditional volume-controlled ventilation, the clinician sets a tidal volume and often an inspiratory time or inspiratory flow pattern; the ventilator delivers that volume over the set time. In pressure-controlled ventilation, the clinician sets an inspiratory pressure and an inspiratory time; the ventilator holds the target pressure for that duration for each breath. BiPAP in a purely spontaneous mode doesn’t force a particular inspiratory time because the patient is in control, but in any mode where the machine assists or guarantees breaths, the principle is the same as pressure control: the machine will maintain the IPAP for the Ti duration on a mandatory breath. Thus, BiPAP’s Ti setting makes it behave like a pressure-controlled ventilator during timed breaths.
   4. Broad range (0.3 – 3.0 s): The device’s Ti range is wide to accommodate a spectrum of patient types:
      • Very short Ti (around 0.3 – 0.5 s) is used for infants or small children who naturally have very fast, small breaths.
      • Short Ti (0.6 – 0.8 s) might be employed in adults with obstructive lung disease (like asthma or COPD) to ensure they do not get breath-stacked; these patients often need shorter inhalation times to allow more exhalation time and avoid trapping air.
      • Typical Ti for an adult starts around 1.0 s, which generally aligns with a comfortable adult inspiratory phase in many situations.
      • Longer Ti (1.2 – 1.5 s, up to 2 – 3 s in extreme cases) can be beneficial in restrictive lung diseases (such as pulmonary fibrosis) or severe hypoxaemia, where a longer inspiratory time may improve oxygenation by giving more time for air to distribute in the lungs and for gas exchange to occur.
   5. Clinical tuning: When initiating bi-level support in an adult, a Ti of about 1.0 s is a reasonable starting point. From there, adjustments are made based on the patient’s condition: if the patient has COPD and there are signs of air-trapping (or if the patient appears uncomfortable getting air out), reducing Ti to perhaps 0.8 s or even less can help. If the patient has a stiff chest wall or poor oxygenation (as in ARDS or fibrosis), lengthening Ti to 1.2 or 1.5 s might improve ventilation distribution and oxygenation. It’s uncommon to use extremely short or long Ti settings in adults (like below 0.5 s or above 2.0 s) except in special scenarios (e.g., an adult on a device meant for paediatric use, or a prolonged Ti for certain lung-protective strategies). Throughout this tuning, monitoring is essential: the clinician watches for patient-ventilator synchrony on the waveforms and listens to patient feedback. The optimal Ti is one that complements the patient’s natural breathing timing—too long, and the patient will try to exhale against the machine; too short, and the patient may feel the breath was cut off.

5. Backup Respiratory Rate (RR) and Timed Breath Ti

   Backup rate (RR): This setting defines the minimum breaths per minute the ventilator will deliver in a mode that includes a backup (like S/T mode). If the patient’s own respiratory rate falls below this value, the ventilator will intervene by providing additional breaths. For example, a backup RR of 12 means the ventilator aims to deliver at least 12 breaths every minute. If the patient is breathing 15 times a minute spontaneously, the backup breaths won’t activate because the patient’s rate exceeds the backup. But if the patient pauses or slows down to, say, 8 breaths per minute, the device will start adding extra breaths to reach the rate of 12. It does so by monitoring the time between patient-initiated breaths and if a breath is missing, it triggers one.

   Timed inspiratory time (Ti) for backup breaths: When the ventilator provides a mandatory breath (because the patient didn’t initiate one in time), it uses the set Ti to determine how long the inhalation lasts. Essentially, the machine-delivered breath in S/T mode is a pressure-controlled breath with a fixed inspiratory duration. For instance, with RR set to 12 (5-second cycle time) and Ti set to 1.0 s, if the patient fails to inhale after 5 seconds, the ventilator will give a breath that lasts 1.0 second at the IPAP level before cycling back to EPAP. This ensures that the delivered breath has an appropriate length to be effective (neither too brief to deliver volume nor too long to encroach excessively on exhalation time).

   Behaviour in different modes: In pure spontaneous (S) mode, there is no backup rate; the ventilator will not trigger a breath on its own no matter how slow or irregular the patient’s breathing becomes. Likewise, there is no fixed Ti because the patient decides how long to inhale. In pure timed (T) mode, the ventilator uses the RR and Ti for every breath (completely controlling the respiratory cycle). In S/T mode, which is a blend, the device is usually in S mode (following the patient) but will switch to T mode temporarily whenever the patient falls below the set RR or has an apnoea. At that moment, the preset RR and Ti govern the breath timing. After that breath, if the patient resumes breathing above the backup rate, the ventilator goes back to pure support mode.

   Clinical setting considerations: A common initial backup rate for adult patients on bi-level support is around 12 breaths per minute, which is close to a normal resting respiratory rate. This ensures that if the patient becomes apnoeic or markedly bradypnoeic (breathes too slowly), the ventilator will not let too much time pass without delivering a breath. However, this rate can be tailored: for someone with known chronic hypercapnia who might benefit from a lower rate and longer exhalation (to fully clear CO ₂ each breath), the backup rate might be set a bit lower (e.g., 10). For someone who needs higher minute ventilation (e.g., obesity hypoventilation syndrome), a backup of 14–16 might be chosen to guarantee more breaths per minute. Similarly, the Ti for backup breaths is usually set around 1.0 s for an adult initially, but if the backup rate is high (meaning each breath cycle is shorter), Ti might need to be slightly shorter to allow enough time for exhalation. Conversely, if the backup rate is low or the patient’s comfortable breathing pattern includes longer inspirations, Ti could be set longer. The key is ensuring that the combination of RR and Ti leaves sufficient time for exhalation within each cycle (i.e., Ti must be shorter than the total cycle length, which is 60/RR). For example, at RR 12, the cycle length is 5 s; a Ti of 1.0 s leaves 4.0 s for exhalation, which is usually adequate. At RR 20 (cycle length 3 s), one would not set Ti to 3 s because that permits no exhalation; instead, Ti might be set around 1.0–1.5 s, leaving the remainder of each cycle for exhale.

6. Rise Time Adjustment

   Rise time controls how quickly the ventilator goes from the expiratory pressure (EPAP) to the inspiratory pressure (IPAP) at the start of each breath. Many devices allow the user to select qualitative rise settings (for example, “Fast,” “Medium,” or “Slow”) instead of specifying an exact time in milliseconds. The HFT700 provides adjustable rise time settings to optimise patient comfort and synchrony. The effects of these settings can be generalised as follows:

   Rise time setting	Approximate rise time	Clinical effect
   Fast	≈ 50 – 100 ms	Very rapid pressurisation. This quickly delivers full support at the start of a breath. It is often beneficial for patients with obstructive lung disease (e.g., COPD) because it helps overcome intrinsic PEEP and any initial airway resistance, reducing the work to initiate a breath. However, if set too fast for a given patient, it can feel like a sudden blast of air and may cause discomfort or even overshoot the patient’s inspiratory flow demand momentarily.
   Medium	≈ 100 – 200 ms	Moderate pressurisation. This is a balanced default that provides support promptly but with a slightly gentler slope than the fastest setting. Many adult patients find this comfortable and effective. It offers a good compromise: it doesn’t delay pressure delivery much, so it still assists in unloading inspiratory effort, but it’s less abrupt than the fastest rise, potentially improving tolerance.
   Slow	≈ 200 – 300 ms	Gradual pressurisation. The pressure takes longer to ramp up to the full IPAP. This can be more comfortable for patients who are sensitive to rapid pressure changes, such as those with stiff lungs (restrictive disease) or paediatric patients. It can also help if a patient is prone to overshooting the pressure (where a fast rise might cause a pressure spike that the patient doesn’t need). The downside is that if the rise is too slow, a patient might feel they are not getting enough air at the beginning of the breath, leading to a sense of air hunger or “starved flow” until the pressure finally builds up.

   Proper rise time adjustment can significantly improve patient-ventilator synchrony. A general approach is to start with a medium setting and adjust based on patient feedback and observed waveforms. If the patient seems uncomfortable at the start of each breath or if inspiratory flow does not meet their demand (e.g., they are drawing in deeply, indicating they want more support faster), the rise time may be increased (faster). If the patient appears comfortable but is startled by or fights the initial rush of air, slowing the rise time might help. Importantly, rise time does not change the total amount of pressure support given—just the delivery profile. Therefore, it should be tuned to the patient’s comfort and the characteristics of their disease (fast for obstructive diseases needing quick help, slower for comfort in restrictive diseases), all while ensuring the patient’s own effort is effectively assisted.

7. Auto Start / Stop feature

   1. Operating principle

      With Auto Start enabled, the HFT700 remains in standby until its flow-sensor detects spontaneous breathing against the circuit (typically > 2 – 3 L min^-1 differential). Once detected, the device automatically initiates the last-used therapy mode (HFOT, CPAP, or NIV) without pressing the start button.

   2. Clinical benefits

      • Facilitates rapid deployment in emergency settings where staff hands-off time is critical.
      • Minimises alarm fatigue by preventing false “no-patient” flow alarms during mask fitting.

   3. Safety considerations

      • Auto-Stop (optional paired function) terminates flow when a sustained leak or zero flow is detected (e.g. mask removal), reducing aerosolisation and O₂ wastage.
      • Disable Auto Start in paediatric patients unable to generate sufficient effort to trigger the algorithm.

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BiPAP vs Pressure Support (PS): Key Differences

BiPAP (Bilevel Positive Airway Pressure) and PS (Pressure Support) are closely related concepts in positive-pressure ventilation, but they refer to different things:

• BiPAP is a non-invasive ventilation therapy/device that delivers two preset pressure levels:
  • IPAP (Inspiratory Positive Airway Pressure) – a higher pressure during inhalation
  • EPAP (Expiratory Positive Airway Pressure) – a lower pressure during exhalation

  This alternating pressure support helps splint the airway open, improve tidal volume and reduce work of breathing without an endotracheal tube.

• Pressure Support (PS) is the difference between IPAP and EPAP (i.e. PS = IPAP − EPAP). It’s the actual “boost” of pressure that assists each spontaneous breath. On an ICU ventilator in PS mode, the machine delivers a set PS above the baseline PEEP (which is functionally equivalent to EPAP) whenever the patient initiates a breath.

In practice, a BiPAP machine implements pressure support ventilation in a non-invasive form, but PS itself is the mode/parameter that any ventilator (invasive or non-invasive) uses to augment spontaneous breaths.

Written on July 3, 2025

Reference

Amato, M. B. P. et al. (2015). Driving pressure and survival in the acute respiratory distress syndrome. The New England Journal of Medicine, 372(8), 747-755. https://doi.org/10.1056/NEJMsa1410639

ARDSnet. (2008, July). Mechanical Ventilation Protocol Summary. NIH NHLBI ARDS Clinical Network. View Protocol.

Cairo, J.M. (2023). Pilbeam's Mechanical Ventilation: Physiological and Clinical Applications (8th ed.). Elsevier.

Tobin, M. J. (Ed.). (2013). Principles and Practice of Mechanical Ventilation (3rd ed.). McGraw-Hill Education.

The Acute Respiratory Distress Syndrome Network. (2000). Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The New England Journal of Medicine, 342(18), 1301-1308. https://doi.org/10.1056/NEJM200005043421801

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