Pressure Triggering in Auto-PEEP: Recommendations for Avoidance
Introduction to Auto-PEEP and Triggering Mechanisms
- In patients with auto-PEEP, flow triggering should be used instead of pressure triggering because pressure triggers worsen patient-ventilator asynchrony and increase the work of breathing required to initiate a breath, as recommended by the British Thoracic Society 1, 2
- Intrinsic PEEP (auto-PEEP) creates an inspiratory threshold load that patients must overcome before any breath can be triggered, with the patient needing to generate sufficient negative pressure to overcome the auto-PEEP pressure, which can be significant in severe conditions 1, 2, 3, 4
Triggering Mechanisms and Auto-PEEP
- The auto-PEEP pressure can be substantial, ranging from 10-15 cm H₂O in severe cases, requiring patients to exert considerable effort to initiate breathing 5, 6, 7
- Flow triggering systems are more sensitive to patient effort in the presence of auto-PEEP, detecting changes in the ventilator's bias flow rather than requiring pressure changes 1, 2
Clinical Guidelines and Recommendations
- The BTS/ICS guidelines state that "flow rather than pressure triggers reduce the incidence of asynchrony," highlighting the importance of using flow triggers in patients with auto-PEEP 1, 2
- Applying external PEEP to offset intrinsic PEEP reduces the effort required for triggering, but setting PEEP greater than intrinsic PEEP can be harmful 1, 2, 3, 4, 5, 6, 7
- Typically, external PEEP of 5 cm H₂O or less is used, even when auto-PEEP is higher, to avoid exacerbating hyperinflation and potential hemodynamic compromise 5, 6, 7
Critical Considerations for Patient Management
- It is crucial to monitor for auto-triggering when using sensitive flow triggers, as cardiogenic oscillations or circuit leaks can cause false triggering, although specific guidance on this aspect is not provided in the cited references 1, 2, 3, 4
- Never set external PEEP levels in excess of intrinsic PEEP, as this can worsen hyperinflation and potentially cause hemodynamic compromise, emphasizing the need for careful PEEP management 1, 2, 3, 4
Auto-PEEP Development and Management in Mechanical Ventilation
Mechanism and Contributing Factors
- The time required to decompress the lungs to elastic equilibrium volume is shorter than the available expiratory time before the next inspiration, leading to auto-PEEP development, as noted by the American Thoracic Society 8
- Air trapping occurs due to incomplete expiration, leading to breath stacking, according to the American Heart Association 9, 10
- High airway resistance, such as in COPD or asthma, further increases the risk of auto-PEEP by slowing expiratory flow, as reported by the American Thoracic Society 8, 11
Clinical Implications
- Auto-PEEP can lead to increased work of breathing as patients must overcome the auto-PEEP pressure threshold to trigger the ventilator, as stated by the American Thoracic Society 8
- Hemodynamic instability due to decreased venous return and reduced cardiac output can occur as a result of auto-PEEP, according to the American Thoracic Society 8 and other sources 12
- Barotrauma from hyperinflation is a potential complication of auto-PEEP, as noted by the American Heart Association 9, 10
- Patient-ventilator asynchrony and ineffective triggering efforts can result from auto-PEEP, as reported by the American Thoracic Society 8
Prevention and Management
- Decreasing respiratory rate to allow more expiratory time can help minimize auto-PEEP, as recommended by the American Heart Association 10
- Using shorter inspiratory times with higher flow rates (adult inspiratory flow rate 80-100 L/min) can also help reduce auto-PEEP, according to the American Heart Association 10
- Aiming for longer expiratory times (I:E ratio of 1:4 or 1:5) can help prevent auto-PEEP, as suggested by the American Heart Association 10
- Considering decreasing tidal volumes (6-8 mL/kg) may also be beneficial in reducing auto-PEEP, as recommended by the American Heart Association 10
PEEP Concepts and Clinical Implications
Pathophysiology and Measurement
- Intrinsic PEEP (PEEPi) develops due to incomplete lung emptying before the next breath begins, which can occur due to high airway resistance, insufficient expiratory time, or inappropriate ventilator settings, as noted by the American Journal of Respiratory and Critical Care Medicine 13
- The end-expiratory occlusion technique is the gold standard for measuring PEEPi, requiring the patient to be passive to avoid artifacts from expiratory muscle activity, as recommended by the American Journal of Respiratory and Critical Care Medicine 13
Clinical Implications and Management
- Both extrinsic PEEP and intrinsic PEEP can decrease cardiac output by increasing intrathoracic pressure, which reduces the pressure gradient for venous return to the right ventricle, as reported by the American Journal of Respiratory and Critical Care Medicine 13
- Intrinsic PEEP poses an inspiratory pressure threshold load that must be fully counterbalanced by the patient's inspiratory muscles before the ventilator can be triggered, as noted by the American Journal of Respiratory and Critical Care Medicine 13
- Application of low levels of external PEEP (5-10 cm H₂O) can significantly improve patient-ventilator interaction and reduce the magnitude of inspiratory effort during assisted ventilation and weaning by counterbalancing PEEPi, as recommended by the American Journal of Respiratory and Critical Care Medicine 13
- Decreasing respiratory rate, using shorter inspiratory times with higher flow rates, and aiming for longer expiratory times can help minimize auto-PEEP, as suggested by Circulation 14
- Hyperventilation can exacerbate auto-PEEP by not allowing sufficient time for complete exhalation, worsening gas trapping, as reported by Circulation 14
Monitoring for Auto-PEEP and Air Trapping
Primary Monitoring Techniques
- Monitor pressure-time and flow-time scalars as recommended monitoring parameters for all ventilated patients, as suggested by the European Society of Intensive Care Medicine 15
- Measure peak inspiratory pressure, plateau pressure, mean airway pressure, and positive end-expiratory pressure near the Y-piece in patients, as recommended by the American Thoracic Society 15
- Consider measuring intrinsic PEEP as part of routine respiratory mechanics monitoring, according to the European Respiratory Society 15
Advanced Monitoring Methods
- Electrical impedance tomography can detect changes in end-expiratory thoracic gas volume that reflect auto-PEEP development, as reported by the National Institute of Health 16, 17
- Monitor EELI stability at each PEEP step during titration procedures, as recommended by the American Association for Respiratory Care 16
Clinical Signs Suggesting Auto-PEEP
- Sudden hypotension or cardiovascular collapse in mechanically ventilated patients with obstructive airway disease suggests significant auto-PEEP, according to the American Heart Association 18, 19
- If auto-PEEP results in significant hypotension, disconnect the patient from the ventilator circuit and press on the chest wall to assist exhalation, as recommended by the American College of Cardiology 18, 19
Systematic Approach to Detection
- Patients with severe asthma, COPD, or receiving high minute ventilation are at highest risk, as reported by the Global Initiative for Chronic Obstructive Lung Disease 18
- Use the DOPE mnemonic (Displacement, Obstruction, Pneumothorax, Equipment failure) plus auto-PEEP when a ventilated patient deteriorates, as suggested by the Society of Critical Care Medicine 18, 19
Clinical Management
- Document the measured auto-PEEP value and correlate with clinical signs, as recommended by the European Society of Intensive Care Medicine 15
- When auto-PEEP is detected, immediately assess for tension pneumothorax, especially if ventilation is difficult, according to the American Thoracic Society 18, 19
- Do not delay intervention—if severe hypotension occurs, disconnect from the ventilator immediately and assist exhalation, as recommended by the American Heart Association 18, 19
PEEP Dynamics and Management of Auto‑PEEP
Fundamentals of PEEP Mechanics
- External positive end‑expiratory pressure (PEEP) remains constant throughout the respiratory cycle, but the total end‑expiratory pressure and its physiological effects differ between inspiration and expiration, especially when intrinsic (auto‑)PEEP is present. 20
- During inspiration, external PEEP creates an inspiratory threshold load that must be overcome before inspiratory flow can begin. [20][21]
Auto‑PEEP Generation in Obstructive Disease
- In patients with airflow obstruction, auto‑PEEP develops when the expiratory time is insufficient to allow complete lung emptying before the next breath, producing a positive alveolar pressure even without external PEEP. 21
Impact of Expiratory Muscle Activity on Measured PEEP
- Active expiratory muscle contraction can artificially elevate measured end‑expiratory pressure; the pressure drop at end‑expiration may reflect muscle relaxation rather than true dynamic hyperinflation, potentially misleading clinicians. [22][20]
- To obtain the true value of dynamic hyperinflation in such patients, the rise in gastric pressure caused by expiratory muscle activity should be subtracted from the measured PEEP. [22][20]
Inspiratory Threshold Load and Patient‑Ventilator Interaction
- Patients with auto‑PEEP must generate sufficient negative pleural pressure to offset the intrinsic PEEP before the ventilator can be triggered or spontaneous inspiratory flow can start. 20
- This threshold load is quantified as the decrease in pleural pressure preceding inspiratory flow, termed dynamic auto‑PEEP (PEEPi,dyn), which is markedly lower than static auto‑PEEP. [20][23]
- Applying external PEEP counterbalances auto‑PEEP, reducing the inspiratory pressure gradient that patients must overcome, thereby improving patient‑ventilator synchrony and lowering work of breathing. 21
Measurement of Auto‑PEEP
- The gold‑standard method for measuring auto‑PEEP is an end‑expiratory airway occlusion performed while the patient is passive, avoiding artifacts from active expiratory muscles. 20