What is Auto-PEEP (Auto-Positive End-Expiratory Pressure) and how to check for it in a patient with Acute Respiratory Distress Syndrome (ARDS)?

Auto-PEEP in ARDS: Definition and Detection Methods

What is Auto-PEEP?

Auto-PEEP (intrinsic PEEP or PEEPi) is positive alveolar pressure that persists at end-expiration when the lungs fail to deflate completely to their elastic equilibrium volume before the next breath begins. 1 This occurs when expiratory time is insufficient for complete lung emptying, resulting in progressive air trapping and breath stacking. 1

Mechanism of Development

• Incomplete expiration occurs when the time required to decompress the lungs to elastic equilibrium volume exceeds the available expiratory time before the next inspiration. 1
• High airway resistance (as in COPD or asthma) slows expiratory flow and increases auto-PEEP risk. 1
• In ARDS patients specifically, auto-PEEP can develop from ventilatory settings (high respiratory rate, short expiratory time, high minute ventilation) rather than primarily from airway obstruction. 2

Clinical Significance in ARDS

• Auto-PEEP creates an inspiratory threshold load that patients must overcome before triggering the ventilator, significantly increasing work of breathing. 1
• Hemodynamic instability results from decreased venous return and reduced cardiac output due to increased intrathoracic pressure. 1
• Patient-ventilator asynchrony and ineffective triggering efforts are common complications. 1
• Barotrauma from hyperinflation is a potential life-threatening complication. 1

How to Check for Auto-PEEP in ARDS

Primary Method: End-Expiratory Occlusion Technique

The end-expiratory occlusion technique is the gold standard for measuring auto-PEEP and requires the patient to be passive (sedated and paralyzed) to avoid artifacts from expiratory muscle activity. 1

Procedure:

• Occlude the expiratory port at end-exhalation in a non-assisting, passively ventilated patient. 3
• Observe the rise in airway pressure displayed on the ventilator—this pressure represents the auto-PEEP value. 4, 5
• Perform multiple measurements (typically 5) and use the average value for accuracy. 5
• This measurement is only accurate during controlled mechanical ventilation, not during assisted modes (PSV, BIPAP). 5

Alternative Methods When Sedation/Paralysis Not Feasible

• Flow-time scalar monitoring: Observe if expiratory flow returns to zero before the next breath—if flow has not reached zero, auto-PEEP is present. 4
• Simultaneous recordings of airflow, airway pressure, and esophageal pressure can estimate auto-PEEP in spontaneously breathing patients. 4
• Electrical impedance tomography can detect changes in end-expiratory thoracic gas volume reflecting auto-PEEP development. 1
• Respiratory inductance plethysmography (RIP) measuring changes in end-expiratory thoracic gas volume provides a noninvasive alternative in both controlled and spontaneously ventilating patients. 6

Routine Monitoring Parameters

Monitor pressure-time and flow-time scalars as standard practice for all ventilated patients. 1

• Measure peak inspiratory pressure, plateau pressure, mean airway pressure, and PEEP near the Y-piece. 1
• Monitor for sudden hypotension or cardiovascular collapse in mechanically ventilated patients, which strongly suggests significant auto-PEEP. 1
• Use the DOPE mnemonic plus auto-PEEP (Displacement, Obstruction, Pneumothorax, Equipment failure) when a ventilated patient deteriorates. 1

Clinical Context in ARDS

In sedated, paralyzed ARDS patients without known obstructive disease receiving lung-protective ventilation, auto-PEEP is typically negligible (median 1.1 cmH₂O). 2 However, this study found:

• 40% of ARDS patients had detectable auto-PEEP at 5 cmH₂O external PEEP. 2
• Higher respiratory rates (18 vs 15 bpm) were associated with auto-PEEP presence. 2
• Auto-PEEP decreased significantly when external PEEP increased from 5 to 15 cmH₂O (1.1 to 0.6 cmH₂O). 2
• The amount of auto-PEEP did not correlate with airway resistance, respiratory system compliance, or lung recruitability in ARDS. 2

Critical Action Steps When Auto-PEEP Detected

If auto-PEEP results in significant hypotension, immediately disconnect the patient from the ventilator circuit and press on the chest wall to assist exhalation. 1

• Do not delay intervention—severe hypotension requires immediate action. 1
• Assess for tension pneumothorax, especially if ventilation is difficult. 1
• Document the measured auto-PEEP value and correlate with clinical signs. 1

Management Strategies to Reduce Auto-PEEP

• Decrease respiratory rate to allow more expiratory time. 1
• Use shorter inspiratory times with higher flow rates (80-100 L/min in adults). 1
• Aim for longer expiratory times (I:E ratio of 1:4 or 1:5). 1
• Consider decreasing tidal volumes to 6-8 mL/kg predicted body weight. 1
• Apply external PEEP cautiously (typically 5 cmH₂O or less) to counterbalance intrinsic PEEP and reduce triggering effort, but never exceed the measured auto-PEEP level. 1, 4

Important Pitfall

Never set external PEEP levels in excess of intrinsic PEEP, as this worsens hyperinflation and can cause hemodynamic compromise. 1 The goal is to counterbalance auto-PEEP to reduce work of breathing, not to exceed it.