PEEP Concepts for DrNB Theory Exam Preparation
Core Definitions
Extrinsic PEEP (Applied PEEP) is the positive end-expiratory pressure deliberately set on the mechanical ventilator to maintain positive alveolar pressure at end-expiration, preventing alveolar collapse and improving functional residual capacity 1, 2. This is the PEEP value you dial into the ventilator settings.
Intrinsic PEEP (PEEPi), also termed Auto-PEEP or Occult PEEP, is the end-expiratory elastic recoil pressure that develops when end-expiratory alveolar pressure remains positive even without external PEEP application 3. This occurs when the time required to decompress the lungs to elastic equilibrium volume is shorter than the expiratory time available before the next inspiration 3, 4.
Auto-PEEP is synonymous with intrinsic PEEP—these terms are interchangeable and refer to the same phenomenon of unintended positive pressure at end-expiration 3, 5.
Pathophysiology of Intrinsic PEEP Development
The fundamental mechanism involves incomplete lung emptying before the next breath begins 4. This air trapping occurs due to:
- High airway resistance (COPD, asthma) that slows expiratory flow 4
- Insufficient expiratory time relative to the time constant of the respiratory system 3, 4
- Inappropriate ventilator settings including high respiratory rates, high tidal volumes, or short expiratory times 3
PEEPi levels can be substantial, ranging from 10-15 cm H₂O in severe COPD exacerbations or asthma 3, 4. In obstructive lung disease, median values of 6.4 cm H₂O (range 5.0-9.6) are common, compared to 2.3 cm H₂O (range 0.8-3.0) in restrictive disease 6.
Critical Clinical Implications
Hemodynamic Effects
Both extrinsic PEEP and intrinsic PEEP decrease cardiac output by increasing intrathoracic pressure, which reduces the pressure gradient for venous return to the right ventricle 3, 1, 2. Auto-PEEP creates a gradual increase in end-expiratory volume and pressure that is transmitted to the great veins, depressing venous return and cardiac output 3.
High PEEP levels can increase pulmonary vascular resistance by creating West zone 2 conditions where alveolar pressure exceeds pulmonary venous pressure, further afterloading the right ventricle 2.
Work of Breathing and Triggering
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 3, 4. The patient must generate sufficient negative pressure to overcome the auto-PEEP pressure before any breath can be initiated 4.
The total effort needed to trigger the ventilator does not correspond to the small negative pressure (1-2 cm H₂O) usually set 3. High levels of PEEPi significantly increase the magnitude of patient effort to trigger the ventilator 3.
Ineffective triggering (wasted efforts) occurs when the achieved decrement in pleural pressure is smaller than the level of PEEPi, resulting in failure to trigger the ventilator despite patient effort 3, 4.
Barotrauma and Overdistension
Auto-PEEP predisposes to barotrauma from hyperinflation 3, 5. This is a critical complication that can lead to pneumothorax in mechanically ventilated patients with significant air trapping.
Measurement Techniques for Exam
End-expiratory occlusion technique is the gold standard: occlude the expiratory port at end-expiration and measure the plateau pressure that develops 3, 7, 6. This requires the patient to be passive (not actively breathing) to avoid artifacts from expiratory muscle activity 3, 1.
Alternative method for spontaneously breathing patients: The difference between maximum inspiratory pressure (MIP) and maximum esophageal pressure (Ppl max) obtained with a Mueller maneuver from end-expiratory lung volume accurately measures static PEEPi 7.
Flow-volume curve inspection can suggest the presence of dynamic hyperinflation and expiratory flow limitation by examining the shape of the expiratory limb 8.
Management Strategies
Ventilator Adjustments to Minimize Auto-PEEP
Decrease respiratory rate to allow more expiratory time 3, 4. This is the most direct intervention to prevent incomplete exhalation.
Use shorter inspiratory times with higher flow rates (adult inspiratory flow rate 80-100 L/min) 3, 4. This maximizes the I:E ratio in favor of expiration.
Aim for longer expiratory times with I:E ratio of 1:4 or 1:5 3, 4. This allows adequate time for lung decompression.
Consider decreasing tidal volumes to 6-8 mL/kg predicted body weight 3, 4. Lower volumes require less time to exhale completely.
Application of External PEEP to Counterbalance Auto-PEEP
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 3, 1, 8.
The mechanism works only in the presence of expiratory flow limitation: External PEEP reduces the pressure gradient the patient must overcome to initiate flow, without necessarily increasing lung volume further 8.
Critical rule: Never set external PEEP levels in excess of intrinsic PEEP, as this worsens hyperinflation and potentially causes hemodynamic compromise 4. Typically, external PEEP of 5 cm H₂O or less is used, even when auto-PEEP is higher 4.
The pressure difference between PEEPi and external PEEP decreases with increasing external PEEP, but external PEEP may not eliminate PEEPi completely 6. In some instances, PEEPi may paradoxically increase with increasing external PEEP because the measured PEEPi represents a mean value of different regional PEEPi values 6.
Triggering Mode Selection
Flow triggering should be used instead of pressure triggering in patients with auto-PEEP, as recommended by the British Thoracic Society 4. 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 4.
Pressure triggers worsen patient-ventilator asynchrony and increase the work of breathing required to initiate a breath 4.
Key Exam Pitfalls to Avoid
Do not confuse the presence of positive end-expiratory pressure with its etiology: A phase lag between the onset of inspiratory pressure decay and when flow reaches zero indicates positive alveolar pressure, but this can result from either elastic recoil pressure (true auto-PEEP) or expiratory muscle activity 3.
Patient-ventilator asynchrony leads to inaccurate PEEPi measurements: Ensuring the patient is not actively breathing during measurement is crucial 1. Expiratory muscle activity can falsely elevate measured values 3.
Hyperventilation exacerbates auto-PEEP: Sustained hypocapnia from hyperventilation does not allow sufficient time for complete exhalation, worsening gas trapping 3. This is particularly deleterious in all low-flow states including CPR and hypovolemia 3.
Monitor for auto-triggering when using sensitive flow triggers: Cardiogenic oscillations or circuit leaks can cause false triggering 4.
In cardiac patients, balance respiratory benefits against hemodynamic effects: When mechanical ventilation is required in patients with cardiovascular compromise, PEEP should be applied with caution as positive intrathoracic pressure may reduce venous return and worsen right ventricular failure 1. Lower PEEP levels may be preferable in patients with right ventricular dysfunction or pulmonary hypertension 1.