How Extrinsic PEEP Counteracts Auto-PEEP
Extrinsic PEEP counteracts auto-PEEP by reducing the inspiratory threshold load that patients must overcome to trigger the ventilator, effectively "counterbalancing" the intrinsic positive pressure without necessarily increasing total end-expiratory pressure when applied judiciously. 1
The Fundamental Mechanism
Auto-PEEP (intrinsic PEEP or PEEPi) creates an inspiratory pressure threshold load during both spontaneous breathing and patient-triggered mechanical ventilation modes 1. Before a patient can trigger the ventilator or initiate inspiratory flow, their inspiratory muscles must fully counterbalance this PEEPi by generating sufficient negative pleural pressure 1. This means if a patient has 10 cm H₂O of auto-PEEP, they must generate at least 10 cm H₂O of negative pressure before any breath can begin—far exceeding the typical 1-2 cm H₂O trigger sensitivity set on the ventilator 1.
Application of low levels of external PEEP (typically 5-10 cm H₂O) significantly improves patient-ventilator interaction and reduces the magnitude of inspiratory effort during assisted ventilation by counterbalancing PEEPi at least in part 1, 2. The key insight is that external PEEP narrows the pressure gradient between end-expiratory alveolar pressure and central airway pressure 3.
Why External PEEP Works Without Worsening Hyperinflation
The critical principle is that when external PEEP is applied at levels below the existing auto-PEEP, it does not substantially increase total PEEP or worsen hyperinflation 3, 4. Research demonstrates that in patients with chronic airflow obstruction, PEEP levels of 5 and 10 cm H₂O improved expiratory resistance without substantially increasing peak static pressure 3.
The relationship is inverse: the change in total PEEP caused by applying external PEEP correlates inversely with the preexisting level of auto-PEEP (r=-0.84) 4. This means that when significant auto-PEEP exists, adding external PEEP up to approximately 80-85% of the measured auto-PEEP value increases total PEEP minimally while dramatically reducing the work required to trigger breaths 3, 4.
Clinical Application Algorithm
Step 1: Measure auto-PEEP using the end-expiratory airway occlusion technique, which is the gold standard and requires the patient to be passive 1, 5. Most modern ventilators have an end-expiratory hold button for this purpose 1.
Step 2: Apply external PEEP conservatively at approximately 50-85% of the measured auto-PEEP value 5, 6. The American Thoracic Society guidelines support using external PEEP of 5 cm H₂O or less even when auto-PEEP is higher, to avoid exacerbating hyperinflation and potential hemodynamic compromise 5.
Step 3: Never exceed the measured auto-PEEP level with external PEEP, as setting PEEP greater than intrinsic PEEP can be harmful and worsen hyperinflation 5.
Step 4: Monitor for improvement in triggering sensitivity, reduced ventilatory drive, and decreased mechanical work of breathing 3. At low PEEP levels (50% of auto-PEEP), expect improved PaO₂ and slightly decreased PaCO₂ due to increased mean VA/Q ratio 6.
Physiological Benefits Beyond Triggering
External PEEP provides multiple benefits in the presence of auto-PEEP:
- Improved lung mechanics: PEEP improves expiratory resistance while inspiratory resistance remains unchanged 3
- Enhanced gas exchange: Application of PEEP at 50% of auto-PEEP increases PaO₂ (from 103±25 to 112±30 mmHg) and decreases PaCO₂ (from 42±4 to 40±3 mmHg) by improving ventilation-perfusion matching 6
- Reduced work of breathing: The mechanical work of breathing during machine-assisted cycles diminishes significantly 3
- Improved ventilator triggering sensitivity: The effective triggering sensitivity improves, reducing wasted efforts and patient-ventilator asynchrony 1, 3
Critical Pitfalls to Avoid
Do not apply external PEEP blindly without measuring auto-PEEP first 5, 4. The total PEEP delivered to the patient should be measured whenever external PEEP is applied 4.
Avoid excessive external PEEP as both extrinsic PEEP and intrinsic PEEP can decrease cardiac output by increasing intrathoracic pressure and reducing venous return 1, 2. This is particularly important in patients with right ventricular dysfunction or pulmonary hypertension 2.
Use flow triggering instead of pressure triggering in patients with auto-PEEP, as pressure triggers worsen patient-ventilator asynchrony and increase work of breathing 5. Flow triggering systems are more sensitive to patient effort in the presence of auto-PEEP 5.
Monitor for auto-triggering when using sensitive flow triggers, as cardiogenic oscillations or circuit leaks can cause false triggering 5.
Complementary Strategies
While external PEEP counterbalances auto-PEEP, also address the underlying causes:
- Decrease respiratory rate to allow more expiratory time 5
- Use shorter inspiratory times with higher flow rates (80-100 L/min in adults) 5
- Aim for longer expiratory times with I:E ratios of 1:4 or 1:5 5
- Consider decreasing tidal volumes to 6-8 mL/kg 5
These ventilator adjustments reduce auto-PEEP generation while external PEEP mitigates its effects on triggering and work of breathing.