Why does end expiration in mechanically ventilated patients undergoing positive pressure ventilation (PPV) correspond to the highest intrathoracic pressure point?

Understanding Intrathoracic Pressure Dynamics in Positive Pressure Ventilation

The Fundamental Misconception

End-expiration during positive pressure ventilation does NOT correspond to the highest intrathoracic pressure point—this is a common misunderstanding that confuses positive pressure ventilation with the presence of PEEP and auto-PEEP. In fact, the highest intrathoracic pressure occurs at end-inspiration, not end-expiration, during the normal respiratory cycle with positive pressure ventilation.

Pressure Dynamics During the Respiratory Cycle

Normal Positive Pressure Ventilation Pattern

• During inspiration in positive pressure ventilation, the ventilator delivers positive pressure that inflates the lungs, progressively increasing intrathoracic pressure throughout the inspiratory phase 1
• Peak intrathoracic pressure occurs at end-inspiration when the lungs are maximally inflated and airway pressure reaches its highest point 1
• During expiration, intrathoracic pressure decreases as gas flows out of the lungs and the respiratory system returns toward its resting elastic equilibrium volume 2
• At end-expiration in a passively ventilated patient without applied PEEP, intrathoracic pressure returns to its lowest point in the respiratory cycle 1

The Exception: PEEP and Auto-PEEP

The confusion arises when considering end-expiratory pressure specifically:

• Applied PEEP maintains positive alveolar pressure at end-expiration, preventing complete decompression of the lungs and keeping end-expiratory intrathoracic pressure elevated above atmospheric pressure 3, 4
• Intrinsic PEEP (auto-PEEP) occurs when end-expiratory alveolar pressure remains positive because insufficient expiratory time prevents complete lung decompression before the next breath 2
• In patients with auto-PEEP, end-expiratory pressure can be substantial (10-15 cmH₂O in severe cases), creating an elevated baseline intrathoracic pressure at end-expiration 5

However, even with PEEP or auto-PEEP present, end-inspiratory pressure still exceeds end-expiratory pressure because inspiration adds additional positive pressure on top of the baseline PEEP level 1.

Clinical Implications of Elevated End-Expiratory Pressure

Hemodynamic Effects

• Both applied PEEP and auto-PEEP increase mean intrathoracic pressure, which decreases cardiac output by reducing the pressure gradient for venous return to the right ventricle 2, 3, 4
• The elevated intrathoracic pressure at end-expiration (when PEEP is present) reduces venous return throughout the cardiac cycle, but this effect is most pronounced when combined with the even higher pressures during inspiration 3, 6

Work of Breathing Implications

• Auto-PEEP creates an inspiratory threshold load that patients must overcome before triggering the ventilator, requiring them to generate sufficient negative pleural pressure to counterbalance the positive end-expiratory alveolar pressure before inspiratory flow can begin 2, 5
• This threshold can require substantial patient effort, particularly in conditions like COPD or asthma where auto-PEEP levels are high 5

Monitoring and Detection

Identifying Elevated End-Expiratory Pressure

• The presence of positive alveolar pressure at end-expiration is indicated by a phase lag between the onset of inspiratory pressure decay and when flow reaches zero 2
• This positive end-expiratory pressure can result from either elastic recoil pressure generated by hyperinflation (auto-PEEP) or from active expiratory muscle contraction 2
• The end-expiratory occlusion technique is the gold standard for measuring intrinsic PEEP, requiring the patient to be passive to avoid artifacts 5

Clinical Monitoring Recommendations

• Monitor pressure-time and flow-time scalars as routine parameters for all ventilated patients to detect auto-PEEP development 5
• Measure peak inspiratory pressure, plateau pressure, mean airway pressure, and PEEP near the Y-piece 5
• Patients with severe asthma, COPD, or receiving high minute ventilation are at highest risk for developing significant auto-PEEP 5

Management Strategies

Minimizing Auto-PEEP

• Decrease respiratory rate to allow more expiratory time, preventing incomplete exhalation 5
• Use shorter inspiratory times with higher flow rates (80-100 L/min in adults) to maximize expiratory time 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 to reduce the volume that must be exhaled 5

Optimizing Trigger Settings

• In patients with auto-PEEP, flow triggering should be used instead of pressure triggering because 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, detecting changes in bias flow rather than requiring pressure changes 5

Application of External PEEP

• Applying external PEEP to offset intrinsic PEEP (typically 5-10 cmH₂O) reduces the effort required for triggering by counterbalancing the auto-PEEP threshold load 5, 4
• Never set external PEEP levels in excess of intrinsic PEEP, as this can worsen hyperinflation and cause hemodynamic compromise 5

Common Pitfalls to Avoid

• Do not confuse end-expiratory pressure (PEEP) with the timing of peak intrathoracic pressure in the respiratory cycle—peak pressure occurs at end-inspiration, not end-expiration 1
• Recognize that while PEEP elevates baseline intrathoracic pressure at end-expiration, this is still lower than the peak pressure reached during inspiration 1
• When severe hypotension occurs in a patient with suspected auto-PEEP, disconnect from the ventilator immediately and press on the chest wall to assist exhalation 5
• Monitor for auto-triggering when using sensitive flow triggers, as cardiogenic oscillations or circuit leaks can cause false triggering 5