Intrathoracic Pressure During Positive Pressure Ventilation
During positive pressure ventilation (PPV), intrathoracic pressure is closest to atmospheric pressure at end-expiration, just before the next mechanical breath is delivered. 1
Physiologic Basis
The respiratory cycle during PPV fundamentally differs from spontaneous breathing in how intrathoracic pressure changes relative to atmospheric pressure:
During spontaneous breathing, pleural pressure (Ppl) decreases below atmospheric pressure during inspiration, creating negative intrathoracic pressure that facilitates venous return 1
In contrast, PPV reverses this pattern: positive airway pressure increases Ppl during inspiration, raising intrathoracic pressure above atmospheric levels 1
At end-expiration, after the positive pressure breath has been delivered and before the next breath begins, intrathoracic pressure returns toward its baseline resting state, which approximates atmospheric pressure 1
Pressure Dynamics Throughout the Ventilatory Cycle
The intrathoracic pressure trajectory during PPV follows a predictable pattern:
At end-expiration (baseline): Intrathoracic pressure is at its lowest point in the PPV cycle and closest to atmospheric pressure, particularly when no PEEP is applied 1
During inspiration: Positive airway pressure increases Ppl, elevating intrathoracic pressure significantly above atmospheric levels 1
At end-inspiration: Intrathoracic pressure reaches its peak, furthest from atmospheric pressure 1
During expiration: As the positive pressure is released, intrathoracic pressure gradually decreases back toward atmospheric levels 1
Impact of PEEP on Baseline Pressure
The presence of positive end-expiratory pressure (PEEP) modifies this relationship:
When PEEP is applied, end-expiratory alveolar pressure remains positive even at the end of expiration 1
This means intrathoracic pressure at end-expiration will be elevated above atmospheric pressure by an amount related to the PEEP level and chest wall compliance 1
Approximately 50% of alveolar pressure changes are transmitted to the pleural space in normal lungs, though this percentage decreases with stiffer, diseased lungs 1
Therefore, with PEEP applied, intrathoracic pressure never truly reaches atmospheric pressure during the respiratory cycle 1
Clinical Implications
Understanding when intrathoracic pressure approximates atmospheric pressure has important hemodynamic consequences:
The pressure gradient for venous return is normally only 4-8 mmHg, so small changes in intrathoracic pressure from PPV can significantly affect cardiac output 1
During PPV, the transient increase in Ppl decreases venous return to the right ventricle and eventually to the left ventricle 1
At end-expiration (when intrathoracic pressure is closest to atmospheric), the impediment to venous return is minimized compared to other phases of the ventilatory cycle 1
High levels of PEEP maintain elevated intrathoracic pressure throughout the respiratory cycle, persistently decreasing the gradient for venous return 1, 2
Auto-PEEP Considerations
Intrinsic PEEP (auto-PEEP) further complicates the pressure dynamics:
Auto-PEEP develops when expiratory time is insufficient for the lungs to decompress to their elastic equilibrium volume before the next breath 1, 3
This creates positive end-expiratory alveolar pressure even without externally applied PEEP, elevating intrathoracic pressure above atmospheric at end-expiration 1
The presence of auto-PEEP means intrathoracic pressure remains elevated throughout the cycle, never returning to true atmospheric pressure 3, 4
Practical Monitoring
Clinicians should recognize that:
Measurements of right atrial pressure and pulmonary artery occlusion pressure are affected by intrathoracic pressure changes during PPV 5
End-expiratory measurements, when intrathoracic pressure is closest to atmospheric, provide the most accurate reflection of true intravascular pressures 5
Transmission of airway pressure to the pleural space varies with lung compliance, with stiffer lungs transmitting less pressure 1, 6