Transpulmonary Pressure vs Transrespiratory Pressure
No, transrespiratory pressure is not the same as transpulmonary pressure—they represent fundamentally different pressure gradients across different anatomical structures. Transpulmonary pressure is the pressure difference across the lung parenchyma (alveolar pressure minus pleural pressure), while transrespiratory pressure is the pressure difference across the entire respiratory system (alveolar pressure minus body surface pressure). 1
Key Definitions and Distinctions
Transpulmonary pressure represents the distending pressure of the lung itself:
- Calculated as alveolar pressure minus pleural pressure (Palv - Ppl) 1, 2
- Reflects the elastic recoil pressure of the lung at very slow breathing frequencies 1
- When divided by tidal volume, yields lung elastance; when divided by flow, yields lung resistance 1
- Essential for understanding lung-specific mechanics independent of chest wall properties 2, 3
Transrespiratory pressure encompasses the entire respiratory system:
- Represents pressure across both lung and chest wall combined 1
- Measured as the difference between alveolar and atmospheric pressure 4
- Reflects the mechanical properties of the respiratory system as a whole, not just the lung 4
Clinical Significance of the Distinction
The differentiation between these pressures is critical in mechanically ventilated patients because:
- Airway pressure alone (what ventilators display) represents transrespiratory pressure and cannot distinguish between pressure spent distending the lung versus the chest wall 2, 3
- In patients with stiff chest walls (obesity, abdominal distension, chest wall edema), a high airway pressure may not indicate high transpulmonary pressure—much of the pressure may be dissipated across the chest wall 2, 3
- Conversely, in patients with compliant chest walls, seemingly acceptable airway pressures may mask dangerously high transpulmonary pressures that risk lung overdistension 2, 3
Measurement Considerations
Esophageal pressure monitoring allows clinicians to estimate pleural pressure and calculate transpulmonary pressure:
- Esophageal pressure serves as a surrogate for pleural pressure 2, 5
- This enables calculation of transpulmonary pressure: Palv - Pes 2, 5
- A 2008 randomized trial demonstrated that ventilator strategies guided by esophageal pressure measurements (to estimate transpulmonary pressure) significantly improved oxygenation and respiratory system compliance compared to standard care in acute lung injury/ARDS 5
Practical Implications for Mechanical Ventilation
Monitoring transpulmonary pressure specifically (not just transrespiratory/airway pressure) helps:
- Prevent lung overdistension by ensuring end-inspiratory transpulmonary pressure remains safe 1, 2, 3
- Guide PEEP titration to maintain positive end-expiratory transpulmonary pressure and prevent alveolar collapse 1, 5, 3
- Avoid right ventricular failure by preventing excessive transpulmonary pressure that increases RV afterload 1
- Account for chest wall mechanics, particularly important in obesity, abdominal compartment syndrome, or chest wall edema 2, 3
Common Pitfalls
Do not assume that acceptable airway pressures (transrespiratory pressure) guarantee safe transpulmonary pressures:
- Plateau pressure ≤30 cmH₂O is a standard target 6, 7, but this represents transrespiratory pressure
- In patients with increased chest wall elastance, higher plateau pressures (≤29-32 cmH₂O) may be acceptable because transpulmonary pressure remains lower 6, 7
- Without esophageal pressure monitoring, clinicians cannot definitively know the transpulmonary pressure 2, 3
The 2017 Paediatric Mechanical Ventilation Consensus Conference recommends considering transpulmonary pressure measurement as part of comprehensive monitoring, though it acknowledges this requires appropriate training 6