Gradient of Opening Airway Pressure to Alveolar Pressure in Normal Adults
In healthy adults without pre-existing respiratory conditions, approximately 50% of alveolar pressure is transmitted to pleural pressure, creating a transpulmonary pressure gradient of roughly 4-8 mmHg under normal breathing conditions. 1
Understanding the Pressure Gradient
The key concept here is transpulmonary pressure (TP), which represents the difference between alveolar pressure and pleural pressure (Ppl). 1 This gradient determines lung volume in conjunction with lung compliance and reflects the actual distending pressure across the lung parenchyma. 1
Normal Transmission Characteristics
In normal subjects, approximately 50% of changes in alveolar pressure are transmitted to pleural pressure. 1 This means if alveolar pressure increases by 10 cmH₂O, pleural pressure typically increases by about 5 cmH₂O, maintaining a transpulmonary pressure gradient.
The normal pressure gradient for venous return from the systemic circulation to the right atrium is only 4-8 mmHg, which provides context for understanding how small changes in intrathoracic pressures can significantly affect hemodynamics. 1
Clinical Context and Variations
Diseased lungs behave differently—stiffer lungs transmit less pressure to the pleural space. 1 This is critical because:
In ARDS patients, lung compliance is dramatically reduced (often ≤20 mL/cmH₂O, less than one-fourth of normal), and the stiff lungs transmit less alveolar pressure to surrounding structures. 1, 2
In COPD patients, dynamic hyperinflation and increased airway resistance alter the normal pressure relationships, with airway resistance reaching 33-110 cmH₂O/L/s at low lung volumes during forced maneuvers compared to low and constant resistance during normal inspiration. 3
Mechanical Ventilation Implications
When positive airway pressure is applied during mechanical ventilation, the 50% transmission rule in normal lungs means that increasing mean airway pressure simultaneously affects both lung recruitment and cardiovascular function. 1
Transpulmonary pressure determines whether alveolar units remain open or collapse, with West zone conditions developing when pleural pressure exceeds pulmonary venous pressure (zone 2) or pulmonary arterial pressure (zone 1). 1
Normal respiratory system compliance ranges from 50-100 mL/cmH₂O, which reflects the combined elastic properties of both lung and chest wall. 2
Critical Pitfall
Do not confuse airway opening pressure with alveolar pressure—these are distinct measurements. Airway opening pressure includes resistance from the conducting airways, while alveolar pressure represents the actual pressure at the gas exchange surface. 3 The gradient between these two pressures reflects airway resistance, which remains low and constant during normal inspiration in healthy subjects. 3