Alveolar Pressure Dynamics: Normal vs Dynamic Compression Expiration
During normal passive expiration, alveolar pressure (Palv) drops smoothly to zero as air flows out driven solely by elastic recoil, whereas during dynamic compression expiration (forced expiration), alveolar pressure remains elevated while intraluminal airway pressure drops below pleural pressure due to viscous resistance, creating transmural pressure gradients that compress airways and paradoxically maintain higher driving pressures despite airway narrowing. 1
Normal Expiration Mechanics
During relaxed, passive expiration:
- Alveolar pressure decreases exponentially following the respiratory system's time constant (τrs), which represents the time for lung volume to passively decrease by 63% 1, 2
- Pressure drops smoothly to zero at end-expiration as the lung returns to its elastic equilibrium volume (EEV) 1, 3
- The driving force is purely elastic recoil of the lung tissue, with no active muscle contraction 2
- Transmural pressure at the alveolus equals lung recoil pressure, as pleural pressure returns to its baseline negative value 1
- Intraluminal airway pressure remains higher than extraluminal (pleural) pressure throughout the airways, preventing collapse 1
Dynamic Compression Expiration Mechanics
During forced expiration (cough, Valsalva, forced vital capacity):
- Pleural pressure becomes markedly positive (up to 300 mmHg during cough) due to expiratory muscle contraction 1, 4
- Alveolar pressure remains elevated because the increased pleural pressure is transmitted directly to the alveolus, maintaining high driving pressure 1
- Intraluminal airway pressure drops progressively as air flows from alveoli toward the mouth due to viscous resistance and friction losses 1
- Critical pressure point develops where intraluminal pressure falls below extraluminal (pleural) pressure, creating negative transmural pressure 1
- Airways downstream from this point undergo dynamic compression, with the compression point moving progressively toward peripheral airways as lung volume decreases 1, 4
The Pressure Paradox Explained
The fundamental difference lies in transmural pressure dynamics:
- In normal expiration: Transmural pressure = Intraluminal pressure - Pleural pressure remains positive (airways stay open) 1
- In dynamic compression: Transmural pressure = Intraluminal pressure - Pleural pressure becomes negative (airways compress) 1
The key mechanism is that viscous resistance causes intraluminal pressure to drop below the elevated pleural pressure as flow continues, creating the conditions for airway compression 1. This is described by the American College of Chest Physicians: "as expiratory flow ensues, the intraluminal pressure becomes lower, due to the viscous forces, while extraluminal pressure (pleural pressure) remains elevated" 1.
Functional Consequences
Dynamic compression serves important physiological purposes:
- Increases linear gas velocity by reducing airway cross-sectional area, potentially increasing velocity fivefold when tracheal area compresses to one-fifth normal 1
- Kinetic energy increases 25-fold with this degree of compression, enhancing mucus clearance 1
- Ensures progressive airway clearance as the compression point moves peripherally with decreasing lung volume, allowing the "whole length of the tracheobronchial tree to be coughed" 1
Clinical Pitfalls
Common misunderstanding: Clinicians may assume that airway compression during forced expiration reduces driving pressure. In reality, alveolar pressure remains high because pleural pressure elevation is transmitted to alveoli, maintaining the pressure gradient for flow despite downstream airway narrowing 1.
Pathological dynamic compression: In patients with weakened airway walls (emphysema, COPD) or excessive pleural pressure, premature airway closure can occur, leading to air trapping and intrinsic PEEP (auto-PEEP) 5, 4, 3. This occurs when airways collapse before complete lung emptying, trapping air at positive pressure 1, 3.
Measurement considerations: Body plethysmography can distinguish between high alveolar pressure with airway compression versus low alveolar pressure with poor effort, as the pattern of pressure change differs fundamentally 6, 7.