What happens to pleural pressure during normal expiration vs forced expiration and can it collapse bronchioles?

Pleural Pressure Changes During Normal vs Forced Expiration and Bronchiolar Collapse

During normal expiration, pleural pressure remains negative (approximately -5 cmH₂O at end-expiration), while during forced expiration, pleural pressure becomes markedly positive, which can lead to dynamic compression and collapse of bronchioles through transmural pressure changes. 1

Pleural Pressure Dynamics

Normal (Passive) Expiration

• Pleural pressure remains negative throughout the respiratory cycle 2
• During quiet exhalation, the thorax reduces in size but both the chest wall and lungs remain in an extended state, maintaining negative pleural pressure 2
• The negative pressure decreases (becomes less negative) as the thorax reduces, but never reaches positive values 2
• The recoil force of the lung and the spring-out force of the rib cage maintain this negative pressure gradient 3

Forced Expiration

• Pleural pressure becomes markedly positive during forced expiratory efforts 1
• The lung pleura and chest wall pleura actively compress the pleural cavity, generating positive pressure 2
• This positive pleural pressure is sustained throughout the forced expiratory effort to ensure effective airway clearance 1
• Pleural pressure swings in humans can range from approximately -150 to +150 mmHg during maximal efforts 1

Mechanism of Bronchiolar Collapse

Dynamic Airway Compression

• Dynamic compression of airways occurs when extraluminal (pleural) pressure exceeds intraluminal airway pressure 1
• During forced expiration, the transmural pressure at the alveolus equals the recoil pressure of the lung, with the increased pleural pressure transmitted to the alveolus 1
• As expiratory flow ensues, intraluminal pressure decreases due to viscous forces while extraluminal (pleural) pressure remains elevated 1
• Airways downstream from the alveolus become subject to dynamic compression when the positive pleural pressure exceeds the intraluminal pressure 1

Progressive Airway Involvement

• Dynamic compression is normally initiated in the trachea and mainstem bronchi at high lung volumes 1
• As lung volume decreases during forced expiration, dynamic compression extends progressively to more peripheral airways and bronchioles 1
• This ensures that the entire tracheobronchial tree undergoes the compressive forces necessary for effective cough and secretion clearance 1
• High intrathoracic pressures must be sustained throughout the expiratory effort for this progressive compression to occur 1

Clinical Implications

Physiologic Purpose

• The dynamic compression and partial collapse of airways during forced expiration serves a protective function by increasing linear gas velocity 1
• When airway cross-sectional area is compressed to one-fifth of its static area, linear velocity increases fivefold, and kinetic energy increases 25-fold 1
• This enhanced kinetic energy is essential for effective mucus clearance from the airway walls 1

Pathologic Considerations

• In patients with airway disease, excessive dynamic compression can lead to premature airway closure and air trapping 1
• Small amounts of positive end-expiratory pressure (5-10 mmHg) may develop during forced expiration when the upper airway closes, which can help prevent bronchial and alveolar collapse 1
• This intrinsic positive end-expiratory pressure is similar in magnitude to that seen in chronic obstructive pulmonary disease 1

Important Caveats

• The extent of bronchiolar collapse depends on the balance between pleural pressure elevation and airway structural support 1
• In healthy individuals, airways possess sufficient structural integrity to prevent complete collapse despite positive pleural pressures 1
• Patients with weakened airway walls or reduced elastic recoil are more susceptible to excessive dynamic compression and premature airway closure 1