What is the pathophysiology of pneumothorax, particularly in patients with underlying lung disease such as chronic obstructive pulmonary disease (COPD) or asthma?

Pathophysiology of Pneumothorax

Pneumothorax occurs when air accumulates in the pleural cavity—the potential space between the parietal and visceral pleura—disrupting the normal negative pressure (−4 mmHg at rest) that allows the lung to expand smoothly within the chest cavity. 1

Basic Mechanism

The pleural cavity normally contains only 5–10 mL of pleural fluid and maintains subatmospheric pressure that keeps the lung inflated against the chest wall. 1 When air enters this space, the negative pressure is lost, causing varying degrees of lung collapse depending on the volume of air accumulated. 1

Primary vs. Secondary Pneumothorax: Critical Pathophysiologic Differences

Primary Spontaneous Pneumothorax

• Occurs in patients without underlying lung disease, though subpleural blebs and bullae are found in up to 90% of cases during thoracoscopy and in up to 80% of CT scans. 2
• Smoking dramatically increases risk, with a lifetime risk of 12% in male smokers versus 0.1% in non-smokers. 2
• The rupture of these apical blebs allows air to enter the pleural space, typically causing minimal physiologic compromise in otherwise healthy lungs. 2

Secondary Spontaneous Pneumothorax

In patients with underlying lung disease (COPD, asthma, ILD, cystic fibrosis), the pathophysiology is far more severe and life-threatening. 1, 2, 3

• COPD patients have pre-existing compromised respiratory reserve, making even small pneumothoraces potentially catastrophic. 3
• The diseased lung parenchyma has reduced elastic recoil and increased air trapping, which perpetuates air leaks once they occur. 3
• Clinical symptoms are generally more severe than in primary pneumothorax due to baseline hypoxemia and limited compensatory capacity. 2
• Bullae rupture in emphysematous lungs creates larger, more persistent air leaks that resist spontaneous closure. 3

Tension Pneumothorax: The Life-Threatening Variant

A one-way valve mechanism can develop, allowing air to enter the pleural space during inspiration but preventing escape during expiration. 1 This creates a progressive, life-threatening cascade:

1. Intrathoracic pressure increases progressively with each breath. 1
2. The mediastinum shifts toward the opposite side, compressing the contralateral lung. 1
3. Venous return to the heart is reduced due to compression of the great veins. 1
4. Hemodynamic instability and cardiovascular collapse ensue if untreated. 1

Critical Clinical Point

The development of tension is not dependent on pneumothorax size—even small pneumothoraces can become tension pneumothoraces, particularly in mechanically ventilated patients where positive pressure continuously forces air into the pleural space. 1, 4

Special Pathophysiologic Considerations

Mechanical Ventilation

Positive pressure ventilation creates large pleural air leaks that exceed the capacity of smaller drainage systems and dramatically increases the risk of tension physiology. 4 The continuous positive pressure acts as a pump, forcing air through any pleural defect and preventing spontaneous sealing. 4

Iatrogenic Pneumothorax

Procedures like transthoracic needle aspiration (24% of cases), subclavian puncture (22%), and thoracocentesis (22%) create direct communication between atmosphere and pleural space. 1 COPD and deeper lesion depth are the two primary risk factors for post-procedural pneumothorax. 1

Underlying Disease Impact

• Cystic fibrosis patients develop more severe and potentially life-threatening disease due to chronic infection, inflammation, and structural lung damage. 2
• HIV/AIDS patients, especially with Pneumocystis pneumonia, have friable lung tissue prone to rupture and poor healing. 2
• Pulmonary tuberculosis causes cavitary disease and pleural adhesions that alter normal pleural mechanics. 2

Compensatory Mechanisms and Failure

In healthy individuals, small pneumothoraces may be asymptomatic as the contralateral lung compensates. 2 However, in patients with COPD or asthma, baseline hyperinflation and air trapping eliminate this reserve, making even small air collections clinically significant. 1, 3

The inability of diseased lungs to re-expand after air removal (due to loss of elastic recoil) explains why secondary pneumothorax requires more aggressive management with chest tube drainage rather than simple aspiration. 1, 3

Common Pitfalls in Understanding Pathophysiology

• Physical activity has no apparent relationship to pneumothorax onset—the rupture of blebs occurs spontaneously, often at rest. 2
• Clinical manifestations are not reliable indicators of pneumothorax size, particularly in tension scenarios where hemodynamic compromise can occur with radiographically small collections. 2
• Expiratory chest radiographs do not improve diagnostic sensitivity and are not routinely recommended. 2