What is the mechanism of a pneumothorax (collapsed lung)?

Mechanism of Pneumothorax (Collapsed Lung)

A pneumothorax occurs when air enters the pleural space between the lung and chest wall, causing the lung to collapse due to the loss of negative intrapleural pressure that normally keeps the lung expanded. 1

Primary Mechanisms

Spontaneous Pneumothorax

• Primary Spontaneous Pneumothorax

  • Occurs in otherwise healthy individuals without apparent lung disease
  • Pathophysiology:
    • Rupture of subpleural blebs and bullae (found in up to 90% of cases at thoracoscopy and 80% on CT scans) 1
    • These blebs/bullae create a one-way valve system that allows air to enter the pleural space but not exit
  • Risk factors:
    • Smoking significantly increases risk (lifetime risk of 12% in smoking men vs 0.1% in non-smoking men) 1
    • Tall, thin body habitus (commonly seen in young males)

• Secondary Spontaneous Pneumothorax

  • Occurs in patients with underlying lung disease (COPD, asthma, tuberculosis, etc.)
  • Symptoms are typically more severe than in primary pneumothorax 1

Traumatic Pneumothorax

• Results from direct chest trauma or iatrogenic causes
• Common iatrogenic causes include:
  • Transthoracic needle aspiration (24%)
  • Subclavian vessel puncture (22%)
  • Thoracocentesis (22%)
  • Pleural biopsy (8%)
  • Mechanical ventilation (7%) 1

Pathophysiological Process

1. Initial Air Entry:

   • Air enters the pleural space through a breach in the visceral pleura (lung surface) or parietal pleura (chest wall)
   • In spontaneous pneumothorax, this typically occurs from rupture of subpleural blebs/bullae 1, 2

2. Loss of Negative Pressure:

   • Normal pleural space maintains negative pressure (-3 to -5 cm H₂O) relative to atmospheric pressure
   • When air enters, this negative pressure is lost, disrupting the normal adhesion between lung and chest wall

3. Lung Collapse:

   • The elastic recoil of the lung causes it to collapse away from the chest wall
   • Degree of collapse depends on the amount of air and whether a one-way valve effect develops

4. Tension Pneumothorax Development:

   • In some cases, a one-way valve mechanism develops where air can enter during inspiration but cannot exit during expiration
   • This causes intrapleural pressure to exceed atmospheric pressure throughout the respiratory cycle
   • Results in progressive lung collapse, mediastinal shift, and cardiovascular compromise 1
   • Presents with rapid deterioration, respiratory distress, cyanosis, sweating, and tachycardia
   • Can occur regardless of initial pneumothorax size 1

Clinical Implications

• The size of pneumothorax does not reliably correlate with clinical symptoms 1
• Secondary pneumothoraces typically cause more severe symptoms than primary ones, with dyspnea disproportionate to pneumothorax size 1
• Tension pneumothorax is a medical emergency requiring immediate decompression to prevent cardiopulmonary collapse 1

Important Considerations

• Smoking cessation is crucial for preventing recurrence in patients with pneumothorax 1
• There is no apparent relationship between physical activity and the onset of pneumothorax 1
• Modern diagnostic approaches include ultrasound, which has higher sensitivity than chest X-ray for detecting pneumothorax 3
• Management has evolved toward more conservative approaches, including observation for small pneumothoraces and use of smaller drainage catheters 3, 4