Pathophysiology
Clinical meaning
Pneumothorax occurs when air enters the pleural space, disrupting the negative intrapleural pressure (-3 to -5 cmH2O) that maintains lung expansion. Primary spontaneous pneumothorax (PSP) occurs without underlying lung disease, typically in tall, thin males aged 15-35 due to rupture of subpleural apical blebs. Secondary spontaneous pneumothorax (SSP) occurs in patients with underlying lung disease (COPD, CF, Pneumocystis pneumonia, Marfan syndrome). Tension pneumothorax occurs when a one-way valve mechanism allows air entry during inspiration but prevents exit during expiration, causing progressive mediastinal shift, IVC compression, and hemodynamic collapse. The clinician must recognize tension pneumothorax as a clinical diagnosis requiring immediate needle decompression before imaging. Management of non-tension pneumothorax depends on size (small < 2 cm apex-to-cupola vs large >= 2 cm) and symptoms.