pneumothorax

The lung normally stays inflated because the pleural space around it is sealed and held at a slight vacuum, like a suction cup gripping the chest wall. A pneumothorax is what happens when air gets into that sealed space: the vacuum is broken, and the springy lung shrinks away from the chest wall — partly or completely collapsing.

Air can enter from outside through a chest wound, or from inside when the lung's own surface ruptures and leaks air into the pleural space. The result is sudden, sharp, one-sided chest pain and breathlessness, often with reduced breath sounds over the affected side. A small pneumothorax may cause little trouble; a large one meaningfully impairs breathing.

Management depends on size, symptoms and cause. A small, stable pneumothorax in someone comfortable may simply be observed while the leak seals and the air is reabsorbed. Larger or symptomatic ones need the air removed — by needle aspiration or by a chest tube. The dangerous variant to recognize quickly is a tension pneumothorax, where pressure keeps building.