POIS is characterized by which condition?

• A. Fluid in the lungs
• B. Excessive pressure inside the lungs, failure of expanding gas to escape the lungs
• C. Bacterial infection
• D. Collapse of a lung

Answer: (B)

Pulmonary Over-Expansion Syndrome happens when gas inside the lungs expands during ascent but cannot escape, causing excessive pressure inside the lungs and leading to barotrauma. That exact mechanism—excessive pressure inside the lungs, failure of expanding gas to escape the lungs—best captures what POIS is about. It’s different from fluid in the lungs (pulmonary edema) or a bacterial infection (pneumonia), and while barotrauma can produce a pneumothorax (collapse of the lung) as a possible outcome, the defining feature of POIS is the trapped gas causing lung injury from overexpansion.