Pulmonary Extravascular Fluid Volume and Hypoxaemia in Myocardial Infarction**

Pulmonary extravascular fluid volume (P.E.V.) obtained by a radioisotope technique, arterial blood gases, and veno-arterial shunting were measured within 48 hours of myocardial infarction in 14 patients admitted to a Coronary Care Unit. Six of the 14 patients showed elevated values for PEV. As a group these patients showed greater initial hypoxaemia, greater veno-arterial shunting, and clinical evidence of more severe disease than did those whose P.E.V. was not elevated. Cardiac output, intrathoracic blood volume, anatomical shunting and alveolar atelectasis or airways closure with distal air trapping was not significantly different. Hypoxaemia after myocardial infarction results mainly from veno-arterial shunting. The mechanisms differ from patient to patient, but increased PEV and airways closure are important factors.