The Mechanisms of Abnormal Gas Exchange in Acute Massive Pulmonary Embolism1,2

Abstract

Hypoxemia usually accompanies acute pulmonary embolism in humans, but its mechanism remains poorly understood. We studied 2 patients with acute, massive pulmonary embolism (APE) documented by pulmonary angiography. Both patients had a markedly increased alveolar-arterial oxygen difference (AaPO₂). The technique of multiple inert gas elimination was used to determine the distribution of ventilation-perfusion ratios . An increase in inequality was found in both patients, but this increased inequality was caused entirely by an increase in the ventilation of lung units with high ratio. No blood flow was found perfusing lung units with a ratio of less than 1.0. Both patients, however, had a large amount of blood flow (20 and 39% of the cardiac output) perfusing unventilated lung units (shunt), and the percent of minute ventilation to unperfused lung units as well as the Vd/Vt determined from the Bohr equation were increased. We conclude that in these 2 patients with APE, inequality did not play a major role in their hypoxemia and that the widened AaPO₂ is explained by the large shunts that were found.