We investigated interactions between cardiac output, VA/Q distribution pattern, pulmonary gas exchange, O2 transport, and tissue oxygenation in 16 patients during the acute phase of pulmonary embolism (PE). The effects of breathing room air, O2 therapy (FIO2 = 0.40) (11 patients), and dobutamine (four patients) were studied after right catheterization using the multiple inert gas elimination technique. The pattern of VA/Q ratio distributions was found to depend essentially on cardiac output level. The individual blood flow perfusing ventilated areas was found to be inversely related to the mean VA/Q ratio of blood flow distribution. PVO2 was directly related to cardiac index (p less than 0.02), and negatively related to the mean VA/Q of blood flow distribution. In view of the influence of low VA/Q ratios and PVO2 on arterial hypoxemia, our results showed that the heart's response to PE conditioned the strategy of pulmonary gas exchange and O2 transport. Oxygen breathing led to a slight but consistent fall in cardiac output (-0.6 +/- 0.5 L/min, p less than 0.01). However, although PaO2 remained normal and PVO2 was slightly improved, we found no evidence for a role of hypoxic pulmonary vasoconstriction in the pulmonary hypertension observed during the acute phase of PE. Administration of dobutamine improved O2 transport and tissue oxygenation, although PaO2 remained constant or even fell in some cases because of increased VA/Q mismatch.