- Obstructive apnoeas could cause flow limitation to venous return, resulting in a decrease in cardiac output and a change in the distribution of flow from the upper and lower body. In 14 anaesthetized dogs, we studied the effects of obstructive apnoeas on inferior and superior vena caval flows under baseline conditions and with intra-abdominal pressure increased by approximately equal to 5 torr by binding the abdomen. During obstructive apnoeas in the two groups, respiratory rate decreased by 30% (P < 0.02) and inspiratory airway pressure decreased by approximately equal to 15 torr (P < 0.01). At baseline, the ratio of inferior to superior vena caval flow was 2.4:1 and did not change with abdominal binding or apnoeas. During apnoeas there was no change in cardiac output or in the ratio of inferior to superior vena cava flow either with baseline or abdominal binding conditions. Preservation of total inferior vena caval flow during apnoeas and cardiac output occurred, even though inspiratory flow limitation was found with the abdomen bound. We conclude: (1) there was no change in either cardiac output or the distribution of venous return during apnoeas; (2) there was substantial inspiratory/expiratory variation in venous return during obstructive apnoeas. The large inspiratory-increase in venous return may have implications for the development of pulmonary hypertension during obstructive apnoeas.