The reason for cardiac output reduction after aortic cross-clamping.

The hypothesis that a decrease in cardiac output during infrarenal aortic cross-clamping is related to a decrease in oxygen consumption in the perfused tissues (cross-clamp-adapted oxygen consumption) rather than to deterioration of myocardial performance has been tested. Twenty-two patients undergoing excision of an aortic abdominal aneurysm were randomly divided into two groups of equal number. During aortic cross-clamping, Group 1 patients received nitroglycerin infusion, 1 to 2 micrograms.kg-1.min-1, whereas Group 2 patients did not receive a nitroglycerin infusion. During aortic cross-clamping, cross-clamp-adapted body oxygen consumption decreased equally in both groups by 40 to 42 percent of baseline values, whereas cardiac output decreased by 17 percent in Group 2 but did not change significantly in Group 1. Mixed venous oxygen content increased significantly after induction of anesthesia and prior to aortic cross-clamping in both groups. During cross-clamping, the values of mixed venous oxygen content remained increased in Group 2 and increased further in Group 1. The data support our hypothesis since a decrease in cardiac output was not associated with an increase in filling pressures during aortic cross-clamping, but was instead associated with an increase in mixed venous oxygen content and a decrease in the arteriovenous oxygen content difference. Nitroglycerin infusion was associated with a further increase in mixed venous oxygen content during aortic cross-clamping and a decrease in the arteriovenous oxygen content difference, without a concomitant increase in oxygen utilization.