Dobutamine increases oxygen consumption during constant flow cardiopulmonary bypass

We have studied the effects of flow and dobutamine on systemic haemodynamic variables, oxygen delivery (DO2) and oxygen consumption (V overdot O-2) in 20 patients during cardiopulmonary bypass (CPB) with mild hypothermia (34 degrees C). In a subgroup of seven patients, we also studied the effects on gastric microcirculatory blood flow (MCF) using laser Doppler flowmetry. During CPB, measurements were made before and after two interventions: the first consisted of increasing flow from 2.4 to 3.0 litre min(-1) m(-2) for 10 min; the second consisted of an infusion of dobutamine at a rate of 6 mu g kg(-1) min(-1) for 10 min during constant flow CPB. There were no significant differences in DO2, V overdot O-2 or haemodynamic variables between the two baseline measurements. The increase in flow raised DO2 (27%, P<0.001), mean arterial pressure (P<0.01) and MCF (P<0.01), but failed to increase V overdot o(2). In contrast, dobutamine infusion increased lie, (11%, P<0.001) during constant flow CPB without significant changes in DO2, systemic haemodynamic variables or MCF. These results indicate that increases in V overdto O-2 during dobutamine may be flow-independent.