Although milrinone effectively increases cardiac function, few studies have specifically evaluated its efficacy during cardiac surgery. We investigated the effects of milrinone on hemodynamics and left ventricular function in cardiac surgical patients who were already treated with catecholamines. Thirty-seven patients undergoing cardiac surgery were studied. Immediately after emergence from cardiopulmonary bypass (CPB), patients were randomly assigned to a control group (n = 10) or to one of these milrinone groups: milrinone 50 microg/kg intravenously (n = 8), 50 microg/kg + 0.5 microg x kg(-1) x min(-1) (n = 10), or 75 microg/kg + 0.75 microg x kg(-1) x min(-1) (n = 9). Hemodynamics and transesophageal echocardiogram were recorded while constant filling pressures were maintained by volume reinfusion from the CPB reservoir. Arterial blood samples were obtained for the measurement of milrinone plasma concentrations and to determine the dose response curve. In all three milrinone groups, cardiac index and velocity of circumferential fiber shortening (Vcfc) significantly increased from the baseline, and both were significantly higher at 5 and 10 min than those in the control group. The plasma concentration of milrinone with half of maximum increase in Vcfc was 139.3 ng/mL based on the dose-response curve. Thus, milrinone improves hemodynamics and left ventricular function when constant loading conditions are maintained.