The authors investigated the effects of pump flow rate on oxygen use during a moderate hypothermic cardiopulmonary bypass (CPB) in 31 patients during either coronary artery bypass grafting or valve replacement and aortic cross clamping. Intravenous anesthesia was performed with high dose fentanyl. A neuromuscular blockade was achieved with an intravenous infusion of pancuronium. After a stable rectal temperature of 29 degrees C was obtained by both surface cooling with a blanket and core cooling with CPB, the pump flow rate was changed from 2.4 L/min/m2 to 2.2, 2.0, 1.8 and 1.6, keeping the same flow rate for at least 5 min. Both arterial and venous blood was sampled for blood gas analysis; serum lactate measurement and hemodynamic changes were monitored and recorded at each flow rate. In 10 of 31 patients, whose aortic cross clamping time was longer than 90 min, the same measurements were followed 10 min after the induction of prostaglandin E1 (PGE1) (40 +/- 13 ng/kg/min) at pump flow rates of 2.4, 2.0, and 1.6 L/min/m2. Oxygen consumption significantly decreased at 1.6 L/min/m2 compared to the other flow rates (52.4 +/- 13.6, 54.3 +/- 11.8, 56.4 +/- 14.7, and 56.7 +/- 11.0 ml/min/m2, at flow rates of 2.4, 2.2, 2.0 and 1.8 L/min/m2, respectively, versus 48.4 +/- 10.9 ml/min/m2, at 1.6 L/min/m2, p < 0.01, paired t-test). After the induction of PGE1, both aortic pressure and systemic vascular resistance decreased significantly (p < 0.01, paired t-test) at flow rates of 2.0 and 1.6 L/min/m2.(ABSTRACT TRUNCATED AT 250 WORDS)