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 °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 prost/aglandin E, (PGE,) (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 PGE,, 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. There was a significant increase in oxygen consumption at 1.6 L/min/m2 from 46.0±12.5 to 56.2±8.5 ml/min/m2 (p<0.01, paired ttest) after the induction of PGE,. No significant changes in oxygen consumption were revealed by PGE1 administration at 2.4 and 2.0 L/min/m2. These results suggest that the critical flow rate to maintain adequate tissue oxygenation is somewhere between 1.6 and 1.8 L/min/m2 during moderate hypothermic CPB, and that PGE, increased the oxygen consumption at a lower rate than the critical flow rate, probably by improving tissue level microcirculation.
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