Evidence for a significant myocardial contribution to total metabolic burden during hypothermic cardiopulmonary bypass: a study of continuously measured oxygen consumption and arterial lactate levels in pigs

Abstract

Objective: We assessed the causes of imbalance of oxygen transport by continuously measuring oxygen consumption (VO2) during hypothermic cardiopulmonary bypass (CPB) in pigs. Methods: Six pigs (17.2±1.6 kg) underwent hypothermic (328C) CPB for 180 min with 120 min of aortic crossclamping (ACC). An AMIS 2000 mass spectrometer was adapted for the on-line measurement of VO2. Arterial lactate was measured at the beginning of CPB, the end of hypothermia, before and 10 min after ACC release, 20 min later, and at the end of CPB. Results: Arterial lactate increased from 1.8±0.7 to 5.1±1.8 mmol/L during CPB. Hypothermia reduced VO2by 0.63±0.29 ml/min/kg per 8C, but lactate increased to 4.2±1.5 mmol/L ( p <0.05). The most rapid rise of VO2and lactate occurred during the first 10 min after ACC removal, accounting for 26% and 68%, respectively, of the total rise during rewarming. Conclusions: Inadequate tissue oxygenation persists throughout hypothermic CPB. The rise in systemic VO2 and lactate immediately after ACC release may reflect inadequate oxygen transport within the myocardium during ischemia and manifest on reperfusion. This simple technique may be used to provide important information regarding the dynamic balance of systemic and myocardial oxygen transport during ischemia - reperfusion.