The effects of cardiopulmonary bypass on postoperative oxygen metabolism

The relationships between oxygen delivery (DO2), oxygen consumption (VO2), and the extraction rate (ER = VO2/DO2 x 100) in patients undergoing cardiopulmonary bypass (CPB) may differ from the normal physiologic state due to the oxygen debt acquired during CPB. Blood gas analysis and hemodynamic parameters were repeatedly measured for the determination of DO2 and VO2 in 40 patients undergoing CPB, every 8 h during the first 48 h postoperatively. As a control, 20 patients who had suffered acute myocardial infarction (AMI) were also studied using the same protocol. In the CPB group, a regression analysis showed that VO2 was significantly dependent on DO2, even within the physiologic range of DO2 (>500 ml/min per m2); VO2 = 121.4 + 0.0844 x DO2 (r = 0.254, P = 0.023). Conversely, in the AMI group, no such supply-dependent consumption was observed within the same range of DO2. At an ER of 30%, which is the optimal value in general, the DO2 of the CPB group was 575 ml/min per m2 and that of the AMI group was 493 ml/min per m2. All these results suggest that patients undergoing CPB need a much higher oxygen supply to recover from the oxygen debt acquired during open heart surgery.