Oxygen metabolism after cardiopulmonary bypass

We hypothesized that the relationship between oxygen delivery (DO2) and oxygen consumption (VO2) in patients undergoing cardiopulmonary bypass (CPB) may differ from the normal physiologic state because of oxygen debts acquired during CPB. Blood gas analysis and hemodynamic parameters were repeatedly measured for determination of DO2 and VO2 in 40 patients undergoing CPB every 8 hours during the first 48 hours postoperatively. Twenty patients of acute myocardial infarction (AMI) also were studied in the same protocol as controls. 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/m2); VO2=121+0.0844 DO2 (R=0.25, p=0.023). However, such supply-dependent oxygen consumption was not observed in the AMI patients. Perioperative parameters which had a significant relationship with oxygen extraction rate (VO2/DO2 x 100) were Hb concentration, cardiac index, systemic vascular resistance, age of the patients, duration of CPB, SaO2 and SvO2. These results suggest that patients undergoing CPB need a much higher oxygen supply to recover from the oxygen debt during open heart surgery.