The oxygen debt during routine cardiac surgery: illusion or reality?

Abstract

Background: Patients undergoing cardiac surgery with the use of cardiopulmonary bypass (CPB) are often thought to have tissue hypoxia and intraoperative oxygen debt accumulation despite the lack of sufficient data to support this assumption. Methods and results: Oxygen uptake and related parameters, including the plasma lactate and pyruvate concentrations, were studied during the peri-operative period in a group of 15 consecutive patients who underwent coronary artery bypass graft surgery. The actual oxygen uptake (VO2) and delivery (DO2) were compared with the individual expected (computed) oxygen transport values. The mean values of DO2 and VO2were in the range of the expected values. Our results demonstrate a leading role for body temperature in perioperative changes of oxygen consumption rate ( r2 = 0.65, p< 0.001). Plasma lactate and pyruvate did not exceed the physiological range in any patient. However, with initiation of CPB, the lactate to pyruvate (LA/PVA) ratio increased (from 9.87± 2.43 at T1 to 12.08± 1.51 at T2, p< 0.05). The mean value of the LA/PVA ratio was elevated during surgery. Later, upon lowering of the plasma lactate concentration in the postoperative period, the LA/PVA ratio decreased to normal values. Without any other evidence of hypoxia, this increase in the LA/PVA ratio could be explained by washout of lactate from previously hypoperfused tissues and intraoperative decrease of lactate clearance. Conclusion: Systemic oxygenation was not impaired during CPB, or during 18 h after surgery in the studied group of patients.