Theoretical analysis of cerebral venous blood hemoglobin oxygen saturation as an index of cerebral oxygenation during hypothermic cardiopulmonary bypass: A counterproposal to the 'luxury perfusion' hypothesis

Abstract

Background: Jugular venous catheters and near-infrared spectroscopy can measure cerebral venous blood hemoglobin oxygen saturation (SV(O2)). We used computer simulation to characterize the relation between SV(O2) and cerebral metabolic rate for oxygen (CMR(O2)) during hypothermic cardiopulmonary bypass (CPB). Methods: We developed a theoretical model of cerebral oxygen consumption and blood-brain oxygen transfer. Our model included the temperature dependence of blood and brain oxygen solubility; the temperature, age, and acid-base dependence of hemoglobin oxygen dissociation; and the temperature and age dependence of CMR(O2). We simulated cerebral blood flow reductions that decreased SV(O1) and CMR(O2). Results: Our model predicts the relation between CMR(O2) and Sv(O2) to be dependent on temperature, because of a shift of the oxygen partial pressure at which hemoglobin oxygen saturation equals 0.50. For example, during normothermic CPB, Sv(O2), can decrease to 30% before CMR(O2), will decrease to less than 90% of normal. In contrast, for α-stat management of infants at 17°C, Sv(O2) must be maintained at greater than 95% to maintain CMR(O2) at greater than 90% of its temperature appropriate value. Conclusions: High Sv(O2), observed during hypothermic CPB may indicate impaired oxygen transfer from hemoglobin to brain, not 'luxury perfusion.' The relation between Sv(O2) and CMR(O2) depends dramatically on the temperature of the patient. Svo(O2) per se may not be a reliable index of normal CMR(O2), during hypothermic CPB.