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.

BACKGROUND: Jugular venous catheters and near-infrared spectroscopy can measure cerebral venous blood hemoglobin oxygen saturation (SvO2). We used computer simulation to characterize the relation between Sv02 and cerebral metabolic rate for oxygen (CMR02) 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 CMRO2. We simulated cerebral blood flow reductions that decreased Sv02 and CMR02.
RESULTS: Our model predicts the relation between CMR02 and Sv02 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, Sv02 can decrease to 30% before CMR02 will decrease to less than 90% of normal. In contrast, for alpha-stat management of infants at 17 degrees C, Sv02 must be maintained at greater than 95% to maintain CMR02 at greater than 90% of its temperature appropriate value.
CONCLUSIONS: High Sv02 observed during hypothermic CPB may indicate impaired oxygen transfer from hemoglobin to brain, not "luxury perfusion." The relation between Sv02 and CMR02 depends dramatically on the temperature of the patient. Sv02 per se may not be reliable index of normal CMR02 during hypothermic CPB.