Combined effects of propofol and mild hypothermia on cerebral metabolism and blood flow in rhesus monkey: a positron emission tomography study.

PURPOSE: Propofol reduces the cerebral metabolic rate for oxygen (CMRO2), regional CMRO2 (rCMRO2), cerebral blood flow (CBF), and regional CBF (rCBF), but maintains the coupling of cerebral metabolism and blood flow. Under mild to moderate hypothermia, the coupling is maintained, while rCBF is reduced, but no direct measurement of rCMRO2 has yet been reported. This study aimed to evaluate the effects of propofol under normothermic and mild hypothermic temperatures upon rCMRO2, rCBF, and their regional coupling, through direct measurement by positron emission tomography.
METHODS: Rhesus monkeys were anesthetized with 65% nitrous oxide and propofol. Then rCBF and rCMRO2 were measured under four sets of conditions: infusion of a low-propofol dose (12 mg.kg(-1) x h(-1)) at normothermic temperatures (38 degrees C), a high dose (25 mg x kg(-1) x h(-1)) at normothermic temperatures, a low dose under mild hypothermia (35 degrees C), and a high dose under mild hypothermia. The ratio of rCBF/rCMRO(2) was calculated from these data.
RESULTS: Reductions in CMRO2 and rCMRO2 in most regions were associated with two factors: the higher propofol dose and the induction of hypothermia, but there was no interaction between these factors. Concerning blood flow, no significant reduction was observed, except for CBF by the induction of hypothermia. The ratio of rCBF/rCMRO2 was constant in this study setting.
CONCLUSION: During propofol anesthesia, it is possible to reduce cerebral metabolism throughout the entire brain as well as in any brain region by increasing the propofol dose or inducing hypothermia. The concurrent use of these two interventions has an additive effect on metabolism, and can be considered as safe, as their combination does not impair the coupling of cerebral metabolism and blood flow.