Hypothermia and Barbiturates: Individual and Combined Effects on Canine Cerebral Oxygen Consumption

Abstract

Following establishment of total spinal anesthesia, the cerebral metabolic effects of progressive hypothermia (37, 28, 18, and 14° C) were studied initially in six awake dogs. The EEG became iso-electric at temperatures below 18° C. At 14° C, CMROl was reduced to 7% of control. Thereafter, 40 mg/kg thiopental, iv, was given and the dogs were rewarmed while an isoelectric EEG was maintained by a continuous thiopental infusion. The CMROl was then compared at the different temperatures with and without thiopental. The CMRO, was unaffected by the barbiturate at 14 and 18° C. At 28 and 37° C the CMRO, was significantly reduced by the barbiturate (at 37° C to 55% of the 37° C value without thiopental). The change in CMROl with temperature in the absence of EEG activity (due to barbiturates) closely approximated an Arrhenius curve (relating log CMRO, to the reciprocal of absolute temperature). In the presence of EEG activity (no barbiturates) such a simple relationship was less apparent. The results support the following conclusions: barbiturates only affect CMRO, in the presence of neuronal electrical activity; the combined effect of hypothermia and barbiturates on CMROl cannot be expressed as a simple additive relationship; and in the presence of electrical activity, the relationship between temperature and CMRO, cannot be denned by any simple mathematical function.