Hypothermia and barbiturates: individual and combined effects on canine cerebral oxygen consumption

Following establishment of total spinal anesthesia, the cerebral metabolic effects of progressive hypothermia (37, 28, 18, and 14 degrees C) were studied initially in six awake dogs. The EEG became isoelectric at temperatures below 18 degrees C. At 14 degrees C, CMRO2 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 CMRO2 was then compared at the different temperatures with and without thiopental. The CMRO2 was unaffected by the barbiturate at 14 and 18 degrees C. At 28 and 37 degrees C the CMRO2 was significantly reduced by the barbiturate (at 37 degrees C to 55% of the 37 degrees C value without thiopental). The change in CMRO2 with temperature in the absence of EEG activity (due to barbiturates) closely approximated an Arrhenius curve (relating log CMRO2 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 CMRO2 in the presence of neuronal electrical activity; the combined effect of hypothermia and barbiturates on CMRO2 cannot be expressed as a simple additive relationship; and in the presence of electrical activity, the relationship between temperature and CMRO2 cannot be defined by any simple mathematical function.