Anesthetic-induced inhibition of thermoregulation causes hypothermia which can decrease respiratory drive, alter drug metabolism, increase protein catabolism, cause hemodynamic instability, and prolong recovery from surgery. Generalized postoperative tremor (which clinically resembles shivering) can also cause complications including an approximate 150% increase in metabolic rate, hypoxemia, would dehiscence, dental damage, and disruption of delicate surgical repairs. Prevention and treatment of hypothermia and postoperative tremors may decrease morbidity, and minimize the duration and cost of postanesthetic care. Effective intervention has been hindered because little is known of their causes. Results from these studies will contribute to the understanding of intraoperative hypothermia and postoperative tremor and may permit development of techniques to prevent and treat these anesthetic complication. Thermoregulatory responses to hypothermia are believed to be inhibited by concentrations of anesthesia sufficient for surgery. Following surgery, anesthetic concentrations in the brain decrease rapidly, and normal compensation for hypothermia triggers vasoconstriction, shivering, and nonshivering thermogenesis. The investigators' preliminary studies indicate that: 1) thermoregulatory vasoconstriction and nonshivering thermogenesis do occur in surgical patients who become sufficiently hypothermic (core temperature approximately 34 degrees celsius); 2) thermoregulatory vasoconstriction and normal shivering do not occur during recovery from general anesthesia in moderately hypothermic patients (core temperature 35-37 degrees celsius); and 3) the electromyographic characteristics of postoperative tremors resemble pathologic clonus, not those of thermoregulatory shivering. These observations cannot be reconciled with existing theories of anesthetic influence of thermoregulation, nor with the conventional explanation for postanesthetic tremors. The proposed studies will determine the magnitude of anesthetic-induced inhibition of thermoregulation and investigate the etiology and control of generalized postoperative tremors. First, the investigators will determine the extent to which intraoperative core temperature must decrease to activate thermoregulatory vasoconstriction and nonshivering thermogenesis. They will also determine the extent which these responses are effected by different anesthetic drugs and doses. Next, they will determine whether postanesthetic thermoregulatory inhibtion is caused by: 1) a very low anesthetic concentration (similar to that observed during recovery); or 2) a long-term effect of a brief anesthetic-induced unconsciousness. Finally, the investigators will determine whether postoperative tremor requires both an appropriate anesthetic concentration and triggering by stimulation of region cutaneous cold receptors.
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