Induction of anesthesia with ketamine reduces the magnitude of redistribution hypothermia

Hypothermia after induction of general anesthesia results largely from core-to-peripheral redistribution of body heat. Both central inhibition of tonic thermoregulatory vasoconstriction in arteriovenous shunts and anesthetic-induced arteriolar and venous dilation contribute to this redistribution. Ketamine, unique among anesthetics, increases peripheral arteriolar resistance; in contrast, propofol causes profound venodilation that other anesthetics do not. We therefore tested the hypothesis that induction of anesthesia with ketamine causes less core hypothermia than induction with propofol. Twenty patients undergoing elective surgery were randomly assigned to anesthetic induction with either 1.5 mg/kg ketamine (n = 10) or 2.5 mg/kg propofol (n = 10). Anesthesia in both groups was subsequently maintained with sevoflurane and 60% nitrous oxide in oxygen. Forearm minus finger, skin-temperature gradients <0 degreesC were considered indicative of significant arteriovenous shunt vasodilation. Ketamine did not cause vasodilation just after induction, whereas propofol rapidly induced vasodilation. Core temperatures in the patients given ketamine remained significantly greater than those in the patients induced with propofol. These data suggest that maintaining vasoconstriction during induction of anesthesia reduces the magnitude of redistribution hypothermia.