NIFEDIPINE AND INTRAOPERATIVE CORE BODY-TEMPERATURE IN HUMANS

Background: Initial anesthetic-induced hypothermia results largely fro m core-to-peripheral redistribution of heat. Nifedipine administration may minimize hypothermia by inducing vasodilation well before inducti on of anesthesia. Although vasodilation would redistribute heat to per ipheral tissues, thermoregulatory responses would maintain core temper ature. After equilibration, the patient mould be left vasodilated, wit h a small core-to-peripheral temperature gradient. Minimal redistribut ion hypothermia may accompany subsequent induction of anesthesia, beca use heat flow requires a temperature gradient. In contrast, similar va sodilation concurrent with anesthetic-induced vasodilation may augment redistribution hypothermia. Accordingly, the authors tested the hypot hesis that nifedipine treatment for 12 h before surgery would minimize intraoperative redistribution hypothermia, whereas nifedipine treatme nt immediately before induction of anesthesia would aggravate hypother mia. Methods: Patients undergoing hip arthroplasty were randomly assig ned to: (1) 20 mg long-acting nifedipine orally 12 h before surgery, a nd 10 mg sublingually 1.5 h before surgery (n=10); (2) nifedipine 10 m g sublingually just before induction of anesthesia (n=10); and (3) no nifedipine (control, n=10). Anesthesia was maintained with isoflurane and 60% nitrous oxide. Administered intravenous fluids were heated, bu t the patients were not otherwise actively warmed. Results: Core tempe rature decreased 0.8 degrees C in the first hour of surgery in the pat ients given nifedipine the night before and the morning of surgery, wh ich was significantly less than in the control group (1.7 degrees C in the first hour). In contrast, core temperature decreased 2.0 degrees C in the first hour of surgery in the patients given nifedipine immedi ately before induction of anesthesia. During the subsequent 70-130 min of anesthesia, core temperature decreased at roughly comparable rates in each group. After 130 min of anesthesia, core temperature in the t wo nifedipine-treated groups differed by 1.6 degrees C, and the temper atures in all three groups differed significantly. Conclusions: Vasodi lation induced by nifedipine well before induction of anesthesia minim ized redistribution hypothermia, presumably by decreasing the core-to- peripheral tissue temperature gradient. In contrast, redistribution hy pothermia was aggravated by administration of the same drug immediatel y before induction of anesthesia. Drug-induced modulation of vascular tone thus produces clinically important alterations in intraoperative core temperature.