Spinal anesthesia speeds active postoperative rewarming

Background: Redistribution of body heat decreases core temperature more during general than regional anesthesia. However, the combination of anesthetic- and sedative-induced inhibition may prevent effective upper-body thermoregulatory responses even during regional anesthesia. The extent to which each type of anesthesia promotes hypothermia thus remains controversial. Accordingly, the authors evaluated intraoperative core hypothermia in patients assigned to receive spinal or general anesthesia. They also tested the hypothesis that the efficacy of active postoperative warming is augmented when spinal anesthesia maintains vasodilation. Methods: Patients undergoing lower abdominal and leg surgery were randomly assigned to receive general anesthesia (isoflurane and nitrous oxide; n = 20) or spinal anesthesia (bupivacaine; n = 20). Fluids were warmed to 37°C and patients were covered with surgical drapes. However, no other active warming was applied during operation. Ambient temperatures were maintained near 20°C. After operation, patients were warmed with a full-length, forced-air cover set to 43°C. Shivering, when observed, was treated with intravenous meperidine. Results: The mean spinal analgesia level, which was at the sixth thoracic level during surgery, remained at the T12 dermatome after 90 min after operation. Core temperatures did not differ significantly during surgery and decreased to 34.4 ± 0.5°C and 34.1 ± 0.4°C, respectively, in patients given spinal and general anesthesia. After operation, however, core temperatures increased significantly faster (1.2 ± 0.1°C/h vs. 0.7 ± 0.2°C/h, mean ± SD; P < 0.001) in patients given spinal anesthesia. Consequently, patients given spinal anesthesia required less time to rewarm to 36.5°C (122 ± 28 min vs. 199 ± 28 min; P < 0.001). Conclusions: Comparable intraoperative hypothermia during general and regional anesthesia presumably resulted because the combination of spinal anesthesia and meperidine administration obliterated effective peripheral and central thermoregulatory control. Vasodilation increased the rate of core rewarming in patients after operation with residual lower-body sympathetic blocks, suggesting that vasoconstriction decreased peripheral-to-core heat transfer after general anesthesia.