Enflurane Decreases the Threshold for Vasoconstriction More than Isoflurane or Halothane

Intraoperative hypothermia results largely from anesthetic-induced inhibition of tonic thermoregulatory vasoconstriction. Sufficient hypothermia, however, triggers peripheral vasoconstriction, which usually prevents further decrease in core temperature. The thermoregulatory effects of all volatile anesthetics have been tested in adults and/or children, but different anesthetics have not been directly compared. We therefore evaluated thermoregulatory responses during enflurane, isoflurane, and halothane administration. Anesthesia was maintained with 1 minimum alveolar anesthetic concentration (MAC) of halothane, isoflurane, or enflurane in 27 patients undergoing intraabdominal surgery. Patients were maintained normovolemic and normocapnic but were allowed to cool passively. A forearm minus fingertip, skin-temperature gradient of 4 degrees C identified significant vasoconstriction; the core temperature triggering vasoconstriction identified the threshold. Morphometric characteristics, initial core temperatures, ambient operating room temperatures, blood pressures, and anesthetic potencies were similar in each group. All eight patients given halothane vasoconstricted at a core temperature of 35.5 +/- 0.6 degrees C. Eight of the patients given isoflurane vasoconstricted at a core temperature of 35.2 +/- 0.5 degrees C. However, two others did not at minimum core temperatures of 34.0 and 33.8 degrees C. Only one patient given enflurane vasoconstricted at a core temperature of 34.6 degrees C. The other six patients never vasoconstricted, at minimum core temperatures of 33.6 +/- 0.4 degrees C. Our data indicate that enflurane profoundly inhibits thermoregulatory responses in children. The mechanism for this extraordinary inhibition remains unknown but does not result from any obvious anesthetic pharmacology or thermoregulatory physiology. We conclude that unwarmed pediatric patients will become colder when anesthetized with enflurane than with halothane or isoflurane.