Isoflurane and Sevoflurane Produce a Dose-Dependent Reduction in the Shivering Threshold in Rabbits

All general anesthetics markedly impair thermoregulatory responses; nonetheless, sufficient hyperthermia or hypothermia will trigger most protective reflexes.Shivering, however, remains an exception among thermoregulatory responses: it is common during postanesthetic recovery, but is rare at typical anesthetic concentrations. This observation suggests that general anesthesia impairs shivering far more than other thermoregulatory defenses. Accordingly, we tested the hypothesis that low concentrations of isoflurane and sevoflurane would virtually obliterate shivering. Japanese white rabbits were anesthetized with isoflurane or sevoflurane at end-tidal concentrations of 0.2, 0.3, and 0.4 minimum alveolar anesthetic concentration (MAC) (n = 6 in each group); the normal core temperature for these rabbits is approximate equals 39 degrees C. Core temperatures were subsequently reduced by a water-perfused thermode positioned in the colon. The core temperature triggering shivering identified the threshold for this response. Five of the six rabbits given 0.2 MAC isoflurane shivered at a mean core temperature of 36.3 +/- 0.3 degrees C (mean +/- SD), and one rabbit failed to shiver at a minimum core temperature of 35.0 degrees C. Four of the six rabbits given 0.3 MAC isoflurane shivered at a mean core temperature of 36.2 +/- 0.6 degrees C, and two of these rabbits failed to shiver at a minimum core temperature of 35.0 degrees C. However, no rabbit given 0.4 MAC isoflurane shivered, even at minimum core temperatures of 35.0 degrees C. All of the rabbits given 0.2 MAC sevoflurane shivered at a mean core temperature of 36.6 +/- 0.7 degrees C. Three of the six rabbits given 0.3 MAC sevoflurane shivered at a mean core temperature of 35.8 +/- 0.3 degrees C, and three of these rabbits failed to shiver at a minimum core temperature of 35.0 degrees C. Only one rabbit given 0.4 MAC sevoflurane shivered at a core temperature of 35.2 degrees C. At least in rabbits, low concentrations of volatile anesthetics profoundly impair shivering. (Anesth Analg 1995;81:581-4)