All
general anesthetics markedly impair thermoregulatory responses; nonetheless, sufficient
hyperthermia or
hypothermia will trigger most protective reflexes. Shivering, however, remains an exception among thermo-regulatory 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 approximately 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.(ABSTRACT TRUNCATED AT 250 WORDS)