ISOFLURANE AND SEVOFLURANE PRODUCE A DOSE-DEPENDENT REDUCTION IN THE SHIVERING THRESHOLD IN RABBITS

All general anesthetics markedly impair thermoregulatory responses; no netheless, sufficient hyperthermia or hypothermia will trigger most pr otective reflexes. Shivering, however, remains an exception among ther moregulatory responses: it is common during postanesthetic recovery, b ut is rare at typical anesthetic concentrations. This observation sugg ests that general anesthesia impairs shivering far more than other the rmoregulatory defenses. Accordingly, we tested the hypothesis that low concentrations of isoflurane and sevoflurane would virtually oblitera te shivering. Japanese white rabbits were anesthetized with isoflurane or sevoflurane at end-tidal concentrations of 0.2, 0.3, and 0.4 minim um alveolar anesthetic concentration (MAC) (n = 6 in each group); the normal core temperature for these rabbits is approximate to 39 degrees C. Core temperatures were subsequently reduced by a water-perfused th ermode positioned in the colon. The core temperature triggering shiver ing identified the threshold for this response. Five of the six rabbit s 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 g iven 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 minimu m core temperature of 35.0 degrees C. However, no rabbit given 0.4 MAC isoflurane shivered, even at minimum core temperatures of 35.0 degree s C. All of the rabbits given 0.2 MAC sevoflurane shivered at a mean c ore temperature of 36.6 +/- 0.7 degrees C. Three of the six rabbits gi ven 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 mini mum 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 imp air shivering.