MAC for halothane and isoflurane during normothermia and hypothermia in the newborn piglet.

BACKGROUND: Halothane and isoflurane are frequently used in studies of perinatal hypoxia and ischemia. Little information exists on the minimum alveolar concentration (MAC) necessary to prevent movement to a painful stimulus in newborn pigs and no information on the effects of hypothermia on MAC in pigs. Hypothermia is currently investigated as a posthypoxic neuroprotective intervention.
METHODS: The MAC of halothane and isoflurane necessary to prevent movement when a 25 cm hemostatic clamp was applied to the tail were determined in six 20-48-hour-old piglets, and when the same stimulus was applied to the hoof. MAC for halothane was first determined at 39 degrees C, then at 35 degrees C, whereafter halothane was discontinued and MAC for isoflurane determined first at 35 degrees C and then at 39 degrees C.
RESULTS: In all six piglets MAC was lower at 35 degrees C than at 39 degrees C for both anesthetics with both tail and hoof determination, lower for halothane than isoflurane for both stimuli at both temperatures, and lower for tail than hoof determination for both anesthetics at both temperatures. For halothane at 39 degrees C, mean (SD) MAC hoof was 0.82 (0.05)% vs tail 0.60 (0.12)%, and at 35 degrees C, hoof 0.65 (0.06)% vs tail 0.42 (0.10)%. For isoflurane at 39 degrees C, MAC hoof was 2.47 (0.28)% vs tail 1.83 (0.28)%, and at 35 degrees C, hoof was 1.83 (0.18)% vs tail 0.85 (0.25)%.
CONCLUSION: In the newborn piglet, MAC should be determined by hoof clamp, MAC of isoflurane is approximately three times that of halothane, and both are reduced during hypothermia.