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 mini mum alveolar concentration (MAC) necessary to prevent movement to a pa inful stimulus in newborn pigs and no information on the effects of hy pothermia on MAC in pigs. Hypothermia is currently investigated as a p osthypoxic neuroprotective intervention.Methods: The MAC of halothane and isoflurane necessary to prevent movement when a 25 cm hemostatic c lamp was applied to the tail were determined in six 20-48-hour-old pig lets, and when the same stimulus was applied to the hoof. MAC for halo thane was first determined at 39 degrees C, then at 35 degrees C, wher eafter halothane was discontinued and MAC for isoflurane determined fi rst at 35 degrees C and then at 39 degrees C. Results: In all six pigl ets MAC was lower at 35 degrees C than at 39 degrees C for both anesth etics with both tail and hoof determination, lower for halothane than isoflurane for both stimuli at both temperatures, and lower for tail t han 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 tai l 1.83 (0.28)%, and at 35 degrees C, hoof was 1.83 (0.18)% vs tail 0.8 5 (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.