A focal cryogenic brain lesion does not reduce the minimum alveolar concentration for halothane in rats

Background: A focal cortical cryogenic brain injury has been reported to reduce the brain pentobarbital concentrations needed to prevent movement in response to pain in rats. This occurred despite any apparent behavioral changes in the awake animals. To determine whether this was true with other anesthetics, the authors determined the minimum alveolar concentration (MAC) for halothane in normothermic, normocarbic ventilated Sprague-Dawley rats previously subjected to a freezing injury of the parietal cortex. Methods: Injury was produced in halothane-anesthetized rats by applying a cold (-70° C), 4-mm-diameter brass rod to the exposed dura for 5 or 15 s. Animals then were studied 3 days after injury, a time when cerebral metabolism in the ipsilateral hemisphere reaches a minimum. Minimum alveolar concentration was determined using a tail-clamp stimulus, combined with end-tidal anesthetic sampling. In addition, exploratory activity was measured by the open field test just before MAC determination, and spontaneous nocturnal motility was monitored by an electronic motion sensor during the night before testing. Results: In normal animals subjected only to preparatory surgery, MAC was 1.10 ± 0.07% (mean ± SD). Almost identical values were found in rats subjected to 5- and 15-s cryogenic injuries (1.11 ± 0.07% and 1.08 ± 0.06%, respectively). There were no intergroup differences in open field test results or in spontaneous nocturnal activity. Conclusions: These results indicate that a focal cortical brain injury that has no obvious neurologic or behavioral effects in the awake rat does not alter halothane requirements.