THE EFFECTS OF HALOTHANE, ISOFLURANE, AND ENFLURANE ON THERMOREGULATORY RESPONSES IN THE NEURAXIS OF CATS

Background: Normal thermoregulatory function is believed to be modulat ed by thermosensitive neurons in the preoptic region of the anterior h ypothalamus and other sites within the central nervous system includin g the spinal cord. Previous evidence has demonstrated modulation of se gmental spinal cord thermoregulatory mechanisms from more rostral cent ral nervous system sites. The ability of the volatile anesthetics to d isrupt normal thermoregulatory function and produce shivering-like act ivity during emergence is well documented. The purpose of the current investigation was to examine the action of the volatile anesthetics ha lothane, isoflurane, and enflurane on thermoregulatory responses produ ced at the preoptic region and spinal cord. Methods: Cats were chronic ally instrumented with bilateral cannulas allowing selective heating a nd cooling of the preoptic region. Electrodes were implanted in hindli mb and forelimb muscles for electromyographic (EMG) analysis. Animals underwent selective heating and cooling of the preoptic region in the awake state, during volatile agent anesthesia and during emergence. In a separate series of animals, pontine-transected cats with epidural t hermodes and a thermocouple underwent alternate heating and cooling of the spinal cord. Heating and cooling was performed in the nonanesthet ized state, at graded concentrations of halothane, and during emergenc e. In all animals, deep core peritoneal temperature, epidural spinal c ord temperature, forelimb and hindlimb EMG activity were continuously recorded and digitally processed. EMG responses in both experiments we re quantitated and analyzed for power spectral density. Results: In th e chronically prepared animals, heating and cooling of the preoptic re gion in the conscious state resulted in appropriate thermoregulatory r esponses, including shivering-like activity and increased EMG power wi th preoptic region cooling. Halothane, isoflurane, and enflurane each abolished these thermoregulatory responses. During emergence from anes thesia, however, the typical spontaneous increases in EMG power observ ed at normothermia were significantly attenuated by heating of the pre optic region and augmented by cooling of the preoptic region. In the a cutely prepared animals, cooling of the spinal cord produced graded in creases in EMG activity. Increased concentrations of halothane dose-de pendently diminished this response to cooling of the spinal cord. Duri ng emergence, cooling of the spinal cord resulted in a shivering respo nse similar to those observed during control conditions. Conclusions: The ability of preoptic region heating and cooling to modulate postane sthetic shivering implies that while thermoregulatory pathways remain intact, volatile anesthetics produce an imprecision in the control of thermoregulatory responses at the level of the anterior hypothalamus. Attenuation of shivering-like responses generated at spinal cord level s in pontine-transected cats implies a significant blunting action of thermoregulatory response mechanisms at the level of the spinal cord o r lower brain stem.