The Effects of Halothane on Cardiovascular Responses in the Neuraxis of Cats: Influence of Background Anesthetic State

Background: This study examined the effects of halothane on arterial pressure after central nervous system (CNS) pressor site stimulation in anesthetized cats, cats rendered unconscious by midcollicular transection, and conscious cats.

Methods: Two anesthetized groups and two nonanesthetized groups were used. Cats were anesthetized with either alpha-chloralose and urethane or pentobarbital. Nonanesthetized groups were cats with midcollicular transections or conscious cats with chronically implanted electrodes. Stimulating electrodes were placed into vasomotor areas of the hypothalamus (HYP), reticular formation (RF), and medulla, and arterial pressure responses to increasing stimulus currents were examined during different halothane concentrations. Two groups of cats were also anesthetized with either pentobarbital or urethane and underwent bilateral carotid artery occlusion.

Results: Stimulation at each CNS site produced increases in arterial pressure and heart rate. Halothane attenuated pressor responses evoked by stimulation of all loci in all groups of cats. The inhibition by halothane on these cardiovascular responses was greatest at HYP and RF sites, while the medulla was more resistant to the effects of halothane in the anesthetized animals. Midcollicular transection decreased this medullary resistance. The inhibition of pressor responses by halothane was also greater in pentobarbital- than chloralose urethane-anesthetized animals. In contrast, pressor responses elicited by bilateral carotid occlusion were attenuated by halothane similarly in both anesthetic groups. Reticular formation stimulation in conscious animals resulted in "alerting responses" in addition to pressor effects, both of which were attenuated by halothane.