Halothane and the carotid sinus reflex: evidence for multiple sites of action.

Baroreceptor reflexes have been found to be attenuated during halothane anesthesia in humans and experimental animals. The baroreceptor reflex arc is comprised for a number of components, including receptors, afferent and efferent nerve pathways, central integratory centers, peripheral ganglia, and effector organs, at which halothane might exert an inhibitory effect. This study was performed to determine the effect of halothane at each component in order to identify the site or sites of baroreflex attenuation due to halothane. The baroreflex effects on heart rate initiated by carotid sinus pressure changes were examined in conscious and anesthetized (0.0%, 0.75%, and 1.5% halothane in 50% N2O and O2, pls 25 mg/kg thiopental) dogs. In addition, carotid sinus afferent activity, cardiac sympathetic efferent activity and heart responses to direct sympathetic and parasympathetic efferent stimulation were examined in anesthetized dogs. Preganglionic and postganglionic sympathetic nerve activities were recorded simultaneously during baroreceptor activation to determine ganglionic effects of halothane. All levels of anesthesia significantly (P less than 0.05) attenuated reflex changes in heart rate produced by the pressure changes compared to conscious dogs. Significant decreases in cardiac sympathetic efferent activity were produced at 1.5% halothane (P less than 0.05). The depression in postganglionic activity was significantly greater than that or preganglionic activity, indicating a ganglionic-blocking effect by halothane. Cardiac chronotropic changes produced by direct efferent stimulation of sympathetic and vagal fibers were attenuated significantly by halothane (P less than 0.05). On the other hand, baroreceptor afferent activity was increased at 1.5% halothane. This sensitization of baroreceptors appeared to contribute to decreased levels of sympathetic tone, leading to blunted reflex changes in nerve activity. Therefore, halothane was found to have multiple sites of action, leading to depression of the baroreflex.