Halothane and the Carotid Sinus Reflex: Evidence for Multiple Sites of Action

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Abstract
Baroreceptor reflexes are attenuated during halothane anesthesia in humans and experimental animals. The baroreceptor reflex arc is comprised of 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. The effect of halothane was determined at each component 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, plus 25 mg/kg thiopental) dogs. Carotid sinus afferent activity, cardiac sympathetic efferent activity and heart rate responses to direct sympathetic and parasympathetic efferent stimulation were examined in anesthetized dogs. Pre- and postganglionic sympathetic nerve activities were recorded simultaneously during baroreceptor activation to determine ganglionic effects of halothane. All levels of anesthesia significantly (P < 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 < 0.05). The depression in postganglionic activity was significnatly greater than that for 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 < 0.05). 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. Halothane had multiple sites of action, leading to depression of the baroreflex.

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