The Effects of Short-acting Barbiturates on Arterial Pressure, Preganglionic Sympathetic Activity and Barostatic Reflexes

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The effects of two short-acting barbiturates (thiopental and methohexital) on arterial blood pressure, cervical preganglionic sympathetic activity, and barostatic reflexes have been studied in cats anesthetized with nitrous oxide. Both barbiturates reduced sympathetic nervous activity while failing to extinguish the reflex responses to electrical stimulation of the aortic depressor nerve. In decerebrated animals the same response was seen. Baroreceptor-denervated animals had a similar but exaggerated reaction. Spinal animals developed arterial hypotension without depression of sympathetic activity. It is concluded that the barbiturates inhibit sympathetic nervous activity by inhibiting “pressor” neurons in the medulla oblongata. The medullary “depressor” neurons are relatively unaffected and the barostatic reflexes consequently remain, although they are weakened. The preservation of these reflexes accounts, in part, for the circulatory stability observed during barbiturate anesthesia.

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