Preganglionic sympathetic activity and the effects of anaesthetics.

Preganglionic cervical and splanchnic sympathetic activity was recorded before and during administration of inhalation anaesthetics, in rabbits ventilated with oxygen and given gallamine. During control periods, when light anaesthesia was maintained with pentobarbitone, sympathetic discharge responded to changes in arterial pressure. Increased arterial Pco3 exaggerated the amplitude of the respiratory sympathetic rhythm, and had a more variable effect on the mean impulse discharge rate. Preganglionic activity was increased by 25—50 per cent cyclopropane, which usually raised arterial pressure; by halothane, which caused severe hypotension; and by diethyl ether, which produced smaller circulatory changes. These experiments question the concept of "central vasomotor depression" during inhalation anaesthesia in the rabbit.