Preganglionic Sympathetic Activity and the Effects of Anaesthetics

Abstract

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 Pco 3 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. Changes in preganglionic sympathetic activity produced by inhalation anaesthetics have not been studied directly apart from a brief communication by Martin and Marrazzi (1942), stating that cyclopropane and chloroform do not affect cervical sympathetic discharge in cats. Deutsch, Linde and Price (1962), however, measured an increased plasma adrenaline level during cyclopropane anaesthesia in the dog, while in the same species Millar and Morris (1960) found no increase in circulating catecholamines when halothane was given. The former result suggests that excitation of the sympathetic nervous system by cyclopropane may account for the arterial hypertension which usually occurs. This view is supported by head perfusion experiments (Price et al., 1963), also in the dog, while similar studies suggest that there is depression of central sympathetic discharge during halothane anaesthesia (Price, Linde and Morse, 1963), which accords with the reduced arterial pressure and the catecholamine measurements.

The effects of cyclopropane, halothane and ether on central baroreceptor pathways

“…Doses of 1-2 mg/kg gallamine had negligible effects on preganglionic sympathetic activity, heart rate, or arterial pressure in the rabbit (Millar & Biscoe, 1965). In confirmation of 542 ANAESTHETICS AND BARORECEPTOR PA THWA YS 543 the earlier work of Van Den Ostende (1951), in five experiments studied specifically there was no modification in the arterial pressure or sympathetic responses to aortic nerve stimulation following intravenous injection of gallamine 1-2 mg/kg.…”

“…7B is a typical response to cyclopropane, occurring throughout administration. The loss of the respiratory oscillation in arterial pressure is also characteristic, but in this experiment the pressure was not elevated above the control level, as usually occurs (Millar & Biscoe, 1965). Figure 8 A shows recovery from cyclopropane, later in the same experiment.…”

“…J. BISCOE AND R. A. MILLAR Biscoe, 1965). The increased sympathetic discharge rate illustrated in Fig.…”

Rhythmical and non‐rhythmical spontaneous activity recorded from the central cut end of the sinus nerve

“…2B). Such a disappearance of the synchronization is typical of sympathetic nervous activity (Adrian, Bronk & Phillips, 1932;Millar & Biscoe, 1965) and illustrates the central nervous origin of the rhythm. Adrenaline, 10 ,ug/kg administered intravenously, caused an increase in arterial pressure accompanied by a decrease in the discharge rate ( Fig.…”

The Cardiovascular Effects of Cyclopropane in the Intact, Decerebrate and Pithed Rabbit Preparations

“…Studies on animals with an intact central nervous system invariably show a pressor response to cyclopropane (Millar & Biscoe, 1965;Price et al, 1969). As decerebration eliminated the pressor effect but not the elevation of heart rate, the increases in sympathetic nerve activity in these earlier studies may have been disproportionately related to the observed elevations of mean arterial blood pressure and in fact may be primarily associated with changes in heart rate.…”