Role of baroreceptor reflexes in the hemodynamic and heart rate responses to althesin, ketamine and thiopentone anesthesia.

The effects of i.v. infusions of althesin, ketamine and thiopentone were studied in instrumental rabbits, in doses that produced similar levels of light anesthesia. The main hemodynamic differences were in the rises in mean arterial pressure (MAP) and in total peripheral resistance (TPR) which were in the order of ketamine greater than thiopentone greater than althesin. These rises in MAP and TPR did not occur in sino-aortic denervated rabbits suggesting that in normal rabbits these depended on the integrity of the arterial baroreceptors and /or chemoreceptors. The heart rate increased with all anesthetics mainly owing to reduction in vagal efferent activity, except with althesin where cardiac sympathetic activity increased also. Baroreceptor--heart rate reflex properties were studied by deriving sigmoid curves relating MAP to heart period (HP, i.e. pulse interval). All drugs depressed the following curve parameters: (i) HP range, i.e. the difference in HP plateaux from maximal tachycardia to maximal bradycardia; and (ii) the reflex gain (sensitivity). The order of depression was ketamine greater than thiopentone greater than althesin, i.e. the same as the order in which they evoked pressor effects. The results suggest that the 3 anesthetics produce differing depression of afferent mechanisms related to baroreceptor reflexes and that this accounts for both the differences in pressor effects (through disinhibition of constrictor tone) and in depression of the vagal and sympathetic components of the baroreceptor--heart rate reflex.