Effect of ketamine, althesin, and thiopentone on the Valsalva-constrictor and heart rate reflexes of the rabbit.

The circulatory effects of Valsalva-like manoeuvers were studied before and during i.v. infusions of either ketamine, Althesin and thiopentone given in doses that produced similar levels of light anesthesia. The Valsalva-like manoeuvers were of 30 s duration and consisted of applying Valsalva pressures (VP) from 2.5 to 20 mm Hg to the animal's respiratory valve and to a cuff placed around its thorax and abdomen. In the conscious rabbit the major reflex responses to the Valsalva-like manoeuver were VP-related rises in total peripheral resistance (TPR) and in heart rate which were mainly mediated through intrathoracic baroreceptors and were completely abolished by sino-aortic denervation. Ketamine depressed the Valsalva-TPR response by about 30-40% but Althesin and thiopentone were without effect. Ketamine and thiopentone produced marked depression of the Valsalva-heart rate reflex, but Althesin had relatively little effect. We concluded that ketamine produces greater impairment of blood pressure homeostasis mediated through constrictor and heart rate reflexes evoked through arterial and cardiopulmonary baroreceptors than the other two anesthetics.