Haemodynamic and heart rate reflex responses to propofol in the rabbit. Comparison with althesin.

Propofol was administered to eight rabbits by constant i.v. infusion at 0.2, 0.4 and 0.6 mg kg-1 min-1 to produce light sedation. The lowest dose was compared with an infusion of Althesin 0.1 mg kg-1 min-1. The rabbits had been previously implanted with aortic and vena caval perivascular balloon cuffs to examine the baroreceptor-heart rate reflex and an aortic thermistor catheter for cardiac output (CO) measurements. A silastic catheter was placed in the pericardial sac so that the cardiac nerves could be blocked with local anaesthetic. Mean arterial pressure (MAP) was well maintained with all anaesthetic infusions. At the lower doses of propofol, CO increased by 20% (P less than 0.01) with a corresponding decrease in total peripheral resistance (TPR). However, CO and TPR were not changed significantly by Althesin. Both anaesthetics induced a similar tachycardia. Cardiac nerve blockade did not abolish the different CO and TPR responses observed for the two agents. A dose-related reduction in the range and gain of the baroreceptor-heart rate reflex was observed with propofol (P less than 0.05). The pattern of alteration of the reflex curve, however, differed between the two anaesthetics and the vagal efferent component was more resistant to blockade with propofol. The relative preservation of baroreceptor reflex responses, and the reduction in TPR by a reduction of resting constrictor tone, suggest propofol may have significant clinical advantages when used as a sedative infusion.