Heart rate-systemic blood pressure relationship in dogs during halothane anesthesia

We have quantified the sensitivity and range of response of baroreflex-mediated heart rate changes during halothane anesthesia. Anesthesia was induced in 10 dogs with sodium thiopental (4 mg/kg i.v.), morphine sulfate (1 mg/kg), and succinylcholine (0.5 mg/min) was infused. After instrumentation, we infused sodium nitroprusside (0.1 mg/ml) or gave phenylephrine in divided doses (0.05 mg i.v.) at end-tidal halothane concentrations of 0.5%, 1.0%, and 2.0%. We changed systolic pressure over the range 6.5-26 kPa, measuring systolic pressure and heart rate responses at 1.3-2.6 kPa intervals. We found that the sigmoid curve relating heart period and systolic pressure was displaced leftwards, and the range of reflex response was significantly depressed (P < 0.001) at 2% halothane concentrations. The maximum slope of the curve was significantly depressed (P < 0.001) at increasing halothane concentrations. We conclude that halothane depresses the slope of the relationship between blood pressure and heart rate. However, the reflex response range is only depressed at high halothane concentrations, and the curve is displaced in the direction of lower heart rates at any given systemic pressure.