Effects of halothane, enflurane, and isoflurane on the SA node

Abstract

The present experiments were carried out to study the influence of halothane, enflurane, and isoflurane on the electrophysiologic properties of single sinoatrial (SA) node cells of the guinea pig. The authors used the isolated, spontaneously beating guinea pig SA node superfused with modified Krebs' solution. These anesthetics were studied for their effects on spontaneous rate of discharge of the SA node cell, action potential amplitude, phase 4 dV/dt, phase 0 dV/dt, action potential duration, overshoot, maximum diastolic potential, and threshold potential. In addition, they have studied the interaction of increased extracellular calcium ion concentration with halothane, enflurane, and isoflurane (1 and 2 MAC). Anesthetic concentrations in the superfusing bath were measured using a gas chromatography procedure. Introduction of these anesthetics (1 and 2 MAC) for five minutes produced a significant decrease in the heart rate, decrease in the slope of the phase 4 and phase 0, and a decrease in the action potential duration. At concentrations of 2 MAC, these anesthetics produced a significant decrease in the action potential amplitude and its overshoot. There was no change in the threshold potential, and the maximum diastolic potential was decreased only with 2 MAC halothane. Increasing the extracellular calcium concentration counteracted the negative chronotropic effects of halothane, enflurane, and isoflurane. Addition of the calcium channel blocker, verapamil, potentiated the negative chronotropic effect of halothane. In the present study, a comparison of these anesthetics at equipotent anesthetic doses, indicated that their interaction with calcium does not appear to be competitive and calcium did not completely overcome the direct negative chronotropic effect of these potent anesthetics.