Effects of Halothane, Enflurane, and Isoflurane on the SA Node

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Abstract
The influence of the anesthetics halothane (HA), enflurane (EF) and isoflurane (IF) on the electrophysiologic properties of single sinoatrial (SA) node cells of the guinea pig was studied. Isolated, spontaneously beating SA node superfused with modified Krebs'' solution was used. Effects on spontaneous rate of discharge of the SA node cell, action potential amplitude (AAP), phase 4 dV/dt, phase 0 dV/dt, action potential duration (ADP50), overshoot, maximum diastolic potential (MDP) and threshold potential (TP) were analyzed. The interaction of increased extracellular Ca ion concentration with HA, EV and IF (1 and 2 MAC [minimum anesthetic concentration]) was studied. Anesthetic concentrations in the superfusing bath were measured using a gas chromatography procedure. Introduction of anesthetics (1 and 2 MAC) for 5 min produced a significant decrease in heart rate, slope of the phase 4 and phase 0 and ADP50. At concentrations of 2 MAC, AAP and its overshoot were significantly decreased. There was no change in TP, and the MDP was decreased only with 2 MAC HA. Increasing the extracellular Ca concentration counteracted the negative chronotropic effects of HA, EF and IF. Addition of the Ca channel blocker verapamil potentiated the negative chronotropic effect of HA. At equipotent doses the interaction of these anesthetics with Ca does not appear to be competitive and Ca did not completely overcome the direct negative chronotropic effect of these anesthetics.