Effects of isoproterenol on bradycardia-dependent intra-His and left bundle branch blocks.

The electrophysiologic study of a patient with a history of fainting showed first- and second-degree Mobitz type I intraatrial and intra-His (IH) bundle blocks. Tachycardia and bradycardia-dependent IH block and bradycardia-dependent left bundle branch block were also present. Bradycardia-dependent block was probably caused by slight hypopolarization plus a slow rising slope of phase 4 depolarization and a shift of the threshold potential toward zero. Two months later a second electrophysiologic study was performed before, during and after administration of i.v. isoproterenol (IP). Shortening of atrium-His (AH1) and IH (H1H2) conduction time during faster heart rates caused by IP infusion may be related to its hyperpolarizing effect. Simultaneously, a shifting to the left of both bradycardia-dependent IH and left bundle branch block ranges was recorded during vagally induced cardiac slowing. These findings suggest that IP produces an increase in the slope of phase 4 depolarization of the His bundle and left bundle branch fibers and a simultaneous and concordant effect at both levels of the intraventricular conduction system.