d‐Sotalol and Atrial Reentry.Introduction:In the chronically instrumented animal and the isolated blood perfused heart, atrial reentry via a fixed path around an anatomical obstacle has been described and is terminated by the Class III antiarrhythmic agent, d‐sotalol. The precise mechanism by which d‐sotalol terminates this arrhythmia is not known.Methods and Results:In the present study, right atrial (RA) activation sequences in the isolated, coronary artery perfused canine heart during episodes of sustained flutter (n = 7) and drug administration were determined. A fixed array of bipolar electrodes was used to record endocardial electrograms from 96 sites on the RA simultaneously. Maps of all control flutters showed that the rhythm was due to persistent circus movement of the impulse around the tricuspid valve ring. D‐sotalol was effective in terminating atrial reentry in this model. In all episodes, block of the excitatory impulse in a specific region of the reentrant circuit accompanied these terminations. However, the events preceding the occurrence of block of the impulse were not similar. Two different modes of termination are described.Conclusion:The Class III antiarrhythmic agent d‐sotalol can terminate atrial reentry in several ways. In one mode, complete conduction block of he reentering impulse within the fixed path occurs to terminate the rhythm. In the other mode, interruption of the original reentrant circuit occurs when there is failure of a lateral boundary. Often in this latter case, interruption of the original circuit is by an extra impulse that is secondary to a change in the path of the impulse. In both modes cycle length oscillations are
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