Observations on rapid ventricular pacing during reentrant ventricular tachycardia in canine myocardial infarction--the use of epicardial mapping in demonstrating the modes of resetting, concealed perpetuation and termination of reentry.

The changes in activation sequences in and around the epicardial reentrant circuit were analyzed during rapid pacing against ventricular tachycardia by using a canine infarction model. In 13 episodes of sustained ventricular tachycardia induced by a premature stimulation technique in eight dogs, reentry circuits were located in the superficial subepicardial myocardium in eight. Pacing stimuli were clearly demonstrated to enter the reentrant circuit in two directions: one wavefront collided with the orthodromic reentrant wavefront and the other entered prematurely into the reentrant circuit to reset the tachycardia (resetting phenomenon). With a faster pacing rate, stimuli failed to reset the tachycardia due to slower entry into the circuit despite the fact that most epicardial recording sites were activated by pacing wavefronts (concealed perpetuation). Termination of tachycardia was achieved by a local conduction block in the center of the reentrant circuit. A pacing impulse which encountered the local block was also shown to reinitiate the tachycardia using a different reentrant pathway. These different phenomena could be observed in consecutive pacing beats. These epicardial mapping data provided a direct electrophysiological basis for the mechanisms of reentrant ventricular tachycardia and the mode of its termination.