ACTIVATION TIMES IN AND ADJACENT TO REENTRY CIRCUITS DURING ENTRAINMENT - IMPLICATIONS FOR MAPPING VENTRICULAR-TACHYCARDIA

Myocardial infarct scars giving rise to reentrant ventricular tachycardia can contain ''bystander'' areas of abnormal electrical activity that are difficult to distinguish from reentry circuit sites. Pacing to entrain ventricular tachycardia with analysis of electrograms at the pacing site is useful to identify reentry circuit sites but assumes that electrograms reflect activation times at the recording site. The purpose of this study was to determine whether a similar analysis could be applied to electrograms recorded from sites distant from the pacing site. In computer simulations, activation times at sites in and adjacent to figure-eight reentry circuits were analyzed during entrainment of tachycardia by pacing at various sites. During entrainment, activation at reentry circuit sites activated by the stimulated orthodromic wavefronts maintains the same relation to the QRS complex as that during tachycardia. The return cycle from the last entrained electrogram to the following electrogram equals the tachycardia cycle length. The same findings occur, however, at bystander sites activated by stimulated wavefronts that have propagated orthodromically through the circuit. When a reentry circuit site is activated by stimulated antidromic wavefronts, the electrogram to QRS interval is shorter than that during tachycardia, the return cycle may be less than the tachycardia cycle length, and the site may appear to be dissociated from the tachycardia, despite its location in the circuit. If the entrained electrogram to QRS interval exceeds the tachycardia electrogram to QRS interval and the return cycle length exceeds the tachycardia cycle length, it is likely that both pacing and recording sites are outside the reentry circuit. Thus, during entrainment, failure to dissociate an electrogram from the QRS complex and the return cycle length does not reliably indicate the relation of the recording site to the reentry circuit when the recording and pacing sites are separate.