ANALYSIS OF THE INITIATION OF SPONTANEOUS MONOMORPHIC VENTRICULAR-TACHYCARDIA BY STORED INTRACARDIAC ELECTROGRAMS

Objectives. This study was designed to analyze stored intracardiac electrograms generated during spontaneous monomorphic ventricular tachycardia to examine the possible mechanisms responsible for the initiation of ventricular tachycardia in a group of postinfarction patients. Background. Implantable cardioverter-defibrillators capable of storing electrograms during an arrhythmic event provide an intracardiac electrogram analog to Holter ambulatory electrocardiographic monitoring. Such electrograms are of value in arrhythmia diagnosis and in determining the appropriateness of implantable cardioverter-defibrillator therapy and may aid in understanding the initiation of ventricular arrhythmias. Methods. We studied 73 stored electrograms in 22 postinfarction patients with spontaneous monomorphic ventricular tachycardia. Premature depolarizations before tachycardia were classified by morphology and number. Electrogram morphology was compared with the morphology of the baseline rhythm and ventricular tachycardia. Prematurity was assessed by the coupling interval and a calculated prematurity ratio. Results. During baseline rhythm, ectopic activity was present in 30 (41%) of 73 stored episodes. Ventricular tachycardia was preceded by a short-long-short sequence in 14% of episodes and by a rapid ventricular rhythm in 5.5% of episodes. The onset of ventricular tachycardia was marked by single premature depolarizations in 33 episodes (45%), by pairs in 16 (22%) and by multiple complexes in 24 (33%). Morphology was similar to that of the ensuing tachycardia in 35 episodes (48%). The mean coupling interval was 364 ms, and the mean prematurity ratio was 0.56. In all 10 episodes (14%) where the prematurity ratio was <0.40, a short-long-short sequence was responsible. When classified by morphology, the mean prematurity ratio of depolarizations dissimilar to ventricular tachycardia (0.53) was significantly less than that of the morphologically similar group (0.60, p = 0.035). Conclusions. In this select group of postinfarction patients with recurrent sustained monomorphic ventricular tachycardia treated with implantable cardioverter-defibrillators, ventricular tachycardia was most often preceded by late-coupled premature depolarizations. Not infrequently, a short-long-short sequence occurred before tachycardia. Premature depolarizations with a morphology different from that of the tachycardia occurred earlier in the cardiac cycle than did those with a morphology similar to that of the tachycardia. These findings may reflect different mechanisms of ventricular tachycardia initiation.