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

Objectives. This study was designed to analyze stored intracardiac ele ctrograms generated during spontaneous monomorphic ventricular tachyca rdia to examine the possible mechanisms responsible for the initiation of ventricular tachycardia in a group of postinfarction patients. Bac kground. Implantable cardioverter-defibrillators capable of storing el ectrograms during an arrhythmic event provide an intracardiac electrog ram analog to Holter ambulatory electrocardiographic monitoring. Such electrograms are of value in arrhythmia diagnosis and in determining t he appropriateness of implantable cardioverter-defibrillator therapy a nd may aid in understanding the initiation of ventricular arrhythmias. Methods. We studied 73 stored electrograms in 22 postinfarction patie nts with spontaneous monomorphic ventricular tachycardia. Premature de polarizations before tachycardia were classified by morphology and num ber. Electrogram morphology was compared with the morphology of the ba seline rhythm and ventricular tachycardia. Prematurity was assessed by the coupling interval and a calculated prematurity ratio. Results. Du ring baseline rhythm, ectopic activity was present in 30 (41%) of 73 s tored episodes. Ventricular tachycardia was preceded by a short-long-s hort sequence in 14% of episodes and by a rapid ventricular rhythm in 5.5% of episodes. The onset of ventricular tachycardia was marked by s ingle 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 couplin g 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 me an prematurity ratio of depolarizations dissimilar to ventricular tach ycardia (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 ventricu lar tachycardia treated with implantable cardioverter-defibrillators, ventricular tachycardia was most often preceded by late-coupled premat ure depolarizations. Not infrequently, a short-long-short sequence occ urred 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 tachyca rdia. These findings may reflect different mechanisms of ventricular t achycardia initiation.