Spontaneous compared with induced onset of sustained ventricular tachycardia

Electrophysiological characteristics of the modes of initiation of 16 episodes of spontaneously occurring sustained ventricular tachycardia recorded in 16 patients by two-channel ambulatory electrocardiographic monitoring were compared with the characteristics of ventricular tachycardia induced by programmed electrical stimulation. Eleven episodes of spontaneous ventricular tachycardia began after a single ventricular premature depolarization (VPD), three episodes after two VPDs, and two episodes after five VPDs. By comparison, only four episodes of sustained ventricular tachycardia were induced with a single VPD. Each episode of spontaneous ventricular tachycardia was initiated by a late coupled VPD (RR':QT ratio greater than 1.0). The VPD was often morphologically similar to the ensuing ventricular tachycardia (eight of 11 episodes that began after a single VPD). No correlation was found between the modes of initiation of spontaneous and induced ventricular tachycardia. We hypothesize that concealed decremental slow conduction, reflected in the long coupling intervals of VPDs initiating ventricular tachycardia, is of critical importance in initiating ventricular tachycardia. We conclude that major differences exist in the timing and number of VPDs associated with the onsets of spontaneous and induced sustained ventricular tachycardia.