Ventricular tachycardia: an unusual pacemaker-mediated tachycardia.

A 74 year old man had recurrent ventricular tachycardia, which was well controlled with amiodarone, and complete heart block for which a VVI permanent pacing system had previously been implanted. After an elective increase in the programmed pacemaker rate from 70 to 82 beats/min, there was recurrence of frequent episodes of ventricular tachycardia. Each episode of tachycardia was initiated by a fusion beat consisting of a ventricular extrasystole and a paced beat. When the pacemaker rate was reprogrammed to 70 beats/min the episodes of tachycardia ceased abruptly. It is proposed that the fusion of a ventricular extrasystole with a pacemaker beat may have induced ventricular tachycardia, even though neither of these beats occurring separately was sufficient to cause this.