Importance of pacing site in entrainment of ventricular tachycardia.

Transient entrainment by pacing has been demonstrated during various tachyarrhythmias, including ventricular tachycardia. A patient is described who had two morphologically distinct forms of sustained ventricular tachycardia induced by programmed stimulation. Entrainment of both configurations of ventricular tachycardia was demonstrated. Evidence for entrainment included the presence of different degrees of fusion between paced and ventricular tachycardia complexes at different pacing cycle lengths, and the observation that the last entrained beat was always unfused and identical in configuration to the ventricular tachycardia complexes. Termination of ventricular tachycardia only occurred at pacing cycle lengths at which there was loss of fusion. Catheter endocardial mapping suggested a septal origin of both configurations of ventricular tachycardia. Demonstration of entrainment was dependent on pacing site, being seen only during pacing in the ventricle opposite from that showing earliest activation during ventricular tachycardia. Thus, when attempting to entrain ventricular tachycardia, multiple pacing sites in both ventricles should be used.