Alternate ventriculo-atrial Wenckebach conduction during ventricular tachycardia

Although pacing-induced ventriculo-atrial (VA) Wenckebach conduction has been previously described, the occurrence of this phenomenon during ventricular tachycardia has received little attention. The latter is defined as 2:1 VA block in which the conducted beats show progressive lengthening of VA conduction until the sequence is terminated by two or three blocked ventricular beats. This phenomenon was observed in a 16-year-old boy who underwent electrophysiologic study for ventricular tachycardia as a late complication of surgical correction of tetralogy of Fallot. During pacing-induced ventricular tachycardia with a morphology similar to that of the spontaneous tachycardia, 8:4 alternating VA block was observed. This sequence suggested that the AV node was the site of block, the 2:1 block being located at the upper level, and the VA Wenckebach block at the lower level. Alternate VA Wenckebach conduction appears as a possible cause of variation in atrial depolarization intervals during ventricular tachycardias with short cycle lengths.