Retrograde cycle length alternans during supraventricular tachycardia: an unusual tachycardia mechanism.

Cycle length alternans is occasionally seen during supraventricular tachycardia due to oscillations in the antegrade atrioventricular nodal (AVN) refractoriness. However, alternans due to retrograde variation in AVN conduction has not been reported. This report describes the case of a 36-year-old man with atypical AVN reentry tachycardia (AVNRT) whose episodes of tachycardia were characterized by continuous oscillations in retrograde AVN conduction. Ablation at one spot eliminated the tachycardia. Cycle length alternans due to oscillations in retrograde AVN conduction, although rare, can be seen during atypical AVNRT and should be considered.