[QRS and cycle length alternans during atrioventricular nodal reentry tachycardia: case report].

A 23-year old man with paroxysmal, poorly tolerated and spontaneously terminating palpitations, was referred to our department for electrophysiological study. Burst pacing from high right atrium using a cycle length of 350 ms induced a slowfast atrioventricular nodal reentry tachycardia. We excluded the presence of accessory atrioventricular tracts. During tachycardia a constant alternans of QRS morphology and cycle length was observed. The cycle length alternans could be due to the presence of three nodal pathways and activation circulating in a figure-of-eight pattern using alternatively two slow pathways as the antegrade arm of the reentry loop. The alternans could also originate from altering decremental properties of a single slow pathway that changed its conduction properties with relation to the length of the preceding cycle. The QRS alternans occurred both during burst pacing and tachycardia. As it was related to the rate and the changing cycle length, we concluded that it could reflect aberration in intraventricular conduction.