Alternating between exit sites of retrograde slow pathway during fast-slow atrioventricular nodal reentrant tachycardia: case report.

We report a case of a 57-year-old male with narrow QRS tachycardia exhibiting the alternance of the cycle length. Differential diagnoses include orthodromic atrioventricular reciprocating tachycardia with alternating antegrade atrioventricular (AV) nodal pathways, atrioventricular nodal re-entrant tachycardia (AVNRT) with alternating AV nodal pathways, and atrial tachycardia with alternating antegrade AV nodal pathways or with Wenckebach periodicity. In electrophysiological study the tachycardia showed alternance in the retrograde atrial conduction sequence and the cycle length. The alternation was caused by that of the HA intervals, between the shorter HA interval with the earliest atrial activation recorded in coronary sinus (CS), and the longer HA interval with that in His bundle region. The tachycardia was diagnosed with fast-slow form of AVNRT exhibiting the alternance of the earliest atrial activation sites. Electroanatomical 3D mapping further revealed that the exit site of retrograde slow pathway (SP) alternated between the left inferior extension (LIE) inside the CS, and the right inferior extension (RIE) in the posterior tricuspid annulus although among conventional electrode catheters the earliest site was the His bundle region. After ablation of the exit site of LIE, the alternation disappeared and fast-slow AVNRT showing a uniform retrograde atrial activation for which the earliest atrial activation site was the exit of RIE sustained. A single application of ablation at this point was insufficient, thereafter conventional SP ablation was added. Then, the ventriculoatrial conduction disappeared and no tachycardia was inducible even with isoproterenol administration. This case is followed by a review of the literature.