Cycle length alternans during an atypical atrioventricular nodal re-entrant tachycardia utilizing a superior slow pathway

A 71-year-old man underwent electrophysiological testing for a supraventricular tachycardia (SVT) with cycle length (CL) alternans (Panel A). At baseline, there was no pre-excitation with normal atrio-Hisian and His-ventriculo intervals, two discontinuities in the anterograde atrioventricular conduction curve were observed, ventriculoatrial conduction was concentric and decremental, and para-Hisian pacing showed a nodal pattern (Panel B). Atrial extra-stimulation induced an SVT exhibiting alterations in short and long atrio-Hisian intervals and long His-atrial intervals with an atrial activation sequence identical to that during para-Hisian pacing. During the SVT, an atrial premature depolarization resulted in an atrio-Hisian interval prolongation and subsequent His-atrial interval shortening (Panel C). These findings suggested that the SVT should be an atrioventricular nodal re-entrant tachycardia (AVNRT) with alterations in anterograde fast pathway (FP) and typical slow pathway (SP) conduction and retrograde superior SP (sup-SP) conduction. Cycle length alternans during a typical AVNRT using an FP as the retrograde limb can occur due to two typical SPs. Our case is the first report demonstrating that a sup-SP could be utilized as the retrograde limb during an atypical AVNRT exhibiting CL alternans due to an FP and typical SP. The sup-SP was successfully ablated within the non-coronary cusp without FP injury, rendering ventriculoatrial conduction abolished and no tachycardia inducible (Panel D). The full-length version of this report can be viewed at: http://www.escardio.org/Guidelines-&-Education/E-learning/Clinical-cases/Electrophysiology/EP-Case-Reports.