Variant Preexcitation Syndrome: A True Nodoventricular Mahaim Fiber or an Accessory Atrioventricular Pathway with Decremental Properties?

INTRODUCTION: The differentiation between a nodoventricular fiber and an accessory atrioventricular (AV) pathway with long conduction times and decremental properties could be very difficult even at detained electrophysiologic study. METHODS AND RESULTS: A 20-year-old male with a history of a wide QRS tachycardia underwent electrophysiologic study. Baseline intervals were normal. There was evidence of dual AV pathways, and a sudden increase in AH interval was associated with the emergence of a delta wave. The atrio-delta interval showed a progressive prolongation. The preexcited QRS complex was typical of a posteroseptal pathway, and the earliest ventricular activation site was recorded at the posteroseptal region. Retrograde conduction was exclusively over the normal conduction system. During ventricular extrastimulation, a sudden increase in HA interval was associated with anterograde conduction over the accessory pathway. The intervals between the stimulus artifact and the onset of the delta wave during atrial pacing from two atrial sites (S-Delta) were compared with those between the retrograde atrial electrogram on the His channel and the onset of the delta wave during ventricular pacing (A2HB-Delta). When pacing from the proximal coronary sinus, the shortest S-Delta interval did become shorter than the longest A2HB-Delta interval (155 vs 170 msec). CONCLUSION: The finding that the S-Delta interval could become shorter than the A2HB-Delta interval provides strong evidence that this accessory pathway was not connected to the AV node but arose directly from the atrial tissue of the posteroseptal region.