Differential Diagnosis of Regular Narrow Complex Rhythm During Catheter Ablation for Atrioventricular (AV) Nodal Reentrant Tachycardia

Differentiating atrioventricular nodal reentrant tachycardia (AVNRT) from non-reentrant junctional tachycardia during ablation is essential to avoid unnecessary catheter ablation and to minimize the risk of complete atrioventricular (AV) block. We report the case of a 51-year-old woman with symptomatic narrow-complex tachycardia who was referred for electrophysiological study and ablation. Typical AVNRT was induced and ablated successfully at the slow pathway region. Shortly afterward, a spontaneous regular narrow-complex rhythm (NCR) emerged, mimicking recurrent AVNRT. Careful analysis showed that premature atrial complexes (PACs) advanced the His potential without interrupting the NCR, suggesting a junctional rhythm rather than a recurrence of AVNRT. Overdrive pacing from the proximal coronary sinus produced an A-H-H-A response, confirming the junctional origin. This case highlights the diagnostic challenge of distinguishing recurrent AVNRT from junctional rhythm after ablation, as junctional rhythms may masquerade as recurrent AVNRT during postablation monitoring; therefore, careful interpretation of key maneuvers, including the response to PACs and overdrive pacing, is crucial for accurate diagnosis, prevention of unnecessary repeat ablation, and reduction of AV block risk.