Very slow atrioventricular nodal reentry masquerading as junctional rhythm: Diagnostic challenges and mechanistic insights.

Background: Very slow atrioventricular nodal reentrant tachycardia (AVNRT) can mimic nonreentrant accelerated junctional rhythm (JR) or junctional tachycardia (JT).

Objective: This study aimed to describe the clinical features and mechanism of slow AVNRT masquerading as accelerated JR or JT (collectively called JRs).

Methods: We reviewed all AVNRT and JT ablations performed at Ascension St. Vincent Hospital, Indianapolis, from July 2016 to June 2024. Patients with a tachycardia cycle length (TCL) of ≥500 ms were analyzed, focusing on clinical characteristics and electrophysiological findings.

Results: Among 556 AVNRT ablations, 22 patients had TCL of ≥500 ms. Nine were clinically diagnosed as having JR based on 1 or more of the following: initiation with junctional beats (4), an apparent accelerating JR merging into sinus rhythm (3), slow tachycardia rates at 70-110 beats/min (8), and incessant slow tachycardia with simultaneous atrial and ventricular activation identified on implantable cardioverter-defibrillator interrogation (1). Five of these 9 patients with JR diagnosis also had a supraventricular tachycardia diagnosis. During electrophysiology study, His-refractory premature atrial complex perturbed the next His signal in all 22 cases, confirming AVNRT. The mean TCL was 578 ms (range, 500-670 ms). Two patients each had initiation with junctional beats and isorhythmic dissociation with sinus rhythm at onset of AVNRT. Radiofrequency ablation successfully modified the slow pathway and eliminated AVNRT in 21 of 22 patients.

Conclusion: Clinically apparent JRs are often very slow AVNRT in patients referred for ablation. The mechanism of AVNRT initiation with isorhythmic dissociation with sinus rhythm may be concurrent JR and AVNRT or 2-for-1 atrioventricular nodal conduction and AVNRT competing with sinus rhythm.