Delayed CAVB After SP Ablation for AVNRT: Mechanistic Insights Into FP Injury and Pacing Decision-Making

Abstract

As standard therapy for atrioventricular nodal re-entrant tachycardia (AVNRT), radiofrequency ablation of the antegrade slow pathway (SP) is associated with a low risk of AV block. Delayed complete AV block (CAVB), however, is a rare but clinically challenging complication, particularly concerning permanent pacemaker decision-making. A 79-year-old man with pre-existing first-degree AV block developed delayed CAVB 1 day after successful SP ablation for typical AVNRT. Transient PR interval shortening immediately post-ablation preceded progression to high-grade AV block. Subsequently, AV conduction gradually recovered without permanent pacemaker implantation, as confirmed using serial electrocardiography, treadmill exercise testing, and repeat electrophysiological studies. Electrophysiological evaluation showed impaired antegrade fast pathway (FP) conduction with complete absence of retrograde ventriculoatrial conduction, suggesting differential involvement of FP fibers. Delayed AV block after SP ablation may be reversible, particularly when preceded by PR interval shortening and when AV conduction improves under exercise or pharmacological stress. Careful functional assessment of AV conduction is needed before committing to permanent pacemaker implantation, as delayed AV block may reflect indirect and potentially reversible FP injury.

Keywords: PR interval shortening; atrioventricular nodal re‐entrant tachycardia; delayed atrioventricular block; fast pathway fibers; reversible AV block; slow pathway ablation.