Residual AVNRT Despite Antegrade Slow-Pathway Block Unmasked by Ventricular Extrastimuli

BACKGROUND: Antegrade slow-pathway (SP) block is widely accepted as an endpoint for ablation of atrioventricular nodal reentrant tachycardia (AVNRT). However, whether it reliably excludes residual reentrant circuitry remains uncertain. CASE SUMMARY: A woman in her 40s with recurrent AVNRT and persistent left superior vena cava (PLSVC) underwent a third ablation procedure after 2 prior recurrences following catheter ablation. Sinus-rhythm activation mapping identified a pivot site targeted for SP modification. Although atrial extrastimulus testing demonstrated elimination of antegrade SP conduction after ablation, double ventricular extrastimuli induced AVNRT. Additional midseptal ablation eliminated inducibility, demonstrating persistence of a residual AVNRT circuit despite apparent antegrade SP block. DISCUSSION: This case demonstrates that antegrade SP block does not necessarily indicate elimination of the AVNRT circuit. In anatomically distorted nodal substrates, ventricular extrastimuli may access residual reentrant circuitry through pathways not assessed by atrial stimulation. Programmed ventricular stimulation may provide complementary information after SP ablation.