Demonstration of phase-3 and phase-4 retrograde block in a second concealed accessory pathway after an initial successful radiofrequency ablation of a 'normal' concealed accessory pathway

We report a patient with concealed Wolff-Parkinson-White syndrome who, foll owing catheter ablation, demonstrated phase-3 and phase-4 retrograde block in a concealed accessory pathway. After an initial 'apparently successful' ablation, retrograde conduction was through the atrioventricular node durin g constant ventricular pacing. Ventricular extrastimulus testing was perfor med at a basic drive cycle length of 600 ms. Unexpectedly, ventricular extr astimuli at coupling intervals of 440-380 ms were conducted retrogradely ov er an accessory pathway, consistent with a phase-3 and phase-4 retrograde b lock in the accessory pathway. Residual accessory pathway conduction was el iminated in a single ablation session.