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

Abstract

We report a patient with concealed Wolff-Parkinson-White syndrome who, following 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 during constant ventricular pacing. Ventricular extrastimulus testing was performed at a basic drive cycle length of 600 ms. Unexpectedly, ventricular extrastimuli at coupling intervals of 440-380 ms were conducted retrogradely over an accessory pathway, consistent with a phase-3 and phase-4 retrograde block in the accessory pathway. Residual accessory pathway conduction was eliminated in a single ablation session.

A case of ventricular-atrial conduction near tachycardia rate via a concealed atrioventricular accessory pathway located anterior to the right pulmonary vein

“…The causes of this rare phenomenon are considered catecholamine-induced changes in the AP refractory period, pacing site–dependent conduction block, supernormal retrograde conduction, bradycardia-dependent block, 1 and phase 3 or phase 4 retrograde block. 2 In our case, further differentiation was difficult because we did not use catecholamines, change the RV pacing site, slow down the sinus rhythm, or evaluate VA conduction at slower rates before radiofrequency energy delivery.…”