Pacing site and bradycardia dependent retrograde conduction block over an atrioventricular accessory pathway.

Intermittent retrograde conduction over an accessory pathway (AP) is an uncommon phenomenon. We report a case of Wolff-Parkinson-White syndrome in which retrograde conduction of the bypass tract was present only during right ventricular (RV) outflow tract and left ventricular pacing or during RV inflow tract pacing with a fast pacing rate (pacing site and bradycardia dependent conduction block). The interaction between different branches/fibres of the bypass tract might explain the mechanism of this phenomenon. The AP was successfully ablated at the lateral aspect of mitral valve annulus. It is suggested that careful evaluation by decremental ventricular stimulation should be applied from multiple sites both during baseline study and after apparent AP ablation in order to detect such uncommon cases of intermittent conduction.