Fixed preexcitation during decremental atrioventricular conduction. What is the mechanism?

Fasciculoventricular pathway (FVP) is a rare form of preexcitation syndrome. A FVP connects the fascicle and the ventricle. It is indistinguishable to the common preexcitation pathways on the 12‑lead electrocardiography. We herein present a case of FVP, who was with the same His-ventricular interval and QRS morphology during sinus rhythm, junctional beats and rapid atrial pacing rhythm. Using a mapping catheter at the right ventricular anterior (RV) wall, we can clearly demonstrate the activation of RV with and without FVP conduction. With FVP conduction, the RV was activated from basal wall to apex. Without FVP conduction, the activation direction was reversed.