Antegrade conduction over a concealed accessory pathway after His bundle catheter ablation.

The existence of unidirectional retrograde preexcitation or concealed bypass fibers and their role in paroxysmal supraventricular tachycardia have been known for a long time. However, the exact anatomic and physiological nature of this retrograde limb remains unclear. We report here on a patient who had a concealed accessory pathway (AP) with circus movement tachycardia. After His bundle catheter ablation the AP had the property to conduct in the antegrade direction. The absence of antegrade conduction over the AP before ablation is likely due to its decremental properties of conduction and to repetitive retrograde concealed conduction from the normal pathway.