Figure 8.

Electrograms show a wide complex tachycardia with a 1:1 V:A relationship with the earliest retrograde A occurring in the mid-CS, suggesting eccentric activation. This retrograde activation is identical to the decremental retrograde conduction seen during VEST. An early premature atrial contraction delivered from the A20 catheter in the lateral right atrium pre-excited the ventricle with an identical QRS morphology and reset the tachycardia, ruling out ventricular tachycardia. There was no advancement in the His A, septal A, ruling out nodoventricular tachycardia or AVNRT with a bystander AP. These findings confirm the participation of the pathway as the antegrade limb in this antidromic reciprocating tachycardia. The septal A was not advanced, making activation of the ventricle via a nodoventricular tract unlikely. These findings are consistent with the antegrade limb of the circuit being an atriofasicular (Mahaim) pathway. The A20 atrial electrogram suggests an oblique orientation of the pathway. Catheter positions are detailed in Figure 3.