A Wide Complex Tachycardia in a Structurally Normal Heart: What is the Mechanism? Where to Ablate?

We present a case of wide complex tachycardia (left bundle branch block, superior axis) in a 30-year-old female with a structurally normal heart and no baseline preexcitation on electrocardiogram. Tachycardia initiation and atrial pacing confirmed the existence and participation of a right-sided atriofascicular decrementally conducting Mahaim pathway. Tachycardia mechanism validation by atrial pacing maneuvers was documented as well as was a response to adenosine and preexcited Mahaim atrial fibrillation. The usual ablation site did not record a Mahaim potential; hence, ablation was performed at a site other than the usual one.