A Left Free‐Wall, Decrementally Conducting, Atrioventricular (Mahaim) Fiber: Diagnosis at Electrophysiological Study and Radiofrequency Catheter Ablation Guided by Direct Recording of a Mahaim Potential

A 64‐year‐old female with Wolff‐Parkinson‐White syndrome and an ECG demonstrating a rigbt posterolateral accessory pathway was referred for electrophysiological study. During electrophysiological testing two AV pathways were identified: a right posterolateral pathway that displayed conventional electrophysiological properties: and a left free‐wall pathway tbat conducted only anterogradely and demonstrated decremental properties. Two separate wide complex tachycardias were induced that utilized the left free‐wall pathway anterogradely and either the AV node or the right posterolateral accessory pathway retrogradely. A discrete electrical potential on the free wall of the mitral an‐nulus was identified during tachycardia and was utilized to facilitate mapping and ablation.