Mahaim fibers: electrophysiologic characteristics and radiofrequency ablation.

Mahaim fibers that exhibit decremental atrioventricular (AV) node-like conduction properties are unusual, comprising < 3% of accessory pathways. They are involved in antidromic AV reciprocating (Mahaim) tachycardia with a left bundle branch block morphology. There is a frequent association of Mahaim pathways with Ebstein's anomaly, additional AV accessory pathways (Kent) and dual AV nodal pathway conduction. The majority of these pathways are long right atriofascicular pathways capable of only anterograde conduction. Radiofrequency (RF) ablation of right atriofascicular pathways guided by a distinct Mahaim potential recorded at the anterolateral-to-posterolateral tricuspid annulus or in the right ventricular free wall is safe and highly effective. True nodoventricular or nodofascicular Mahaim pathways usually also capable of only anterograde conduction appear to be rare and are associated with pre-excited tachycardia of left bundle branch block morphology not distinguishable from the preexcitation pattern of right atriofascicular pathways. In these pathways, a reliable method for mapping and safe RF ablation is lacking because no Mahaim potential could be recorded from the tricuspid annulus in the right midseptal region where ablation may result in elimination of both the slow AV nodal pathway and Mahaim conduction. Fasciculoventricular Mahaim pathways have never been reported to be involved in a tachycardia circuit. At our institution, successful RF ablation of unusual Mahaim pathways (concealed nodoventricular and nodofascicular and left anterograde atriofascicular) was performed without impairing the normal AV conduction system.