Characteristics of the ventricular insertion sites of accessory pathways with anterograde decremental conduction properties.

Background: Accessory pathways (APs) with anterograde decremental conduction properties referred to as Mahaim fibers have recently been recognized as originating from the right lateral atrium. Little information is available about their distal insertion. The purpose of this study was to determine the different kinds of APs involved and the characteristics of their distal insertion site.

Methods And Results: Twenty-one patients (mean age, 28 +/- 13 years) with reciprocating tachycardia or atrial fibrillation were studied. Right-sided atrial and/or ventricular endocardial mapping during tachycardia identified different types of APs. (1) Seventeen patients had long APs originating from the right lateral atrium and coursing several centimeters to the right ventricle. In 10 patients, the AP terminated in or close to the right bundle-branch system (atriofascicular AP) and in 7, the AP terminated in the anterior right ventricle (atrioventricular AP). Patients with atriofascicular APs had narrower QRS complexes (133 +/- 10 versus 165 +/- 26 milliseconds, P = .02) and narrower initial r wave in leads V2 through V4 during maximal preexcitation than patients with atrioventricular APs. In addition, they had earlier His-bundle and right bundle-branch retrograde activation, ie, shorter V-His (16 +/- 5 versus 37 +/- 9 milliseconds, P < .01) and V-right bundle intervals (3 +/- 5 versus 25 +/- 6 milliseconds, P < .01). In 6 patients, minimal preexcitation not readily apparent was present in sinus rhythm despite the appearance of a narrow QRS complex. A wide distal insertion site of 0.5 to 2 cm in diameter consistent with arborization of the AP was found in 10 patients. The distal application of radiofrequency current produced a change in the preexcitation pattern in 4 patients and ablated the AP in 2 patients. In the other patients, radiofrequency current was applied more proximally and successfully ablated the AP bundle (n = 9) or AP proximal insertion (n = 6). No recurrence was observed during a follow-up period of 12 +/- 10 months. (2) Four patients had short paratricuspid atrioventricular APs; in one, the decremental conduction property was acquired as demonstrated by two electrophysiological studies performed 7 years apart. Radiofrequency ablation was successfully accomplished in all 4 patients at the tricuspid annulus.

Conclusions: Different types of APs account for tachycardias previously called Mahaim fibers. Long and short atrioventricular APs are observed in 81% and 19%, respectively. Long APs often have a distal arborization and may have either a fascicular or ventricular insertion. Radiofrequency current is more efficient when applied to the AP bundle or AP proximal insertion rather than to the distal insertion in patients with long APs.