Evidence of 2 conduction exits of the moderator band: Findings from activation and pace mapping study

BACKGROUND The moderator band (MB) is an endocavitary structure with only 2 exits to the bulk of the ventricular myocardium. Whether this may lead to specific electrophysiological characteristics remains unknown.OBJECTIVE The purpose of this study was to investigate electrocardiographic (ECG), activation, and pace mapping characteristics of MB-originated ventricular arrhythmias (VAs).METHODS Mapping and ablation of MB-VAs were performed in 12 patients under the guidance of a 3-dimensional electroanatomic mapping system and intracardiac echocardiography and ECG, and mapping data were analyzed. Of these patients, 11 underwent pace mapping study of 6 sites around the MB and the QRS morphology was compared.RESULTS The earliest activation site was free wall (FW) insertion in 8 patients (66.7%) and MB body in 4 patients (33.3%), preceding the QRS onset by 17.8 +/- 4.7 ms, and Purkinje-like potential was observed in 6 (50.0%). VAs were eliminated at the earliest activation site in the procedure, but recurrence was documented in 2 cases (16.7%) during a follow-up of 13.4 +/- 7.8 months. Pacing QRS complex from the MB was characterized by short QRS duration (P<.001), short intrinsicoid deflection time (P<.001), later precordial transition (P = . 025), and notch on the descending limb of the inferior leads (P<.001) as compared with pacing from the adjacent anterior-lateral FW, and that notch could also differentiate MB from the anterior papillary muscle (P = .027). However, pacing QRS is identical between the MB body and the FW insertion in 11 of 11 patients and between the septal insertion and the MB body in 7 of 11 patients.CONCLUSION Bidirectional conduction via the 2 exits during MBVAs contributed to distinct ECG and electrophysiological characteristics, while pace mapping is of limited value in defining the ablation target.