Effect of radiofrequency and ethanol ablation on epicardial conduction through the vein of Marshall: How to detect and manage epicardial connection across the mitral isthmus

BACKGROUND The vein of Marshall (VOM), which is surrounded by the Marshall bundle (MB), behaves as an epicardial connection bypassing the mitral isthmus. The influence of radiofrequency abla-tion and VOM ethanol infusion (VOM-EI) on epicardial MB conduc-tion remains unclear. OBJECTIVE The purpose of this study was to evaluate MB conduc-tion status during mitral isthmus ablation. METHODS Of 57 consecutive patients undergoing mitral isthmus ablation, 50 with electrode catheter cannulation into the VOM were analyzed. MB conduction was investigated by evaluating electrograms inside the VOM. Endocardial ablation was initially per-formed, followed by ablation inside the coronary sinus (CS), if required. Selective VOM-EI was performed if the MB potentials still exhibited early activation after radiofrequency ablation, suggesting the presence of MB connection bridging the mitral isthmus. RESULTS VOM electrograms composed of near -field MB and far -field left atrial potentials were recorded in all patients. Solely with endocardial ablation, 33 patients (66%) achieved entire mitral isthmus block, and 43 patients (86%) achieved an epicardial MB conduction block. MB potentials exhibited early activation in the remaining 7 (14%), even after requiring CS ablation. VOM-EI then was performed. Elimination of MB potentials was verified by elec-trode catheter reinsertion after VOM-EI. Mitral isthmus conduction was successfully blocked during VOM-EI in 4 patients and during additional radiofrequency ablation in the remaining 3. All patients finally achieved entire mitral isthmus block. CONCLUSION MB is effectively ablated by radiofrequency ablation. Continuous evaluation of MB conduction can reveal epicardial conduction and ablation effect. A residual MB epicardial connection is relatively rare but can be ablated by VOM-EI.