Clinical impact of ethanol infusion into the vein of Marshall on the mitral isthmus area evaluated by atrial electrograms recorded inside the coronary sinus

Background: The left atrial myocardium (LAM) and coronary sinus (CS) musculature (CSM) generate atrial electrograms recorded inside the CS (AECSs). The vein of Marshall (VOM) courses the mitral isthmus (MI), and ethanol infusion into the VOM (EI-VOM) is useful to ablate it. However, its detailed effect on the MI, which contains the LAM, CSM, and those connections, is unknown. Objective: The purpose of this study was to investigate the impact of EI-VOM on the MI by assessing the AECS. Methods: Eighty-four consecutive patients with atrial fibrillation undergoing MI ablation with successful EI-VOM were included. After EI-VOM, radiofrequency (RF) catheter touchup ablation was performed at MI gap sites or inside the CS (RFCS), as needed, to achieve bidirectional conduction block. Ablation effects on AECSs were evaluated during the MI ablation procedure. Results: AECSs demonstrated double potentials consisting of low-amplitude LAM components and high-amplitude CSM components in 31 patients (37%). Of those patients, 21 had a distal-to-proximal activation sequence of the LAM along with a proximal-to-distal activation sequence of the CSM during left atrial appendage pacing, suggesting CSM isolation from the LAM due to electrical LAM–CSM disconnection. Only 2 of the 21 patients required RFCS. The remaining 10 patients with distal-to-proximal activation in both CSM and LAM, suggesting incomplete CSM isolation and persistent LAM–CSM conduction, required RFCS. Overall, combined EI-VOM with RF created bidirectional conduction block at the MI in 78 patients (93%). Conclusion: EI-VOM can ablate the LAM and myocardial connections between the LAM and CSM. Careful assessment of AECSs can predict a requirement for RFCS.