Prompt Identification of the Location of Gap Conduction in the Mitral Isthmus Following Vein of Marshall Ethanol Infusion and Endocardial Ablation

BACKGROUND: Mitral isthmus (MI) gap conduction is common despite ethanol infusion into the vein of Marshall (EI-VOM) and endocardial ablation of the MI. This study aimed to investigate the characteristics of electrograms of the distal coronary sinus (CSd) to guide the identification of the gap location in the MI. METHODS: A total of 187 patients who underwent EI-VOM and MI ablation were included in the study. After routine completion of EI-VOM and endocardial MI ablation, the characteristics of the electrogram in the CSd during left atrial appendage pacing were analyzed in unblocked MI conduction. RESULTS: Among the 187 patients, 43.3% (81/187) had unblocked MI following EI-VOM and linear lesion creation in the endocardium. In patients with unblocked MI, 84.0% (68/81) showed double potentials in the CSd during left atrial appendage pacing, among whom 80.9% (55/68) presented with an earlier high-frequency near-field potential followed by a low-frequency far-field potential, suggesting an epicardial gap, whereas 19.1% (13/68) presented with a far-field potential followed by a near-field potential, suggesting an endocardial gap. In patients with single potentials in the CSd (16.0%, n=13), simple activation mapping of the endocardium and CSd revealed the gap location. Intracoronary sinus ablation was necessary in 77.8% (63/81) of the patients, with a mean of 1.3±1.7 sites and 1.1±0.4 minutes of ablation. Eventually, 95.7% (179/187) of the patients achieved MI block. These findings were confirmed in an external validation cohort, which demonstrated the effectiveness and efficiency of CSd potential-guided gap identification. CONCLUSIONS: The characteristics of the electrograms in the CSd could aid in the prompt identification of the gap location(s) in the MI in patients with unblocked MI conduction.