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 (LAA) 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 potential in the CSd during LAA pacing, among whom 80.9% (55/68) presented with an earlier high-frequency near-field potential (NFP) followed by a low-frequency far-field potential (FFP), suggesting an epicardial gap, whereas 19.1% (13/68) presented with FFP followed by NFP, suggesting an endocardial gap. In patients with single-potential in CSd (16.0%, n = 13), simple activation mapping of the endocardium and CSd revealed the gap location. Intra-CS 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 min for ablation. Eventually, 95.7 % (179/187) of the patients achieved MI blockage. These findings were confirmed in an external validation cohort, which demonstrated the effectiveness and efficiency of CSd potential-guided gap identification. Conclusion: The characteristics of the electrograms in the CSd could aid in the prompt identification of the gap(s) location of the MI in patients with unblocked MI conduction. Key words: Atrial fibrillation; Catheter ablation; Mitral isthmus; Vein of Marshall

The authors have declared no competing interest.

This work was supported by the National Natural Science Foundation of China (No. 82300349 and No.81970272).

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