Abstract Funding Acknowledgements Type of funding sources: None. Background Achieving bidirectional mitral isthmus block during radiofrequency (RF) ablation for persistent atrial fibrillation (AF) is still challenging. The conventional ablation method involves RF applications on the endocardial aspect of the Mitral Isthmus (MI), and for a majority of patients, in the distal coronary sinus (CS). Purpose We have evaluated the acute success of obtaining mitral isthmus block by adding another epicardial component using ethanol infusion in the vein of Marshall (EIVOM) in addition to endocardial MI and epicardial CS ablation. Methods We studied 121 patients (pts.) with a mean age of 65 years (range 40-83) 73% men; 119 with longstanding persistent AF (98%) and 2 with perimitral flutter (2%). The mean duration of AF was 53 months (12-244 months). In the majority of patients, additional endocardial (on the ventricular aspect of the MI) and/or epicardial (distal CS) (RF) ablation was performed in order to achieve MIB. The ablation procedure was performed under general anesthesia (GA) for 81 pts (67%). EIVOM was perform with a mean 6 ml ethanol (range 2-10ml) Results Bidirectional MIB was obtained in 114 pts. (94,2%). The 7 patients without MIB were scheduled for another ablation procedure (4 pts under GA during the first procedure). The average RF delivery time to block was 160 seconds (range 42-480 seconds) for the endocardial MI RF ablation (point-by-point application with a power of 50W and an Ablation Index of 450-500, contact force 10-20g) and 156 seconds (range 55-438) for the epicardial MI RF ablation (applications with a power of 20W). Bidirectional endocardial and epicardial MIB was confirmed by conventional pacing maneuvers performed in sinus rhythm. No major complications were observed. The parameters associated with failure for MIB were AF duration, Left Atrial dilatation >200 ml, MI thickness (epicardial endocardial distance on the CARTO maps >15mm). Conclusion Ethanol infusion in the vein of Marshall is a safe approach and is associated with a higher success rate of obtaining acute bidirectional endocardial and epicardial mitral isthmus block when compared with the conventional method. Abstract Figure. Bloc Mitral Endo; Bloc Mitral Epi;