Immediate Block Is Not Stable Block: Early Mitral Isthmus Reconnection Despite Systematic Vein of Marshall Ethanol Infusion and Focal Pulsed Field Ablation With the Sphere-9™ Lattice-Tip Catheter

Background. Durable mitral isthmus (MI) block remains challenging in persistent atrial fibrillation (PeAF) ablation. Recent epicardial vein of Marshall (VoM) recordings have shown incomplete MI transmurality and time-dependent conduction recovery after pulsed field ablation (PFA). Whether systematic VoM ethanol infusion (VoM-EI) followed by focal PFA provides stable acute MI block remains unknown. **Objectives.** To assess the incidence, timing, and procedural implications of early MI conduction recovery after systematic VoM-EI followed by focal Sphere-9 PFA. Methods.In this prospective single-center study, 55 consecutive patients undergoing first ablation for symptomatic PeAF with planned MI ablation were screened. VoM-EI was systematically attempted before left atrial access and successfully performed in 51 (92.7%), who constituted the study cohort. Pulmonary vein isolation, roof-line, and MI ablation were performed with the Sphere-9? lattice-tip catheter. After bidirectional MI block, conduction was systematically reassessed during a standardized 30-minute waiting period. Results.Mean age was 70.3 {+/-} 8.2 years, and 36 patients (70.6%) were men. Initial bidirectional MI block was achieved in 50/51 patients (98.0%). During the waiting period, conduction recovered in 9/50 (18.0%; 95% CI, 9.8%-30.8%), at a median of 16 minutes (IQR, 10-20; range, 8?23). Six of 9 patients with recovery (66.7%) required targeted coronary sinus (CS) ablation. Block was restored in all 9, yielding a final block rate of 50/51 (98.0%). Median procedure duration was 82 minutes (IQR, 73-95), with no major complications. Conclusions. Immediate bidirectional MI block was not synonymous with stable block. Despite systematic VoM-EI followed by focal Sphere-9 PFA, conduction recovered in approximately one in five patients, including beyond 20 minutes, and two thirds required targeted CS ablation. These findings support standardized 30-minute reassessment and targeted CS interrogation rather than reliance on immediate block. Chronic invasive remapping is required to determine whether this strategy improves long-term MI block durability.