Abstract 1. Introduction Pulmonary vein isolation (PVI) is the cornerstone of AF treatment. An accurate estimation of the lesion size and a optimal evaluation of the catheter-myocardium coupling is crucial for achieving an effective and safe radiofrequency (RF) ablation. A few studies have suggested that local impedance (LI) drop is a better predictor than general impedance drop in terms of lesion size, electrical coupling and therefore acute PVI success. An open-irrigated ablation catheter designed to measure contact force (CF) and LI drop while applying RF has been recently developed. 2. Purpose The aim of this study is to evaluate if conduction GAPS are located in certain areas and if LI drop can predict the presence of conduction gaps in each of these segments. 3. Methods Observational prospective study including 55 consecutive patients who underwent PVI. The RF lesions were delivered with 30 W (posterior wall) or 35 W (anterior wall) for a maximum of 30 seg with a CF > 5 g. The applications were stopped if a LI of 35 Ohm was reached on the posterior wall or 40 Ohm on the anterior wall. PVs were divided into seven segments and ablation tags were assigned to a segment according to their anatomical position (figure 1) We categorized the tags included in each segment as successful or not according to the presence of conduction gaps detected with an Ultrahigh density mapping catheter once the WACA was completed. At the same time lesion by lesion were studied regardless of the segment where it was located. 4. Results We analyzed 4002 lesion tags of 55 patients who underwent PVI. 53 Gaps were found in 19/55 (34.5%) left WACA and 16/54 (29,6%) right WACA and were localized predominantly in three PVI segments (L1, L6 and R3) 34(54.85%) vs 19 (35,8%) (L2-L5, L7, R1-R2, R4-R7) (p<0.001) (figure 1). They were mainly circumscribed to anterior segments in left PVs and posterior-superior segment in right PVs (figure1). When examining lesions independently of their location we found a significant difference in LI drop among successful and unsuccessful lesions (24.22±8.31 vs 15.67±7.85) Ohm, p<0.001). In the same line when studying lesions in those segments with > 10% of GAPS (L1, L6 R1 and R3) LI drop was greater in successful lesion tags: L1 25.2 ± 5.35 vs 16.8 ± 5.17 (p<0.001), L6 25.2±5.7 vs 16±5.48 (p<0.001), R1 27.9±4.78 vs 17.1±5.73 (p=0.03) and R3 18.5±5.4 vs 10.7±2.49 (p<0.001). 5. Conclusions Gaps were located predominantly in anterior segments in left PVs and posterior-superior segment in right PVs. A low LI drop was an excellent predictor of conduction gaps, and it performs well regardless of the segment where it was analyzedFig. 1