Abstract Body (Do not enter title and authors here): Background: Contemporary power-modulated radiofrequency (RF) ablation prioritizes power, force and time as variables impacting lesion formation. Targeting specific impedance values remains less well defined but may be important due its effect on current delivery.

Objective: To characterize 1) baseline unipolar RF impedances at PV ostia, and 2) the impact of baseline impedance (BI) and impedance change on procedure success.

Methods: We reviewed one hundred and seventy-five consecutive patients with paroxysmal or persistent atrial fibrillation (AF) undergoing first time RF ablation at our institution between 2020-2021, to accommodate at least one year of follow-up. Two Megadyne impedance patches were placed on the flank of all patients in a standard fashion. BI was measured at PV ostia under intracardiac echocardiography imaging and averaged across all sites. Patients were stratified into low and high impedance groups relative to the median.

Results: Patients with higher BI were slightly younger with larger BMI (p = 0.0001). Mean BI varied by anatomical site, with the roof, ridge, and posterior wall having higher BIs than anterior and inferior regions (125.7 ohms vs 122.8 ohms, p = 0.017). Regions with low BI had higher mean current values achieved (599 mA vs 559 mA, P = 0.000013). There was no significant difference in the BI between groups regarding one year recurrence or first pass isolation. However, the most frequent region requiring additional ablation was the right posterior wall, and there the percent impedance drop was the lowest at 4.87%.

Conclusion: Baseline pulmonary vein impedances are higher at the roof, ridge, and posterior wall. To achieve similar first pass isolation rates, these regions had larger absolute impedance drops resulting in similar percent impedance change. The region associated with worst first pass isolation rates had percent impedance drop <5%. Targeting percent rather than absolute impedance drop may improve atrial fibrillation success.