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Effect of Ablation Electrode Contact Force. Introduction: Ablation electrode–tissue contact has been shown to be an important determinant of lesion size and safety during nonirrigated ablation but little data are available during irrigated ablation. We aimed to determine the importance of contact force during irrigated-tip ablation. Methods and Results: Freshly excised hearts from 11 male pigs were perfused and superfused using fresh, heparinized, oxygenated swine blood in an ex vivo model. One-minute ablations were placed using one of 3 different power control strategies (impedance control—15 Ω target impedance drop, and 20 W or 30 W fixed power) and 3 different contact forces (2 g, 20 g, and 60 g) to give a grid of 9 ablation groups. The force sensing catheter (Tacticath™, Endosense SA) was irrigated at 17 mL/min for all of the ablations. Of a total 101 ablations, no thrombus formation was noted but popping was seen in 17 lesions. The lesion depth and incidence of pops was 5.0 ± 1.3 mm /0%, 5.0 ± 1.6 mm /10% and 6.7 ± 2.5 mm /45% for the 15 Ω, 20 W, and 30 W groups (P < 0.01), respectively, and 4.4 ± 1.8 mm /3%, 5.8 ± 1.6 mm /17% and 6.6 ± 2.0 mm /37% for the 2 g, 20 g, and 60 g groups, respectively (P < 0.01). The impedance drop in the first 5 seconds was significantly correlated to catheter contact force: 9.7 ± 9.9 Ω, 22.3 ± 11.0 Ω, and 41.7 ± 22.1 Ω, respectively, for the 2 g, 20 g, and 60 g groups (Pearson's r = 0.65, P < 0.01). Conclusion: Catheter contact force has an important impact on both ablation lesion size and the incidence of pops. (J Cardiovasc Electrophysiol, Vol. pp. 806-811, July 2010) [ABSTRACT FROM AUTHOR]