Abstract Background/Introduction Recent studies have shown that measuring local impedance (LI) at a single frequency can be useful for real-time characterization of radiofrequency (RF) lesion size during arrhythmia ablation procedures. (1) However, these systems do not take into account the effect of the increase in tissue temperature on the impedance measurement itself, which could be avoided by comparing the impedance at different frequencies. Purpose The aim of this study was to evaluate the ability of local multifrequency impedance (LMI) to characterize the size of RF lesions in real-time, while correcting for the thermal effect on the measurement. Methods Lesions were performed on the epicardium in 5 anesthetized pigs while measuring IML (from 1 to 209 kHz) before, during, and after ablations (20W/30W/40W, 60s, >10g) through a conventional irrigated electrocatheter connected to a cardiac navigation system and a RF generator. In these measurements, the thermal effect on the tissue was corrected by assuming that the changes in impedance at high frequency were only due to the temperature variation and not to the injury itself. Local voltage (Vuni) was also measured before and after ablations, and catheter-measured temperature and contact force were monitored. At the end of the procedure, the animals were sacrificed, and a histological study of the lesions was performed. Results A total of 43 effective lesions (11±2 per animal) were performed. The 40W lesions were deeper and wider than the 20W lesions (6,6 ± 3,3 mm vs. 5,4 ± 2,0 mm vs. 4,0 ± 1,9 mm; ANOVA p<0.05) and presented a significant decrease in unipolar voltage that was similar among the different lesions (40W vs. 20W: 56% vs. 40%; ANOVA p=ns; Figure). Four seconds after the start of the ablation the 40W lesions had a greater decrease in IML than the 30W and 20W lesions (73±14Ω vs. 85±4Ω vs. 100±12Ω; ANOVA p<0.05). After thermal adjustment, IML recorded 30 and 55 seconds after the start of the ablation correlated better with the final depth of the lesion than IL or Vuni (Table). Conclusions The measure of IML during ablation allows a better quantification of the changes in the structural characteristics of myocardial tissue than local impedance at a single frequency. The increase in tissue temperature during ablation influences the values of local impedance and should be considered when creating new impedance-based metrics to predict the chronic size of the RF lesion.Figure.Lesion quantification by LMITable.Correlation analysis