Local multifrequency impedance to characterize RF ablations

Radiofrequency (RF) ablation is one of the principal treatments of cardiac arrhythmias. The electroanatomical systems used to perform RF ablations use specific metrics to approximate the ablation lesion size, but these indicators are based on ablation parameters (catheter pressure, ablation time, and power) and ignore the local characteristics of the tissue. Local multifrequency impedance magnitude (LMI) may overcome these limitations.

The objective of this study was to characterize the changes in LMI magnitude after performing ablations with different powers in the epicardium of anesthetized pigs.

Lesions were performed on the epicardium of the left ventricle in 9 anesthetized pigs while measuring LMI (from 1 to 173 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 a subset of measurements, the myocardial temperature was recorded using an optical fiber inserted into the epicardium. 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. The study followed the "Principles of laboratory animal care" (NIH Publication no. 85-23 revised 1985) and was reviewed and approved by the Institutional Animal Care and Use Committee of our Institution.

A total of thirty-two effective ablations were performed. In 11 of those we also measured the tissue temperature. The 40W lesions were deeper and wider than the rest (Fig 1. ANOVA: p<0.01). The changes in the LMI at high frequencies during the ablation correlated well with the changes in tissue temperature (Fig 2. 20W: LMI magnitude @ 173kHz vs. Tissue temperature, r2=0.62; p<0.01). The LMI of 40W lesions decreased faster and to a lower value during the first five seconds of ablation (p<0.05). After thermal adjustment, LMI recorded 30 and 55 seconds after the start of the ablation correlated better with the final depth of the lesion than generator impedance or local voltage (Fig 2).

The measure of LMI during ablation allows a better quantification of the changes in the structural characteristics of myocardial tissue than the generator impedance. The increase in tissue temperature during ablation influences the values of local impedance and should be adjusted to correctly predict the chronic size of the RF lesion.Fig. 1

RF lesion sizes