Investigation of Myocardial Bioimpedance at Multiple Frequencies for Cardiac Radiofrequency Ablation: Ex-Vivo Experiments.

Abstract

Bioimpedance method is used clinically as a surrogate measure of effective tissue heating during cardiac ablation. It does not, however provide information regarding thermal cellular destruction which is the goal of ablation procedures. This study investigated myocardial impedance (resistance and reactance) at different frequencies in response to the thermal effect and across temperatures where tissue necrosis occurs during ablation. An ex-vivo experiment bench was designed to heat heart muscle tissue blocks uniformly to different targeted temperatures. Tissue resistance and reactance were recorded at three frequencies of 5 k, 50 k and 250 kHz. Impedance was measured in 3 phases (1) the relationship of tissue impedance with thermal effect at temperatures < 50 °C, (2) impedance change at 55 °C to 85 °C at which cellular necrosis occurs, and (3) post-ablation impedance during restitution back to 37 °C$.$ Results: Myocardial resistance and reactance demonstrated a strong linear relationship with temperature effect within the tissue (-1.3%/ °C and -1.96%/ °C on average respectively, frequency dependent). At ≥ 65 °C, the reactance was almost abolished (∼0Ω) and stayed flat during the restitution back to 37 °C, potentially indicative of complete necrosis. Time to reactance abolishment was a few seconds for ≥ 75 °C, 17 seconds for 65 °C and few minutes for 55 °C. Myocardial impedance behaviour thermal and necrotic effects were straightforward and observable. Changes in reactance can be used as a potential indicator of cellular necrosis. Data provided can be used to develop better models and solutions for monitoring the efficacy of cardiac ablation procedures, thus enhancing clinical outcomes.