30. Thermal properties of porcine myocardial tissue at subzero temperatures: Implication for thermal therapy studies

Cryoablation for atrial fibrillation is a clinically approved technique used to control irregular heart rhythms due to abnormal electrical signals conducted from the pulmonary veins to the atria. The ablation catheter rapidly cools the cardiac tissue to freezing temperatures creating a discrete lesion which terminates these abnormal electrical pulses. Benefits of this approach include decreased risk of thrombus formation, well demarcated lesions, and preservation of extracellular matrix and endothelial integrity. However, possible complications include phrenic nerve injury and pulmonary vein stenosis (although these are not unique to the cryoablative approach). Therefore, further understanding of the mechanisms and thresholds of cryoinjury are required. This study begins to address these issues by improving predictive model accuracy through demonstrating the importance of accurate, temperature-dependent tissue properties. The direct relationship between thermal history and treatment outcome, combined with the difficulties of realtime, clinical thermometry, make heat transfer modeling and numerical predictions important aspects of cryoablation studies. The accuracy of the models depends on numerous factors including the kinetics and energy release during phase change phenomena and knowledge of thermal properties as a function of temperature. However, insufficient data for tissue thermal properties in the subzero domain results in a reliance on property estimations, generally based on water–ice data or weight averaged values from known materials. This study focused on expanding the thermal properties database for biological tissue in the subzero regime for use in cryoablation modeling. Results for porcine myocardium as well as ultrasound gel (a substitute phantom used for cryoprobe characterization) are reported. A differential scanning calorimeter was used to measure the specific heat and to observe latent heat effects. The thermal conductivity was measured using self-heating thermistors, employing both transient pulse decay and quasi-steady constant power methods. Specific heats in porcine myocardium were slightly less than those for ice, ranging from 0.93 J/gK at −150 °C to 2.35 J/gK at −21 °C. Specific heat behavior for ultrasound gel could be bracketed into two ranges, in which the values measured were less than those for ice at temperatures below the glass transition temperature (around −110 °C), and progressively higher than those for ice with temperatures above the glass transition. Thermal conductivities of porcine myocardium rose gradually at lower temperatures, ranging from 1.52 W/mK at −10 °C to 2.31 at −148 °C, and were about 50% less than those values for ice. Thermal conductivities of ultrasound gel were similar to those of porcine myocardium, but with an inflection point at the glass transition temperature after which the rise in thermal conductivity was attenuated. The importance of accurate, temperature-dependent thermal properties is demonstrated by comparing the predicted thermal history versus those estimated from constant property values.