Thermodynamic properties of atrial fibrillation cryoablation: a model-based approach to improve knowledge on energy delivery.

The objective of this study is to describe a suitable model of atrial fibrillation cryoablation thermodynamic properties. Three different thermal loads were applied to a cylindrical copper element simulating the cryoprobe, thermally coupled with a Peltier stack producing the freezing effect, and in contact with a bovine liver sample. Thermal events occurring inside the samples were measured using mirror image technique. Heat subtracted flux during ice formation and minimum temperature measured at probe-tissue interface were, respectively, 1.33 W cm-2 and -27.8°C for Sample#0, 1.88 W cm-2 and -35.6°C for Sample#1 and 1.82 W cm-2 and 1.44 W cm-2 before and after the ice trigger, respectively, and -29.3°C for Sample#2. Ice trigger temperature was around -8.5°C for Sample#0 and Sample#2, and -10.4°C for Sample#1. In all the investigated samples, ice front penetration was proportional to the square root of time and its velocity depended on the heat flux subtracted. The fraction of the useful energy spent for ice formation was less than 60% for Sample#0, and about 80% for Sample#1 and for Sample#2, before the reduction of the removed heat flux. Freezing time exceeding a cut-off, according to the heat subtracted flux, does not improve the procedure effectiveness and is detrimental to the surrounding tissues.

Mirror image technique for the thermal analysis in cryoablation: Experimental setup and validation.

The paper presents a set of experiments that were performed to characterize the freezing front propagation in water first, and in an agar-gel solution afterwards. The experimental setup made of Peltier devices, to emulate the cryogenic effect, and a copper cold finger, to mimic the cold probe interface, are described. We claim that by monitoring some temperatures at the generating cryodevice, several pieces of information can be derived through the cold interface to assess the outside thermodynamic changes. The employed technique, known as mirror image, allows determining the occurrence of the initial ice formation outside the cryo-probe and in the surrounding material, also with different magnitudes of the thermal contact resistance at the cold interface. For both water and agar the ice penetration was found to be non linear versus time, and proportional to the square root of time in the performed experiments. The ice drift velocity decreases according to its penetration inside the tested materials. At the beginning of ice formation, the measured drift velocities are approximately 0.11 mm/s and 0.06 mm/s for water and agar, respectively, and after the ice penetrates 2 mm, they become approximately 0.03 mm/s for both materials.