Epicardial Treatment of Atrial Fibrillation Using Cryoablation in an Acute Off-pump Sheep Model

BACKGROUND: The purpose of this study was to test the feasibility and effectiveness of cryoablation therapy (SurgiFrost trade mark CryoCath, Endocare Inc., Irvine, California, USA) for linear epicardial treatment of atrial fibrillation in an acute off-pump sheep model. METHODS: After thoracotomy, we performed epicardial cryoablation (2 min intervals at -160 degrees C) with pacing electrodes positioned at the left atrial appendage (LAA), the pulmonary veins (PVs), the right atrial appendage (RAA), and the vena cava cranialis (VCC) in 8 sheep. Circular epicardial ablations were performed with online temperature measurement in the ascending aorta and in the esophagus. The sheep were sacrificed two hours after ablation procedure, and heart, lungs, and esophagus were retrieved for histological examination. Out of all 8 sheep, histo-pathological analysis was performed on the RAA and VCC in 6 sheep and on the CAA and PV in all 8 sheep. RESULTS: Thin-walled structures such as PVs and VCC showed electrical isolation. No significant changes in temperature in the descending aorta and the esophagus were observed. There was evidence of extensive transmural alteration including vascular lesions, myocardial degeneration and necrosis as well as epi- and endocardial necrosis in the left atria in three of 8 cases, in the right atria in 5 of 6 cases, in the VCC in 6 of 6 cases, and in the PV in 5 of 8 cases. Mild lesions of the muscular layer of the esophagus were found in 7 of 8 cases. CONCLUSIONS: Epicardial cryoablation is not effective on thicker tissues like LAA and RAA due to the rewarming of the current blood flow. However, thin tissues like VCC and PV can be isolated. Further chronic studies are necessary to evaluate the potential for regeneration of adjacent structures.