Endocardial hypothermia and pulmonary vein isolation with epicardial cryoablation in a porcine beating-heart model.

Objective: The objective of this study was to investigate whether epicardial cryoablation could achieve sufficient endocardial hypothermia to create transmural lesions leading to acute and sustained pulmonary vein isolation in a normothermic beating-heart model.

Methods: Thirty-five- to 45-kg pigs underwent median sternotomy. Epicardial cryoablation was performed on the right ventricle after insertion of thermocouples. Endocardial temperatures from thermocouples were recorded continuously and correlated with the thickness of the myocardium. Thirteen animals underwent pulmonary vein isolation as a box lesion by using 5-minute epicardial cryoablation. Endocardial temperatures were measured in 5 of these animals. Ten animals survived for 7 or 30 days. Electrical isolation was tested at the time of surgical intervention and again at death. Hearts were removed en bloc and submitted for gross and microscopic examination.

Results: Endocardial temperature varied inversely with tissue thickness, ranging from -60 degrees C in 5-mm-thick tissue to staying unchanged in tissue more than 10 mm thick. During pulmonary vein isolation, median endocardial temperatures were between -1 degrees C and -22 degrees C. Acute electrical isolation was achieved in all 13 animals. All except one of the animals maintained electrical isolation long-term. Histologic analysis revealed transmurality in 89% of sections, although none of the box lesions were completely transmural.

Conclusion: Epicardial cryoablation can produce long-term pulmonary vein isolation in a beating heart. Dose-response studies demonstrate consistent endocardial hypothermia in tissues up to 7 mm thick. To our knowledge, this is the first report documenting endocardial hypothermia during epicardial cryoablation. This technology holds promise for performing the complete maze procedure on a beating heart.