Histologic study of chronic catheter cryoablation of atrioventricular conduction in swine.

To evaluate the efficacy of catheter cryoablation, we studied histologic changes of the atrioventricular (AV) junction in eight pigs 6 weeks after cryoablation of central conducting tissue. Cryothermia at -60 degrees C was applied to the AV junction through an 8F or 11F cryocatheter with no electrode and was maintained for 3 minutes after complete AV block was obtained. Six weeks later, gross inspection revealed a distinct, oval-shaped scar 5 to 8 mm in length along or above the septal leaflet of the tricuspid valve in each animal. Microscopically, ablated sites were discrete and were replaced by dense connective tissue with no signs of chronic inflammation. Neovascularization was often seen in scar tissue, and small branches of coronary vasculature within lesions appeared patent with mild intimal proliferation. Ablated lesions involved a small area of the summit of the muscular interventricular septum in three animals, but lesions were never found in the base of the aortic wall or aortic valve. The proximal AV conduction system, either the penetrating bundle or a portion of the AV node, was ablated in all five animals with complete AV block. Three animals that did not maintain complete AV block showed scar formation in the atrial septum above the AV node or penetrating bundle; two of these lesions had small islands of viable atrial myocardium embedded within scar tissue. Catheter cryoablation can produce lesions as effectively as those created during open-heart surgery and is a possible procedure for treatment of intractable supraventricular tachycardia. Improvements in technique or in the cryocatheter may be needed to create more reliably located and homogeneous scar.