Transvenous Cold Mapping and Cryoablation of the AV Node in Dogs: Observations of Chronic Lesions and Comparison to Those Obtained Using Radiofrequency Ablation

INTRODUCTION: Radiofrequency (RF) is the most commonly used energy source for the treatment of cardiac arrhythmias. Surgical experience has shown that cryoablation also is effective for ablating arrhythmias. The aims of this study were to (1) investigate the feasibility of inducing permanent complete AV block (CAVB), (2) investigate the value of cold mapping to select the cryoablation site to produce permanent CAVB, (3) study the macro- and microscopic lesion characteristics 6 weeks later, and (4) compare them to those produced with RF energy. METHODS AND RESULTS: A new steerable 8.5-French bipolar electrode catheter having a thermocouple with a 3-mm tip using N2O as the refrigerant controlled by a cryoconsole was used. Six mongrel dogs were anesthetized, and the catheter was positioned via the femoral vein across the tricuspid valve to record a large low right atrial and a small His-bundle potential. After cold mapping (-15 degrees to -20 degrees C tip temperature) resulted in ECG modifications, cryothermia (-70 degrees C) was given twice, lasting 5 minutes each, to create permanent CAVB (Cryo group). Additionally, RF catheter ablation of the AV node was performed in two anesthetized mongrel dogs (RF group). In the Cryo group, a permanent proximal CAVB was created in four dogs (block occurred within 10 to 20 sec of cryothermia). Permanent right bundle branch block was obtained in one dog and transient CAVB in the remaining dog. In both dogs of the RF group, permanent CAVB was obtained. The cryolesions consisted of well-circumscribed, homogeneous areas of fibrotic tissue without viable cardiomyocytes. Lesions produced with RF were less circumscribed and inhomogeneous, with clear evidence of viable cardiomyocytes and cartilage formation (patchy lesions). CONCLUSIONS: (1) Permanent CAVB can be created by using a steerable cryoablation catheter. (2) Histologically, cryoablated sites were homogeneous and showed fibrotic tissue without signs of chronic inflammation and no evidence of viable myocytes. (3) Lesions created with RF were less homogenous and still contained viable myocytes within the lesion and cartilage formation. (4) The arrhythmogenic significance of these differences requires further study. (5) The technology of using reversible cold mapping has the potential to identify the successful ablation site and warrants further clinical study.