Intra-atrial temperatures in radiofrequency endocardial ablation: histologic evaluation of lesions

Background Because of the limited information on the effects of ablation in human tissues, we studied intraatrial temperatures during endocardial radiofrequency applications. We correlated the intra-tissue temperatures with the tissue thickness and with the histologic appearance of the lesions. Methods Radiofrequency currents were delivered to human atrial tissue, simulating conditions in endocardial ablation during surgery at set temperature of 70° and 80°C, and intra-tissue temperatures were measured with thermocouples. Radiofrequency applications at 70°C were performed in patients undergoing mitral valve surgery and biopsy specimens were obtained. Samples from in vitro studies and from patients were assessed histologically. Results The subepicardial temperatures were usually over 60°C in applications in vitro at 70°C and over 70°C in applications at 80°C. Values were higher when the interior of the tissue was warmer than its surface as a result of consecutive radiofrequency applications over the same area. Histologic examination of 12 in vitro samples showed that 10 had transmural lesions. Five of 10 samples from patients with mitral valve surgery had lesions confined to the endocardium, 3 had damaged variable portions of the myocardium, and 2 had transmural lesions. Conclusions Although it is possible to obtain transmural lesions in vitro and in vivo with endocardial applications at 70°C, it is significantly more difficult to achieve transmural lesions in patients with mitral valve disease than in normal atrial tissue in vitro. Consecutive applications can raise the intra-tissue temperatures to values significantly higher than those used for application. Our findings suggest that the composition of the endocardium and of the myocardium is a major determinant in lesion formation.