Post-mortem histologic evaluation of microwave lesions after epicardial pulmonary vein isolation for atrial fibrillation

Background. Surgical pulmonary vein isolation has gained widespread use as a treatment modality for patients with concomitant atrial fibrillation. However, several uncertainties persist concerning the appropriate energy source, approach, and the need for lesion transmurality. In this study, we present an in-depth histologic investigation of epicardial ablation lesions in 3 patients. Methods. Within a large clinical series of adjuvant epicardial beating-heart microwave isolation of the pulmonary veins, with intraoperative measurement of electrical block, 3 nonablation-related deaths allowed detailed histologic investigation of the lesions. All three patients were in sinus rhythm prior to death. Transmural histologic sections from the box lesion encircling the pulmonary veins were microscopically examined for tissue damage, lesion depth, width, and transmurality, as well as for signs of ongoing repair. Results. Three out of 13 tissue samples showed transmural lesions. In three sections no histologic damage was observed and in the remaining samples transmural extent of myocardial damage ranged from 48% to 82% (mean, 64 +/- 13%). Lesion depths varied between 1.2 mm and 5.7 mm (mean 2.6 +/- 1.3 mm). The lesion depth did not differ significantly among patients and was not related to the thickness of the epicardial or myocardial layers. Interestingly, several sections showed clear necrosis of nerve bundles located in the epicardial tissue. Conclusions. This post-mortem histologic study showed that in the majority of samples the lesions were not transmural and that the extent of myocardial damage was highly variable. Even in this validated approach of epicardial beating heart ablation with satisfactory clinical results, transmurality of lesions cannot be assumed.