Ultrastructural insights from myocardial ablation lesions from microsecond pulsed field vs radiofrequency energy

Background Ultrastructural findings immediately after pulsed field ablation (PFA) of the myocardium have not been described. Objectives To elucidate ultrastructural characteristics and differences between microsecond PFA at the 1- and 4-hour timepoint after pulse delivery, and to compare them to irrigated radiofrequency ablation (RFA) lesions. Methods Healthy swine underwent endocardial PFA or RFA followed by necropsy. Discrete microsecond PFA and irrigated RFA lesions were created in the ventricles with a lattice tip ablation catheter. Lesions were delivered in a manner so as to allow sampling to occur 1 and 4 hours after ablation. All lesions were located at necropsy, and samples were carefully obtained from within the lesion core, lesion periphery and from adjacent healthy myocardium. Transmission electron microscopy assessment was performed after fixation using paraformaldehyde and glutaraldehyde. Results One hour after microsecond PF delivery, myocytes were noted to be significantly and uniformly disrupted. Clustered, misaligned and swollen mitochondria coupled with degenerating nuclei and condensed chromatin were visualized. These findings progressed over the subsequent few hours with worsening edema. Similar changes were seen with RFA but reduced in severity. However, there was prominent extravasation of red blood cells with occlusion of capillaries which was not seen in PFA. At the lesion periphery, an abrupt change in the degree of myocyte damage was observed with PFA but not RFA. Conclusion Transmission electron microscopy demonstrates evidence of widespread destruction of myocytes as early as an hour after PFA and corroborates known histological features such as sparing of vessels and sharp lesion margins.