Comparison of the healing mechanisms of myocardial lesions induced by dry radiofrequency and microwave epicardial ablation

Background: Histological assessment of the evolution of lesions induced on a pig's left atrium by microwave (MW) epicardial applications and comparison with dry radiofrequency (RF) lesions. Methods: MW (40 W, 40 seconds) and dry RF (80 degrees C, 2 minutes) were epicardially applied on nine pigs' left atrium. Samples were procured following application (n = 2), at day 3 (n = 2), day 7 (n = 2), day 14 (n = 2), and at 1 month (n = 1). They were fixed in formalin, embedded in paraffin, sectioned (2 mu), stained with histochemical dyes, immunomarked, and histologically analyzed. Results: Histological features of acute stage MW lesion are interstitial hemorrhage, adipose, and muscular tissues' coagulation necrosis, thrombosis of myocardial interstitium small vessels at damaged and optically undamaged areas, epicardial coronary branches, and endocardial parietal thrombosis. Day 3-lymphohistiocytic infiltration (lysosyme+) highlights lesion limits. Day 7-lymphohistiocytic infiltration increases, multi-nucleated giant cells appear surrounding/fagocyting necrotic tissue. Neovessels and scarce myofibroblasts appear. Lesion edges are now better defined. Day 14-myofibroblastic proliferation (actin++, vimentin+) creates "young" scar tissue, as in "healing by second intention." Lesions are deeper and wider than appeared at acute stage. One month-dense fibrous tissue scar appears. Endothelial cells covering endocardium are morphologically intact. RF lesions are histologically identical to MW's, although no vessel thrombosis was identified at acute optically undamaged areas and cytomorphologic elements emerge at later stages in the healing process. Conclusions: (1) Microwave scars are deeper and wider than the lesions observed at the acute stage. (2) Evolution of microwave lesions is faster and induces broader scars than dry radiofrequency. (3) Scar formation (both energies) is "healing by second intention." (4) Endocardial thrombosis may occur despite morphologically intact endothelium.