Histopathology and transmurality of acute microwave lesions on the beating human atrium

Microwave energy allows thoracoscopic beating-heart ablation for the treatment of atrial fibrillation. However, there is a paucity of data on the histologic effects of microwave energy on the beating human heart. This study aims to histopathologically characterize microwave lesions on the beating human atrium. Microwave energy was applied prior to cardiectomy on the beating native right atrium in eight patients undergoing heart transplantation and as a circumferential left atrial 'box' lesion in one patient undergoing heart-lung transplantation. Lesions were applied following heparinization and cannulation, but before initiation of cardiopulmonary bypass. Following cardiectomy, specimens were resected, fixed and subjected to histologic preparation. Grossly, all atrial lesions were 'comma-shaped' with an area of maximum injury on the surface. Microscopically, myocyte injury manifested as acute coagulation necrosis with hypereosinophilic myocytes with both nuclear loss and pyknosis. Contraction bands were noted at the periphery of lesions. The injury was transmural in all right atrial lesions. The left atrial sample contained a circumferential lesion ranging from 0.1 to 0.8 cm in width. The cut edge demonstrated lesion depths of 0.2-0.6 cm, maximum (transmural) in the inferior margin. Microwave ablation represents an acceptable energy source to create characteristic lesions on the beating human atrium.