Histopathology of monopolar transcatheter radiofrequency ablation at the mitral valve annulus.

Although monopolar radiofrequency (RF) catheter ablation is being used to interrupt left-sided accessory pathways in patients with tachyarrhythmia, little is known of the histologic effects from this method of treatment. RF ablation at the mitral valve (MV) annulus was performed in ten dogs to examine the histology of the lesion area. A custom 6 French ablation catheter with a 4 mm distal electrode was positioned beneath the MV adjacent to the annulus. Mean preablation atrial to ventricular electrogram ratio (A/V ratio) was 0.26 +/- 0.17. Thirty +/- 1 watts of RF power were applied for 53 +/- 13 seconds between the distal electrode and a large skin electrode. Nine dogs were sacrificed 6 weeks and one dog 2 days following ablation. Annular lesions were seen in eight of the ten dogs. Lesion volume was 136 +/- 41 mm3 and correlated with the A/V ratio (r2 = 0.74, P = 0.006). Lesions consisted of necrosis of the left ventricle with extension into the atrioventricular groove and left atrium. No injury to the coronary sinus or circumflex artery was observed. A small area of injury was noticed on the mitral leaflet in one dog. Monopolar RF ablation creates lesions at the MV annulus without injury to adjacent vascular structures.