Left Atrial Isthmus:

The success rate of catheter maze ablation procedures for atrial fibrillation depends upon adequate electrical isolation by performing linear ablation lesions. However, recurrence of atrial arrhythmias is not uncommon, particularly in the so-called left atrial isthmus, between the orifice of the left inferior pulmonary vein and the mitral valve annulus. The focus of the present study is the anatomy of this area.Twenty human hearts were studied. The distance between the left inferior pulmonary vein and the mitral valve, the thickness of the left atrial myocardium, and the extent of left atrial myocardium toward the mitral valve were measured. The AV groove contained a fat pad harboring the great cardiac vein. The distance between the vein and the valve varied considerably (range 17-51 mm). The great cardiac vein coursed along the inferior left atrial wall, approximately 1 cm above the level of the mitral valve. Myocardial thickness also varied considerably (distal: range 1.4-7.7 mm, midway: range 1.2-4.4 mm, proximal: range 0-3.2 mm). Left atrial myocardium extended onto the mitral valve in two hearts, and the left atrial myocardium ended above the level of the mitral valve (range 1.8-5.1 mm) in six hearts. An important variable because it raises the question of how much energy should be used--and at which point--to achieve an adequate transmural ablation line.The great cardiac vein is not an adequate marker for the level of the mitral valve, left atrial myocardium may continue onto the mitral valve, the distance between the left inferior pulmonary vein and mitral valve varies considerably, and left atrial myocardial thickness is highly variable and not uniform.