Anatomical and Pathophysiological Considerations on the Left Main Coronary Artery

Abstract

The left main is the first segment of the left coronary artery and is responsible for the blood supply of over 70% of the myocardium. It usually arises from the superior margin of the left aortic sinus of Valsalva and then divides into the left anterior descending artery and the left circumflex artery, while a ramus intermedius can be present in almost 30% of cases. Histologically, it differs from the rest of the coronary tree because of the higher elastic fiber content at the level of its ostium. Mean length of the left main is around 10 mm, while the mean diameter is slightly less than 5 mm. As with other bifurcations, the dimensions of its branches can be estimated by Finet’s law, as it follows fractal geometry. Significant coronary artery disease affecting the left main is found in approximately 5% of coronary angiographies, more often in men than in women. Isolated left main stenosis is quite rare as compared to its association with one-, two-, and three-vessel disease. Plaque distribution mostly depends on the dynamics of blood flow. Short left main arteries are mainly affected by stenosis near the ostium, while long left main arteries show significant disease more frequently near the bifurcation. Coronary artery disease does not usually involve the carina, rather being concentrated in portions with the lowest endothelial shear stress. The Medina classification represents the simplest and most useful definition of bifurcation lesions.