Coronary Arteries and Cardiac Veins
|STERNOCOSTAL AND DIAPHRAGMATIC SURFACES|
The normal heart and the proximal portions of the great vessels receive their blood supply from two coronary arteries. The left coronary artery (LCA) originates from the left sinus of Valsalva near its upper border, at about the level of the free edge of the valve cusp. The LCA usually has a short (0.5-2 cm) common stem that bifurcates or trifurcates. One branch, the anterior inter- ventricular (descending) branch, courses downward in the anterior interventricular groove (largely embedded in fat), rounds the acute margin of the heart just to the right of the apex, and ascends a short distance up the posterior interventricular groove.
The left anterior descending branch of the LCA gives off branches to the adjacent anterior RV wall (which usually anastomose with branches from the right coronary artery) and septal branches (which supply anterior two thirds and apical portions of septum), as well as a number of branches to the anteroapical portions of the left ventricle, including the anterior papillary muscle.
One septal branch originating from the upper third of the anterior interventricular branch is usually larger than the others and supplies the midseptum, including the bundle of His and bundle branches of the conduction system. This branch also may supply the anterior papillary muscle of the right ventricle through the moderator band. The second, usually smaller circumflex branch of the left coronary artery runs in the left AV sulcus and gives off branches to the upper lateral left ventricular wall and the left atrium. The circumflex branch usually terminates at the obtuse margin of the heart, but it can reach the crux (junction of posterior inter- ventricular sulcus and posterior AV groove). In this case the circumflex branch supplies the entire left ventricle and ventricular septum with blood, with or without the right coronary artery.
In cases where the LCA trifurcates, the third branch, coming off between the anterior interventricular and the circumflex branches, is merely an LV branch that originates from the main artery.
The right coronary artery (RCA) arises from the right anterior sinus of Valsalva of the aorta and runs along the right AV sulcus, embedded in fat. The RCA rounds the acute margin to reach the crux in the majority of cases, and it gives off a variable number of branches to the anterior RV wall. A usually well-developed and large branch runs along the acute margin of the heart. The posterior interventricular (descending) branch descends along the posterior interventricular groove, not quite reaching the apex, and supplies the posterior third or more of the interventricular septum. The diaphragmatic part of the right ventricle is largely supplied by small, parallel branches from the marginal and posterior descending arteries, not from the parent vessel itself. The latter generally crosses the crux, giving off the posterior interventricular branch and a small branch to the atrioventricular node. It terminates in a number of branches to the LV wall.
The posterior papillary muscle of the left ventricle usually has a dual blood supply from both the left and the right coronary artery.
Of the right atrial branches of the right coronary artery, one is of great importance. This branch originates from the RCA shortly after its takeoff and ascends along the anteromedial wall of the right atrium. It enters the upper part of the atrial septum, reappears as the superior vena cava branch (nodal artery) posterior and to the left of the SVC ostium, rounds the ostium, and runs close to (or through) the sinoatrial node (see Plate 1-13), giving off branches to the crista terminalis and pectinate muscles.
Variations in the branching pattern are extremely common in the human heart. In about 67% of cases the RCA crosses the crux and supplies part of the LV wall and the ventricular septum. In 15% of cases (as in dogs and many other mammals) the LCA circumflex branch crosses the crux, giving off the posterior interventricular branch and supplying the entire left ventricle, the ventricular septum, and part of the RV wall. In about 18% of cases, both coronary arteries reach the crux. No real posterior interventricular branch may exist, but the posterior septum is penetrated at the posterior interventricular groove by many branches from the LCA, RCA, or both. In about 40% of cases the SVC branch is a continuation of a large anterior atrial branch of the LCA rather than of the anterior atrial branch of the RCA.
Also, the first branch of the RCA may originate independent of the right sinus of Valsalva rather than from the parent artery. Rarely, the second or even the third RCA branch arises independently.
Most of the cardiac or coronary veins enter the coronary sinus. The three largest veins are the great cardiac vein, middle cardiac vein, and posterior left ventricular vein. The ostia of these veins may be guarded by fairly well-developed unicuspid or bicuspid valves. The oblique vein of the left atrium (of Marshall) enters the sinus near the orifice of the great cardiac vein, and its ostium never has a valve. The small cardiac vein may enter the right atrium independently, and the anterior cardiac veins always do.
Small venous systems in the atrial septum (and probably in ventricular walls and septum) enter the cardiac chambers directly, called the thebesian veins. The existence of so-called arterioluminal and arteriosinusoidal vessels is debatable and the evidence inconclusive.