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The coronary circulation consists of the blood vessels that supply blood to, and remove blood from, the heart. The vessels that supply blood high in oxygen to the heart are known as coronary arteries. The vessels that remove the deoxygenated blood from the heart are known as cardiac veins.
The coronary arteries that run on the surface of the heart are called epicardial coronary arteries. These arteries, when healthy, are capable of autoregulation to maintain coronary blood flow at levels appropriate to the needs of the heart muscle. These relatively narrow vessels are commonly affected by atherosclerosis and can become blocked, causing angina or a heart attack. (See also: circulatory system.)
The coronary arteries are classified as "end circulation", since they represent the only source of blood supply to the myocardium: there is very little redundant blood supply, which is why blockage of these vessels can be so critical.
Coronary anatomy
The exact anatomy of the myocardial blood supply varies considerably from person to person. A full evaluation of the coronary arteries requires cardiac catheterization.
In general there are two main coronary arteries, the left and right. Both of these arteries originate from the beginning (root) of the aorta, immediately above the aortic valve. As discussed below, the left coronary artery originates from the left aortic sinus, while the right coronary artery originates from the right aortic sinus.
Left coronary artery
The left coronary artery (LCA) arises from the aorta above the left cusp of the aortic valve as the left main (LM) artery. The left main artery typically runs for 1 to 25 mm and then bifurcates into the left anterior descending (LAD) artery and the left circumflex artery (LCX). If an artery arises from the left main between the LAD and LCX, it is known as the ramus intermedius. The ramus intermedius occurs in 37% of the general population, and is considered a normal variant.
The LAD runs down the anterior interventricular groove. In 78% of cases, it reaches the apex of the heart. It supplies the anterolateral myocardium, apex, and interventricular septum. The LAD typically supplies 45-55% of the left ventricle (LV). The LAD gives off two types of branches: septals and diagonals. Septals originate from the LAD at 90 degrees to the surface of the heart, perforating and supplying the intraventricular septum. Diagonals run along the surface of the heart and supply the lateral wall of the LV and the anterolateral papillary muscle.
The LCX runs across the left atrioventricular groove. It gives off obtuse marginal (OM) branches. The LCX supplies the posterolateral LV and the anterolateral papillary muscle. It also supplies the sinoatrial nodal artery in 38% of people. It supplies 15-25% of the left ventricle in right-dominant systems. If the coronary anatomy is left-dominant, the LCX supplies 40-50% of the left ventricle.
Right coronary artery
The right coronary artery (RCA) originates above the right cusp of the aortic valve. It travels down the right atrioventricular groove, towards the crux of the heart. At the origin of the RCA is the conus artery. In addition to supplying blood to the right ventricle (RV), the RCA supplies 25% to 35% of the left ventricle (LV). In 85% of patients, the RCA gives off the posterior descending artery (PDA). In the other 15% of cases, the PDA is given off by the left circumflex artery. The PDA supplies the inferior wall, ventricular septum, and the posteromedial papillary muscle. The RCA also supplies the SA nodal artery in 60% of patients. 40% of the time, the SA nodal artery is supplied by the LCX.
Coronary artery dominance
The artery that supplies the posterior descending artery and the posterolateral artery (PLA) determines the coronary dominance. If the RCA supplies both these arteries, the circulation can be classified as "right-dominant". If the LCX supplies both these arteries, the circulation can be classified as "left-dominant". If the RCA supplies the PDA and the LCX supplies the PLA, the circulation is known as "co-dominant". Approximately 70% of the general population are right-dominant, 20% are co-dominant, and 10% are left-dominant. [1]
Blood supply of the papillary muscles
The papillary muscles tether the mitral valve (the valve between the left atrium and the left ventricle) and the tricuspid valve (the valve between the right atrium and the right ventricle) to the wall of the heart. If the papillary muscles are not functioning properly, the mitral valve leaks during contraction of the left ventricule. This causes some of the blood to travel "in reverse", from the left ventricle to the left atrium, instead of forward to the aorta and the rest of the body. This leaking of blood to the left atrium is known as mitral regurgitation.
The anterolateral papillary muscle receives two blood supplies: the LAD and LCX, and is therefore somewhat resistant to coronary ischemia. On the other hand, the posteromedial papillary muscle is supplied only by the PDA. This makes the posteromedial papillary muscle significantly more susceptible to ischemia. The clinical significance of this is that a myocardial infarction involving the PDA is more likely to cause mitral regurgitation.
Coronary flow
During contraction of the ventricular myocardium (systole), the subendocardial coronary vessels (the vessels that enter the myocardium) are compressed due to the high intraventricular pressures. However the epicardial coronary vessels (the vessels that run along the outer surface of the heart) remain patent. Because of this, blood flow in the subendocardium stops. As a result most myocardial perfusion occurs during heart relaxation (diastole) when the subendocardial coronary vessels are patent and under low pressure. This contributes to the filling difficulties of the coronary arteries.
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