The supply of the posterior and inferior parts of the heart is quite variable. Sometimes the RCA is the dominant artery and  less commonly the region is supplied by the LCA. Coronary dominance defines the vessel that supplies this area of the heart. The classical definition of dominance has been related to which of the two vessels give rise to the posterior descending artery. The right coronary artery feeds the posterior descending artery in 85% of patients and by inference in this context is dominant in 85% of hearts.However, a more useful definition relates to the posterior circulation of the heart that includes the origins of the PDA,  A-V nodal artery and the posterior left ventricular artery. If all these vessels originate from the right coronary artery, then it is a right dominant system. If the origin of these three vessels is shared by both vessels, it is called a co-dominant or balanced circulation.   When the vessels all arise from the LCA then it is a left dominant system.

With this latter definition, the RCA is dominant in 70%, LCA is dominant in 10%, and co-dominance occurs in about 20% of people.

The detail of each of the vessels will be advanced and exemplified in anatomical and angiographic situations.

There are three major branches of the left coronary artery (LCA):

• left main coronary artery.
• left anterior descending artery
• left circumflex coronary artery

The left main coronary artery is between 1 and 25mm long, typically about 10-20mm, but can be up to 40mm. It is about 4.5mms in diameter (3-6mm), arises from the left coronary ostium and terminates in the bifurcation into the LAD and circumflex coronary artery. It is best viewed in the A-P projection during angiography. Occasionally it is absent (1%) in which case the LAD and circumflex have separate origins (Mill).  In about 35% of cases there is a trifurcation and the third and middle branch is called the ramus medianus, which acts like a first diagonal artery.The left main proceeds in a leftward direction and courses behind the pulmonary artery and right ventricular outflow tract.

The left anterior descending artery (LAD) measures about 4mm in diameter proximally and about 2mm distally. It originates at the bifurcation of the left main and often terminates in characteristic moustache-shaped branching pattern, as it passes around the apex (80%). Less commonly, it falls short of the apex and the posterior diagonal artery (PDA) passes around the apex from the posterior location. The LAD and PDA form an anastomoses at the apex.In about 4% of patients there are two LAD’s that travel in parallel, with one vessel supplying the septal perforators and the other the diagonals (Mills).

The proximal circumflex artery in a right dominant systems measures about 3.5mm in diameter. In left dominant systems it is larger and measures about 4mm.In a right dominant system, it arises as a branch of the left main and terminates as a small vessel in the posterior portion of the heart (85%).

Proximally, it proceeds under the left atrial appendage to enter the anterior portion of the atrioventricular groove. In general, it gives off 3-5 obtuse marginal vessels feeding the anterolateral portion of the heart (usually OM1) , the lateral margin– obtuse marginal proper (usually the largest vessel) as the second branch (OM2) and the third (usually OM3) feeding the posterolateral aspect. Sometimes there are smaller unnamed branches between the obtuse marginals. The number and size of the obtuse marginal vessels is variable. A posterior left ventricular branch may arise as the terminal vessel off the circumflex.

The obtuse marginals typically supply the posterolateral aspects of the LV, as well as the posterolateral papillary muscle.

The circumflex also gives rise to the left atrial branches. In about 40% of individuals it gives rise to the SA nodal vessel, and supplies the A-V node in about 10%.

The proximal right coronary artery (RCA) in a right dominant system is about 4mm and in a left dominant system it is about 3mm. The RCA enters the anterior AV groove under the right atrial appendage. The first branch is the conus branch, and then multiple, unnamed right-sided branches supply the anterior wall of the right ventricle (RV).The acute marginal artery courses along the lateral margin of the RV.

Thereafter, the RCA courses posteriorly in the AV groove, and in 85% of patients it gives rise to the posterior descending artery in a typical 90 degree change in direction.

In 60% of patients the RCA gives rise to the SA nodal artery from its anterior portion and in 90% of patients it gives rise to the AV nodal artery from its posterior portion. As a completely right dominant system it will supply both the AV nodal artery and posterior left ventricular artery.

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The AV nodal artery is a branch of the posterior and distal portion of the right coronary artery in 90% of patients and arises at the posterior crux of the heart, formed by the distal RCA, that often continues to the left side to branch into posterior left ventricular branches, the AV nodal artery that proceeds cranially, and the posterior descending artery that proceeds inferiorly. In 10% of patients the AV node is supplied by the left coronary system. The AV node is situated just anterior to the coronary sinus at the entrance of the IVC.AV block is associated with acute myocardial infarctions involving the right coronary artery.

Atherosclerosis is a degenerative disorder of the vascular wall characterised by a fibrofatty deposition (plaque) in the inner lining of the artery.It is the most common disorder of the coronary arteries and a challenging disease to Western cultures particularly because of the high prevalence of the disease and the associated high morbidity and mortality that attends the disease. Atherosclerosis is a normal aging phenomenon but is accelerated in genetically predisposed individuals and is associated with common diseases such as diabetes and hypertension.

The accumulation of plaque in the wall causes narrowing of the lumen of the arteries progressively restricting flow. In addition, the normal smooth and glistening lining of the arteries are replaced by a rough irregular inner wall that is a factor that predisposes to thrombosis, sometimes an acute life threatening event if the arteries of the heart or brain are affected.
The clinical presentation depends on the organ involved and the degree of narrowing. In the heart for example simple narrowing results in chest pain called angina. When total acute obstruction occurs with thrombosis of the artery, severe unremitting chest pain occurs, and myocardial infarction ensues.

The coronary arteries are essentially end arteries and their branching pattern is tree like so that a specific area of the myocardium is essentially only supplied by one mother vessel. As implied in the text, the presence of collaterals, best defined in the conal arteries and septum are available. These however usually require time to mature these collaterals in order that they can compensate for physiological demands at rest and excercise. In an acute coronary thrombosis in a young patient with little atherosclerosis, the collateral system often fails because the collateral system is innately ill equipped to respond to this situation.

Cause: It occurs as a normal course of aging, but it is accelerated by genetic, dietary and behavioral factors.

Result/Diagnosis: The clinical presentation depends on the organ involved and the degree of narrowing. In the heart for example simple narrowing results in chest pain called angina. When total acute obstruction occurs with thrombosis of the artery, severe unremitting chest pain occurs, and myocardial infarction ensues.Each organ has varied manifestations of clinical presentation, all relating to reduced perfusion of the end organ due to narrowing. Imaging plays an essential role in diagnosis. Ultrasound can directly view the lumen and wall of accesible vessels, while CTscan and MRI are advancing to be the mainstay of diagnosis for the deeper arteries. Angiography was the gold standard of the past, is still used and is invaluable in selected circumstances, particulalrly if therapeutic intervention is needed.

Knowledge of the anatomy and variation of the anatomy of the coronary circulation is the key to the understanding and evaluation of disease in the vessels on imaging studies. The framework of the coronary vessels is one of a cross which corresponds to the interatrial septum and interventricular septum in the vertical plane, and the atrioventricular groove in the horizontal plane.The right and left coronary arteries, although distinctly separate, act in unison via collaterals which are essential when stenosis or occlusions occur.

The most important role of coronary CTA is in the patients with atypical chest pain.

Coronary arteriography remains the gold standard for the evaluation of the coronary arteries.