The heart is the only organ that supplies its own circulation. The arteries that supply blood to the heart are the right and left coronary arteries, which take origin from the right and left coronary ostia at the base of the aorta. The blood vessels travel on or just below the epicardial surface and then enter the myocardium at either an oblique angle or at a 90 degree angle. The capillary network is quite extensive and results in a 1:1 ratio of capillary to myofibril, which is very high, but is necessary to meet the high metabolic demand of the myofibril and the heart. Capillary density is less in the interventricular septum and AV nodal tissue making them more susceptible to ischemia, putting the conduction systems in these areas at a relatively higher risk to ischemic injury.
About 5% of cardiac output is supplied to the coronary arteries. Blood flow is during diastole, since the myocardium around the intramyocardial vessels is relaxed.
The word coronary arises from the Latin word “corona” meaning crown. The reason for this term is exemplified below.
Infrastructure of the Coronary Artery
The infrastructure of the coronary artery conforms to the cross of scaffolding described in the introduction to the applied anatomy of the heart.
The left coronary artery supplies most of the left ventricle and septum together with the left atrium. Blood supply to the posterior portion of the heart is highly variable.
The left coronary artery originates from the left coronary sinus, which is slightly posterior and superior to the right coronary cusp. After the left main coronary artery branches into the LAD and circumflex, the LAD proceeds in the interventricular groove and supplies the anterolateral aspect of the heart and most of the septum, including the anterior fascicle of the left bundle and the right bundle branch. The circumflex proceeds in the left A-V groove first, coursing anteriorly and then posteriorly, and supplies the posterolateral aspect of the left ventricle to variable extent, as well as the left atrium.
The right coronary artery originates from the right coronary sinus. It makes a loop in the anterior groove and then proceeds to the posterior part of the atrioventricular groove, and usually ends as the posterior descending artery in the posterior interventricular groove. Distribution includes the anterior, lateral, and posterior aspects of the right ventricle, the right atrium and the lower one third of the interventricular septum. It also supplies the the posterior fascicle of the left bundle branch, often supplies the AV nodal artery (85-90%), the SA nodal artery (60% of cases), and sometimes a portion of the posterior aspect of the left ventricle.
The vessels enter the myocardium from the surface and proceed into the myocardium at variable angles.
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.
The Left Coronary Artery
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 Coronary Artery: The Left Main 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.
CT Angiography (CTA) allows for the evaluation of the left main coronary artery in any plane. Since the vessel lies posterior to the pulmonary outflow tract, it is necessary to digitally manipulate the image in order to expose left main from the surrounding tissues.
After a short distance the left main terminates usually by bifurcating into the LAD and circumflex vessels.
Left Main Terminates as the LAD and Circumflex
The Left Coronary Artery: The Left Anterior Descending Coronary Artery
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 Left Coronary Artery: Conal Artery
The first branch of the LAD is a small and barely seen conal artery that together with the right coronary artery supplies the right ventricular outflow tract. This anastomosis is a major collateral pathway when there is a stenosis in the left main, proximal LAD or proximal RCA. The conal artery is also called the artery of Vieussens and is named after the French anatomist Raymond de Vieussens (1641 – 1715).
The Left Coronary Artery: Diagonal Arteries
There are usually 2-6 diagonal branches that travel over the anterolateral surface of the heart and supply this region as well the anterolateral papillary muscle. The diagonals initially run on the surface of the heart and therefore can be visualized by CT imaging. At variable distances from their origin they gradually dive into the myocardium. The first diagonal is generally the largest branch, but there is a wide variation in size and number of diagonal arteries.
The Left Coronary Artery: Septal Perforators
Multiple septal perforators (typically between 4-6 in number) originate at right angles from the LAD, and are directed toward the diaphragm. The first septal artery usually arises just after the first diagonal and it is the largest of the septal arteries. The septal vessels arising from the LAD, supply the upper 2/3rds of the septum. The right bundle and the anterior fascicle of the left bundle are supplied by the LAD perforators. In general the density of the capillaries is reduced in the septum making the conduction system in this area susceptible to ischemia. The lower 1/3rd is supplied by the posterior descending artery and these vessels anastomose seamlessly. The anastomosis allows the perforators to act as a major collateral pathway between the left and right circulation. Thus, a significant stenosis in the LAD will result in a lower pressure distally in the vessel and the right coronary artery will be able to supply this area via the septal arteries distal to the stenosis. These collaterals require time to mature and function optimally when an atherosclerotic stenosis evolves slowly. In the event of an acute thrombosis, collaterals are not well developed and this results in more devastating consequences to the myocardium.
The Left Coronary Artery: Circumflex Coronary Artery
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 distal circumflex can be a confusing vessel to define in angiography, since often one of the marginals, usually second, is so large, that the distal true circumflex becomes inordinately small, quite insignificant in appearance. Its position in the A-V groove can be confirmed by identifying the coronary sinus which fills on the later phase of the coronary angiogram, and which also runs in the A-V groove.
The Right Coronary Artery
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.
The Right Coronary Artery: The Conal Artery
The conal artery is the first branch of the right coronary artery and it supplies the right ventricular outflow tract. It collateralizes with its counterpart, the first branch of the LCA. The collateral pathway plays a significant role in patients with evolving, hemodynamically significant stenoses in the left main, proximal LAD and proximal RCA lesions.
The Right Coronary Artery: SA Nodal Artery
In the past it was thought that a single SA nodal artery was present. It is now felt that there often two sources of SA nodal artery supply, and that there are multiple potential routes for the artery to find its way to the SA node. Infarction of the SA node is rare possibly for the reasons stated above. (Kawashima)
The Right Coronary Artery: Acute Marginal Artery
The marginal branch of the right coronary artery, follows the right acute margin of the right ventricle and supplies branches to both the anterior and posterior surfaces of the right ventricle. In angiography it helps define the anterior from the posterior aspects of the AV groove.
The Right Coronary Artery: Posterior Descending Coronary Artery
The posterior descending artery marks the posterior border of the interventricular septum and originates by taking a typical 90 degree turn down toward the interventricular septum. It is smaller than the LAD, usually falls short of the apex but anastomoses with the distal LAD which usually supplies the apex. The PDA sometimes has an early takeoff from the right coronary artery before it reaches the crux of the heart. It occasionally supplies the apex. It usually supplies the posterior fascicle of the left bundle conduction system. It often branches off the posterior RCA before it reaches the apex, and rarely it duplicated. It is distinguished from the posterior left ventricular branches by the septal arteries which originate at 90 degrees from the PDA.
Septal branches of the posterior descending artery
The posterior descending artery supplies vasculature to the lower 1/3 to 1/4 of the interventricular septum, which anastomoses with the branches arising from the LAD. They arise at right angles off the PDA. They supply the post fascicle of the left bundle branch.
The Right Coronary Artery: AV Nodal Artery
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.
The Right Coronary Artery: Posterior LV Branches
The posterior left ventricular branches are also known as the posterolateral arteries. The supply of the posterior aspect of the left ventricle is variable and may arise from the left coronary or right coronary system, or from a combination.
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.
Atherosclerosis of the Aorta
The proximal RCA is the most common site of symptomatic atherosclerosis.
Coronary stenosis is caused by progressive accumulation of atherosclerotic burden in the wall of the vessels resulting in progressive stenosis. Stenosis of greater than 70% is hemodynamically significant. In the left main, a 50% stenosis is clinically relevant. The plaque is variably composed of acute inflammatory changes, fatty cholesterol deposits, fibrous capsules and calcium.
Angiography and more recently CT and MRI are able to define the stenotic lesions.
Coronary thrombosis is an acute occlusive process of the coronary artery, almost universally in the presence of atherosclerotic disease resulting in a variety of syndromes commonly presenting with acute chest pain.
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.
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