Introduction

The coronary arteries run along the coronary sulcus of the myocardium of the heart. Their main function is to supply blood to the heart. This is a crucial function for myocardial function and subsequently homeostasis of the body. The arrangement of coronary arteries varies among people significantly.

Structure and Function

There are two primary coronary arteries, the right coronary artery (RCA) and the left main coronary artery (LMCA). Both of these originate from the root of the aorta. The RCA emerges from the anterior ascending aorta and supplies blood primarily to the right atrium, right ventricle. The sinoatrial nodal artery is a branch of the RCA that supplies the SA node. The RCA also supplies the AV node via a septal perforating branch in 90% of people.[1]  The RCA then classically descends into smaller branches including the right posterior descending artery (PDA) and acute marginal artery. The posterior descending artery is responsible for blood supply to the posterior one-third of the interventricular septum. The left anterior descending artery (LAD) supplies the anterior two-thirds of the septum.[2] The LAD is one of two major branches of the LMCA, with the other being the left circumflex (LCx) coronary arteries. Combined, these two supply blood to the left atrium and left ventricle. The circumflex artery is responsible for blood supply to the left atrium and the posterior-lateral aspect of the left ventricle while the LAD supplies blood to the anterior portion of the left ventricle. Other small branches of the coronary arteries are the obtuse marginal artery (OMA), diagonals, and septal perforator (SP). 

Embryology

The current understanding of coronary artery embryology posits that coronary arteries stem from the epicardial atrioventricular and interventricular grooves with its proximal section merging into the aortic valve sinuses during fetal development. The vascular connection from the atrioventricular valve and the interventricular valve to the aortic valve sinuses gives rise to the right coronary artery and the LMCA. The connection only occurs after the aortopulmonary rotation has been completed. This phenomenon explains the proximity of the vascular connection from the atrioventricular and interventricular grooves to the aortic root since coronary arteries have to pierce through a sinus wall late in the cardiac development and do so via the shortest route possible.[3]

Blood Supply and Lymphatics

The RCA and LMCA extend from the aortic root to supply different regions of the heart. The RCA gives rise to the sinoatrial nodal branch of the right coronary artery, posterior descending artery branch of the RCA, and the marginal branch. The LMCA branches into the circumflex and LAD. The circumflex artery gives rise to the left marginal artery and posterior descending artery (in a left-dominant heart). The left anterior descending artery gives off the diagonal branches.

The RCA supplies blood to the right side of the heart. The sinoatrial nodal branch of the RCA provides blood to the SA node, and the atrioventricular nodal artery delivers blood to the AV node. The marginal branch of the right coronary artery provides blood supply to the lateral portion of the right ventricle. The posterior descending artery branch supplies blood to the inferior aspect of the heart.

The LMCA supplies blood to the left side of the heart. The LAD provides blood to the anterior ventricular septum and the greater portion of the anterior portion of the left ventricle. The LCx supplies blood to the lateral wall of the left ventricle and sometimes to the posterior inferior aspect of the heart when there is left heart dominance.

The heart has a lymphatic system (lymphatic vessels, lymph nodes, and lymphoid organs) just like every other organ in the body. The lymphatic system penetrates through the sub-epicardium, myocardium, and sub-endocardium. The direction of flow starts at the sub-endocardial layer where the lymphatic capillaries absorb interstitial fluid to the collecting lymphatic vessels within the sub-epicardial layer. There are two main collecting lymphatic vessels. The first one runs along with the conal vein then towards the left pulmonary trunk as it approaches towards the mediastinum to drain into the left and right ventricles. The second main collecting lymphatic vessels travel along the left marginal vein, coronary sinus, and left atrium up to the mediastinum.[4]

Physiologic Variants

There are numerous normal physiological coronary artery variances. Starting with the coronary dominance, the PDA in 70% of patients arises from the right coronary artery (known as right dominance); in about 10% of cases, a branch from the LCx supplies the inferior segment of the heart (known as left dominance). In about 20% of cases, there is a dual supply from both the right coronary artery and the LCx (known as codominance).[5]  Though the PDA mainly supplies blood to the inferior wall and inferior third of the interventricular septum, there are known cases where the PDA is a small, relatively redundant artery. In this case, the RCA, LCx, and the obtuse marginal branches supply the inferior wall directly.[6] The atrioventricular nodal artery supplies the AV node and comes from the RCA. The sino-atrial nodal blood supply in 60% of cases is from the proximal RCA, but variants can be seen with blood supply from proximal LCx.[7]

In most cases, epicardial fat surrounds the coronary arteries. However, in 0.15% - 25% of cases angiographically and 5 - 86% on autopsy, the coronary arteries are embedded directly into the myocardium. This phenomenon is known as a myocardial bridge. The LAD is the most commonly affected artery, and the patients are usually asymptomatic even under stress testing.[8]

Other physiologic variants include an acute take-off of the LCx in which the angle between the LMCA and the LCx is less than or equal to 45 degrees. This variant occurs in about 2% percent of the population.[9] Another rare variation is the high take-off of a coronary ostium where the ostium is 5mm or more above the aortic sino-tubular junction. High take-offs may be seen in patients with a bicuspid aortic valve, and the RCA is most commonly affected.[10] In 5% of the population, Shepherd's crook RCA exists. In this situation, the RCA has a normal site of origin from the aorta but immediately courses higher than usual. It is usually clinically insignificant except during percutaneous intervention where patients with this physiological variant are at risk for complications.[5] Finally, there is variation in blood supply to the AV node. The septal perforating branch supplying the AV node has its' origin from the RCA in 90% of patients and from the LCA in the remaining 10%.[1]

Clinical Significance

The limitation of blood flow through the coronary arteries that cause malfunctioning of the myocardium is known as coronary artery disease. Atherosclerosis is the number one cause of coronary artery disease. Other etiologies include Prinzmetal angina and congenital coronary artery abnormalities. Stenosis of the branches of the LMCA or the RCA affects specific locations of the heart. For instance, the LAD stenosis affects the anterior septum, anterior free base, and mid-cavity level, apical segments of the septum, and anterior wall. The LCx stenosis affects the anterolateral wall and the inferolateral wall. The posterior descending coronary artery stenosis affects the inferior septum and inferior free wall. These pathologies can be seen via multiple diagnostic modules such as EKG, CT angiogram, and echocardiogram, although the diagnostic gold standard test is invasive coronary angiography.

Complete occlusion of the LAD results in an anterior wall MI, which appears as ST elevations in the precordial leads (V1-V4). Occlusion of the LCx results in a lateral wall MI, which appears as ST elevations in leads I, aVL, and V5-6. Occlusion of the PDA results in an inferior infarction, which would appear in leads II, III, and aVF.