A thorough knowledge of normal and abnormal coronary anatomy is essential for the interventional cardiologist. The direct application of this knowledge base helps in the selection of the appropriate catheter for coronary engagement in both diagnostic and intervention procedures. Knowledge of anatomy also facilitates the selection and interpretation of angiographic views during cardiac catheterization. Finally, because interventional therapy may be needed in several unusual circumstances, a working knowledge of coronary artery anomalies is also essential. This chapter addresses coronary artery anatomy and related technical issues pertinent to the interventional cardiologist.

The coronary arterial system can be divided into the large vessels, or the epicardial coronary arteries, and the small vessels, or the microvasculature. The microvasculature consists of arterioles, which measure less than 200 μm and are poorly visualized with routine coronary angiography. These arterioles feed a broad capillary network that delivers oxygenated blood to the myocardium. They also regulate coronary pressure and flow through their ability to vasodilate and constrict in response to a variety of stimuli. The importance of the microvasculature in determining patient outcomes in both the acute setting, such as myocardial infarction, and the chronic setting has been highlighted.¹

The Left Coronary Artery

Both the right and left coronary artery ostia arise from their respective aortic sinuses. Both ostia are located more than half the distance between the sinotubular junction and aortic valve annulus (Fig. 2-1). The left main coronary artery originates with an elliptic ostium measuring approximately 3.2 ± 1.1 mm × 4.7 ± 1.2 mm.² This coronary artery continues at an acute angle and travels parallel to the aortic sinus wall, coursing between the pulmonary artery and the left atrium in the region of the left atrial appendage. The length of the left main artery ranges from 0 mm (“double-barrel ostium”) to 20 mm. However, in most cases, the length of the left main coronary artery is between 6 and 15 mm, with an average diameter ranging from 3 to 6 mm.³ In two-thirds of cases, the left main coronary artery bifurcates into the left anterior descending (LAD) and circumflex arteries (Fig. 2-2); in one-third of cases, it trifurcates into the LAD artery, the circumflex artery, and a ramus intermedius artery, which follows a course similar to either the first diagonal artery from the LAD artery or first obtuse marginal artery from the circumflex.⁴

The LAD artery follows the anterior interventricular sulcus and may pass around the cardiac apex. When it passes around the apex, the LAD travels along the posterior interventricular sulcus and can anastomose with branches from the posterior descending branch of the right coronary artery. The LAD artery has characteristic septal perforating branches that serve the anterior two-thirds and the infra-apical portions of the ventricular septum. A common anatomic variant occurs when the LAD artery terminates before reaching the cardiac apex. In these cases, a large diagonal artery or an unusually large posterior descending artery from the right coronary artery should be anticipated to be present and provide the necessary left ventricular perfusion.⁵

In addition to septal perforators, the next major branches of the LAD artery are the diagonal arteries, which course along the anterolateral free wall of the left ventricle in a diagonal fashion, hence the name. On average, two major diagonal arteries are present, although smaller diagonal vessels can be identified.⁶ In addition, small ventricular branches from the LAD can form an anastomotic network with similar branches from the proximal right coronary artery.

The circumflex coronary artery arises at an acute angle from the left main coronary artery and travels in the epicardial fat pad beneath the left atrial appendage, coursing through the left atrioventricular (AV) sulcus.⁷ When a posterior descending artery (PDA) arises from the distal circumflex artery, the coronary circulation is defined as either left dominant or codominant. A “codominant” coronary artery system is one in which a PDA arises from both the right and circumflex coronary arteries. Codominant coronary artery systems, seen in less than 10% of patients,⁸ are usually characterized by smallish PDA branches, which perfuse the posterior interventricular sulcus toward the apex of the heart. The circumflex artery gives rise to as many as three obtuse marginal arteries that supply the left ventricular lateral wall. In more than 80% of patients, the left circumflex artery transitions to a smaller AV groove artery, which is considered the distal continuation of the true circumflex artery. Usually, this artery does not pass the crux cordis, but in the 10% of cases with a left-dominant coronary circulation, the AV groove branch is large and of similar diameter to the main circumflex artery, giving rise to both posterolateral branches and a left posterior descending branch.⁸

The Right Coronary Artery

The right coronary artery originates from the right sinus of Valsalva and follows the right AV sulcus around the acute margin of the heart, giving off several major branches (Fig. 2-3). The conus artery is the first major branch, with a separate ostium from the aorta in 50% of cases. The sinoatrial nodal artery is the second major branch, arising from the right coronary artery in 60% of cases and from the left circumflex artery in 40% of cases.⁵

The acute marginal branches of the right coronary artery arise from the proximal to midportion of the vessel, supply the free wall of the right ventricle, and often anastomose to smaller branches of the LAD artery. The distal right coronary artery follows the posterior interventricular sulcus where it bifurcates into the PDA (in 90% of patients) and major posterolateral branches.^5,6 The PDA continues in the posterior interventricular sulcus and usually terminates in the region of the cardiac apex. The posterolateral artery, a continuation of the right coronary artery, continues in the AV sulcus to the crux of the heart, frequently giving rise to the AV nodal artery. Finally, the posterolateral artery terminates in two or three branches that course toward the cardiac apex. When the right coronary artery is nondominant, it may terminate in the region of the acute margin of the heart (see Fig. 2-3).^5,6 In the situation of total LAD artery occlusion, many of the branches of the right coronary artery may give rise to LAD artery collateral vessels.^1,8 The right coronary conus branch artery may become clinically significant when the LAD artery is occluded proximally. In addition, the acute marginal branches of the right coronary artery anastomose with marginal branches from the circumflex artery as well as the septal perforating branches from the LAD artery. The PDA gives rise to branches that anastomose with the distal branches of the LAD artery. This angiographic visualization of the distal LAD artery by collateral arteries is especially important when performing interventions on chronic total occlusions of that artery. Likewise, the distal segments of the right coronary artery can often be visualized by left coronary angiography via collateral filling.

Myocardial bridging is a congenital variant that can occur in all coronary vessels but is most commonly found in the LAD artery. It is defined as a segment of the major epicardial artery tunneling through the myocardium and then resurfacing. Evidence of bridging has been reported in 40% to 80% of autopsy studies, but it is noted less commonly on invasive coronary angiography. The in vivo diagnosis of bridging can be improved with the use of intravascular ultrasound and/or coronary computed tomographic angiography. The clinical relevance of myocardial bridging is controversial. In most cases, bridges are asymptomatic, benign findings. However, in some patients, they can be responsible for chest pain, arrhythmia, and acute coronary syndromes. In these instances, it is believed that endothelial dysfunction, development of atherosclerosis just proximal to the bridge, left ventricular hypertrophy, and diastolic dysfunction exacerbate the bridge in varying degrees and contribute to the clinical presentation. Symptomatic bridges are typically treated medically. Results of treatment with percutaneous coronary intervention have been poor in general. When medical therapy fails, surgical unroofing of the bridge can be considered.⁹

The Coronary Venous System

Knowledge of the coronary venous system is becoming increasingly important to the interventionalist to facilitate various noncoronary and electrophysiologic cardiac interventions (Fig. 2-4). The venous circulation of the heart is divided into three systems: the anterior cardiac veins, the coronary sinus and its tributaries, and the thebesian veins. The anterior cardiac veins, which originate from the right ventricular free wall surface of the heart,^3,8 drain the right ventricle and empty into the right atrium. The coronary sinus and its tributaries, which course on the surface of the left ventricle, run in parallel to the coronary arterial branches. The major tributary of the coronary sinus is the anterior interventricular branch, which parallels the LAD artery and turns into the great cardiac vein as it enters the AV groove. The coronary sinus enters the right atrium adjacent to the tricuspid valve. Finally, the thebesian veins drain directly into the underlying right-side chambers. Flow is very minimal in these vessels, but these veins could give rise to very small and usually insignificant right-to-left shunts.^3,8

Benign Anomalies

Most coronary artery anomalies are clinically benign incidental findings (Table 2-1). Separate and adjacent ostia of the LAD and left circumflex arteries are the most common anomaly, occurring in 0.41% of cases according to a large series reported from the Mayo Clinic.¹⁰ Separate coronary ostia constitute approximately 30% of all benign coronary anomalies. Of interest, there is an association with a bicuspid aortic valve in these patients. From a practical standpoint, catheter engagement may opacify one vessel and not the other. Occasionally, two separate diagnostic catheters are needed to complete the angiographic study of this anomalous left coronary artery system.

The second most common coronary artery anomaly is the anomalous origin of the circumflex artery from either the right coronary artery or the right sinus of Valsalva.^10,11 This anomaly, seen in 0.38% of diagnostic angiograms and making up 28% of benign coronary artery anomalies, is characterized by a retroaortic course of the circumflex, with the vessel running posterior to the aortic root. The clinical importance of identifying this anomaly is high for several reasons. First, unless the catheterizing physician has an adequate level of suspicion, this vessel may remain unvisualized. Second, for patients undergoing aortic valve replacement or mitral valve repair or replacement, this artery may be at risk for damage during surgery unless the surgeon is aware of its presence and retroaortic course. Angiographically, one imaging hallmark is a long left main coronary artery segment, often longer than 15 mm. Also, the retroaortic course of the circumflex around the aorta produces an angiographic “dot” (the end-on view of the vessel as it courses around the aorta) seen on aortography or ventriculography in the right anterior oblique projection.¹² This coronary anomaly is best engaged with an Amplatz or multipurpose catheter in a steep left anterior oblique view.