Anomalous origin of the left coronary artery from the pulmonary trunk

Anatomical considerations

Anomalous origin of the left coronary artery from the pulmonary trunk is the most common major congenital malformation of the coronary circulation, with an incidence estimated at 1/300,000 live births. More rarely, the right coronary artery ( Fig. 18.2 ), the left anterior descending coronary artery, the circumflex coronary artery , both coronary arteries , or a single coronary artery arise from the pulmonary trunk or its right or left branch. ^, Stenosis of the ostium of the anomalous left coronary artery has been reported, and rarely, the anomalous artery courses within the aortic wall (intramural).

No single terminology encompasses all of these many variations. Anomalous left coronary artery from pulmonary artery (ALCAPA) has been used as a general designation, but the ALCAPA does not include an anomalous right coronary artery, and the pulmonary artery does not distinguish the pulmonary trunk from its right or left branch. In this chapter, the designation Anomalous origin of the left coronary artery from the pulmonary trunk will apply to the most prevalent variation, whereas the less or least prevalent variations will be described individually.

The anomalous left coronary artery is a thin-walled vessel resembling a venous channel ( Figs. 18.1 through 18.4 ). ^, The right coronary artery originates from its normal aortic sinus, is dilated and tortuous ( Fig. 18.5 ; see also Figs. 18.1 and 18.2 ), and, on rare occasions, is aneurysmal. The portion of left ventricle perfused by the anomalous left coronary artery is thin, scarred, and dilated and occasionally forms a ventricular aneurysm. Conversely, the hypoperfused but viable portion of left ventricle increases its mass often appreciably because immature cardiomyocytes replicate in response to the hypoxemic stimulus ( Fig. 18.6 ). The left ventricular endocardium exhibits fibroelastosis and rarely is focally calcified. Interestingly, in adults with typical ischemic heart disease, newly formed elastic fibers appear within 3 or 4 weeks after a myocardial infarction and culminate in endocardial fibroelastosis in the vicinity of the infarct.

Three theories have been proposed to explain the origin of a coronary artery from the pulmonary trunk. The first two theories are related to division of the embryologic truncus arteriosus. In the early embryo, two opposing truncal cushions enlarge and fuse to form the truncal septum which divides the truncus arteriosus into aortic and pulmonary channels. Assuming that the coronary arteries originate as two endothelial buds (see Chapter 29 ), displacement of the origin of one or both of these buds could assign either or both coronary arteries to the portion of the truncus arteriosus destined to become the pulmonary artery. Alternatively, faulty division of the truncus could incorporate one or both coronary artery buds into the pulmonary artery. The higher incidence of anomalous origin of the left rather than the right coronary artery from the pulmonary trunk has been attributed to the proximity of the left aortic sinus to the truncal septum, so a relatively small displacement of the left coronary artery anlage would suffice to cause the left coronary artery ostium to lie within the pulmonary artery. These theories presuppose that human coronary arteries originate as two endothelial buds that develop before or simultaneously with the division of the truncus arteriosus (see Chapter 29 ). Nor do these theories explain the presence of a third (accessory) coronary artery or anomalous origin of one branch of the left coronary artery from the pulmonary trunk or explain why the relative sizes of the pulmonary artery, the aorta, and their valves are not altered by the proposed displacement of the truncal septum.

The involution and persistence theory postulates that there are originally six coronary artery anlagen, three from the aorta and three from the pulmonary artery (see Chapter 29 ). The coronary arteries that are destined to originate normally are believed to arise from two persistent anlagen in two separate aortic sinuses, whereas the anlage in the third aortic sinus, in addition to all three anlagen in the pulmonary sinuses, undergoes involution. According to this theory, anomalous origin of one or both coronary arteries could result from persistence of pulmonary artery coronary anlagen together with involution of the normally persistent aortic coronary anlagen. In the presence of a bicuspid aortic valve, anomalous origin of the left coronary artery from the pulmonary trunk is believed to be the expression of a single morphogenic defect. ^, In the Syrian hamster, a relationship has been proposed between anomalous origin of the left coronary artery and the developmental morphology of the semilunar valves. ^,

Physiological consequences

Myocardial ischemia is a serious sequela of anomalous origin of the left coronary artery from the pulmonary trunk. Ischemia does not stem from the fact that only one coronary artery originates from the aorta because the functional consequences of a congenitally single coronary artery are benign, with few exceptions (see Chapter 29 ). ^, Nor is perfusion of the anomalous coronary artery by unoxygenated blood from the pulmonary trunk a satisfactory explanation because in cyanotic congenital heart disease the oxygen content of coronary arterial blood can be exceedingly low without producing myocardial ischemia. Why then does cardiac muscle become ischemic when the left coronary artery arises from the pulmonary trunk? The answer lies in the direction of blood flow through the coronary bed, ^, as illustrated in the circulatory patterns of Fig. 18.7 . In fetal and early neonatal life, relatively high pulmonary arterial pressure results in antegrade blood flow into the anomalous left coronary artery (see Fig. 18.7 , first panel ). The subsequent fall in pulmonary arterial pressure is accompanied by a parallel fall in blood flow into the anomalous left coronary (see Fig. 18.7 , middle panel ). During this crucial transition, myocardial perfusion depends almost entirely on perfusion from the right coronary artery (see Fig. 18.3 ). ^, Myocardial ischemia is unavoidable unless adequate circulation from right-to-left coronary artery is established via intercoronary anastomoses, on which survival largely depends (see Fig. 18.3 B). ^{, ,} Intercoronary anastomoses represent low-resistance pathways between the aorta and pulmonary trunk, in essence arteriovenous fistulae that have two opposing effects: (1) a desirable effect of reestablishing left coronary arterial perfusion and (2) the undesirable effect of a coronary steal that bypasses the capillary bed and deprives the myocardium of oxygen. ^{, ,} The idea of retrograde flow through an anomalous left coronary artery was originally proposed in 1886 by St. John Brooks:

Here are two arteries belonging to the different circulations—the pulmonary and the systemic—anastomosing with each other. In these circulations, as is well known, the arterial pressure is very much greater in the systemic than in the pulmonary; how then did the blood flow in the anomalous coronary artery? There cannot be a doubt that it acted very much after the manner of a vein, and that blood flowed through it towards the pulmonary artery, and from thence into the lungs.

Physiological, pathological, and clinical derangements arise from the ischemic consequences of the transition from decreased antegrade perfusion of the anomalous left coronary artery, to flow from the right coronary artery through the low-resistance intercoronary anastomoses into the left coronary artery, followed by retrograde flow into the pulmonary trunk (see Fig. 18.7 ). Ischemia causes the left ventricle to labor under three handicaps: first , viable myocardium is compromised, so contractility is depressed, second , mitral regurgitation occurs as a consequence of ischemic papillary muscle dysfunction that adds to the hemodynamic burden, and third , flow via the intercoronary anastomoses constitutes a left-to-right shunt that is occasionally large enough to impose volume overload on the left ventricle. Although regional abnormalities of wall motion characterize anomalous origin of the left coronary artery from the pulmonary trunk, ^, infants tend to exhibit global hypokinesis. ^,