Congenital anomalies of the coronary circulation

Anatomical considerations

Myocardial morphogenesis begins with the emergence of trabeculations in the luminal ventricular layers that permit an increase in mass in the absence of coronary circulation. Cardiac jelly in the embryonic luminal layer is the primitive site of metabolic exchange between cardiac mesenchyma and blood in the ventricular cavity. ^, The human heart begins to beat as early as the 22nd day after conception, and a few days later, an ebb and flow circulation permits metabolic exchange.

The sequence of development of the coronary vascular bed begins with blood islands and coronary venous connections followed by coronary artery-to-aortic connections. ^{, ,} Blood islands are epicardial layers of endothelial cells distended by nucleated erythrocytes. The blood islands proliferate, coalesce, and form rudimentary networks of vascular channels with no discernible connections to other islands or to the cavity of the ventricle. The second stage of development is a venous connection between the network of vascular channels and the coronary sinus, an arrangement that drains the blood islands and reduces their size. The distal coronary bed remains a loose intermingling vascular network until a myocardial mass develops. In the third stage of development, the coronary arteries join the aorta through an arterial connection to the plexus of vascular channels. The third stage is established after aortopulmonary septation and after formation of the semilunar valves. Flow commences from the aorta into the proximal coronary arteries through myocardial capillaries and then into coronary veins and into the coronary sinus. It is unknown why the two coronary arteries originate from the right and left aortic sinuses that face the right ventricular outflow tract ( Fig. 29.2 A). The embryonic great arteries contain six sinuses, three in the aorta and three in the pulmonary trunk. The endothelial outgrowths or anlagen (buds, sprouts) in the third aortic sinus and in all three pulmonary sinuses undergo rapid involution or do not develop. Aortic-to-coronary artery connections become evident before the appearance of precursor connections in the aortic sinuses. It has been speculated that the aortic sinus sites of the two coronary ostia is determined by mural tension that is increased by the catenoid configuration of the right and left sinuses so that endothelium penetrates the aortic wall preferentially in the right and left sinus sites, establishing connections with the epicardial coronary plexus.

The capillary bed interposed between the coronary arterial and coronary venous circulations plays a pivotal role in the transport of oxygen and nutrients to the myocardium. Capillary density, defined as the ratio of the number of capillaries to the number of cardiomyocytes, is basic to the metabolic exchange between blood and tissues. Capillary density depends on angiogenesis—the capacity of capillaries to replicate. Fetal capillaries replicate in response to the hypoxic intrauterine environment. Postnatal angiogenesis is a diminishing continuation of that response. ^,

The coronary circulation includes three separate systems of veins. The largest system terminates in the coronary sinus and drains blood from most of the left ventricle. A second venous system drains most of the blood from the right ventricle. Adam Christian Thebesius, a German anatomist (1686–1732), described a third venous system consisting of tributaries that drain directly into the cardiac chambers. Before the discovery of pulmonary circulation, Thebesian veins were considered pathways of blood flow from the right heart to the left heart.

Patterns of blood flow in the coronary circulation are different in the right and left ventricles. In the normal thin-walled right ventricle, coronary flow is continuous because the pressure in the aorta continuously exceeds the right ventricular intramural pressure. A higher diastolic pressure in the aorta coincides with a diastolic fall in intramyocardial pressure that results in brisk diastolic flow into the epicardial coronary arteries. Isovolumetric left ventricular contraction exerts disproportionate pressure on the subendocardial third of the wall, expressing blood from the subendocardial perforating branches and arresting flow into extramural branches. The pressure in the aorta during ventricular systole generates transient flow into the extramural coronary arteries and into the contiguous subepicardial myocardium because subepicardial pressure is appreciably lower than pressure in the subendocardium.

The round or ovoid ostia of the two coronary arteries are located in the right and left anterior aortic sinuses which are therefore called the coronary sinuses . The posterior aortic sinus is devoid of a coronary ostium and is appropriately called the noncoronary sinus . Normal coronary artery ostia are located in the middle of an aortic sinus just below the sino-tubular junction and just above the upper margin of the aortic cusps. Abnormally located ostia originate above the sino-tubular junction, in the posterior aortic sinus, or eccentrically near a commissure. ^,

There are typically two coronary ostia, but three or four ostia are considered normal variants. A third ostium usually results from a separate conus branch in the right coronary sinus (see Fig. 29.2 A). Less commonly, three coronary ostia are the result of the origin of the left anterior descending and circumflex coronary arteries from the left aortic sinus and the origin of the right coronary artery from the right aortic sinus, an arrangement that has been designated absent left main coronary artery ( Fig. 29.3 A).

Coronary arteries that originate from normally located ostia arise at a right angle from the aortic wall. Coronary arteries that originate from ectopic ostia arise from the aortic wall at an acute angle and run tangential to the aortic wall. Normal proximal coronary arteries are epicardial, but in about a quarter of cases, the proximal left anterior descending coronary artery is intramural (see earlier) and recognized by angiographic narrowing of the intramyocardial segment.

A relatively large left ventricular mass is served by both the left and right coronary arteries. Either the right or left coronary artery can be dominant. Extreme dominance assumes that the dominant and non-dominant coronary arteries arise from separate ostia in separate aortic sinuses, in contrast to dominance that results from a single coronary artery that originates from a single coronary ostium in a single aortic sinus (see later).

Congenital anomalies of the coronary arteries are the subjects of comprehensive clinical and necropsy reviews. ^, The widespread use of selective coronary angiography has gone far in establishing the types and prevalence of these anomalies (see Box 29.1 ). ^{, , , ,} The incidence was 1.3% in 126,595 coronary arteriograms performed at the Cleveland Clinic. Trans-esophageal and trans-thoracic echocardiography and multislice computed tomographic coronary angiography are established procedures for depicting the origin and course of anomalous coronary arteries ( Figs. 29.4–29.6 ).

Anomalous aortic origins of coronary arteries unassociated with congenital heart disease ( Box 29.2 )

A normal coronary ostium is located in the middle of the aortic sinus and is considered anomalous when it is located above the sino-tubular junction, in the posterior sinus, or in close proximity to an aortic commissure. ^{, , , ,} In 30% to 50% of normal human hearts, a small conus artery arises from the right aortic sinus and is of no functional significance (see Fig. 29.2 A). ^{, , , ,} The circumflex coronary artery may arise from a separate ostium in the right aortic sinus and pass behind the aorta (see Fig. 29.2 B) or may arise from the proximal right coronary artery and enter the left atrioventricular groove as if it were a proximal branch of the left coronary artery. ^{, ,} Anomalous origin of the circumflex coronary artery is regarded as benign, but there is one report of myocardial ischemia in the absence of coronary atherosclerosis. Also regarded as benign is absence —the circumflex coronary artery with a dominant right coronary artery that perfuses the lateral and posterolateral left ventricular walls.