Congenital coronary arterial fistula

Anatomical considerations

Coronary arterial fistulas are the most frequent functionally significant congenital malformations of the coronary circulation, comprising 14% of all congenital coronary artery anomalies and 0.2% to 0.4% of all congenital cardiac defects (see Chapter 29 ). The right and left coronary arteries arise from their appropriate aortic sinuses, but a fistulous branch of one or more than one drains into a cardiac chamber or into the pulmonary trunk, coronary sinus, vena cava, or a pulmonary vein. When the fistula drains into a right cardiac chamber or into the pulmonary trunk , it is arteriovenous , an appropriate term because the communication allows arterialized systemic blood— arterio —to mix with unoxygenated blood in the right side of the heart— venous . When the fistula drains into the left atrium or left ventricle , the appropriate term is coronary arterial rather than arteriovenous.

Approximately half of these fistulas arise from the right coronary artery, somewhat less from the left coronary artery, and only 5% from both coronary arteries. Even more rarely, all three coronary arteries are involved, or multiple fistulas arise from one of the three coronary arteries (right coronary artery, left anterior descending [LAD], or circumflex) or from a single origin coronary artery (both left and right emerging from a single ostium). Isolated reports have appeared of fistulas from the conus artery to the right atrium, from the coronary sinus to the left ventricle, from the left circumflex to the coronary sinus, ^, or to the pulmonary artery, or of microfistulae to left ventricle. A significant minority of these fistulas are acquired (i.e., traumatic) because of intravascular, interventional, or surgical procedures. The contralateral coronary artery is absent in about 3% of congenital cases.

The drainage site of a coronary arterial fistula is more important than its site of origin and consists of a single vascular channel, multiple channels, or a maze of fine channels that form a diffuse network or plexus (spongy myocardium), a pattern especially likely when the left ventricle receives the fistula ( Fig. 19.1 B). Over 90% of congenital coronary arterial fistulas drain into the right side of the heart and are therefore arteriovenous ( Fig. 19.2 ) . A substantial majority, in approximate order of frequency, enter the right ventricle (40%) ( Fig. 19.3 ), or right atrium (25%) ( Figs. 19.2 and 19.4 A), less commonly the pulmonary trunk (15%) ( Fig. 19.5 ) or coronary sinus (7%) (see Fig. 19.4 B), and rarely the hepatic vein, or superior vena cava. A dual right coronary artery has been accompanied by a fistulous communication. The coronary sinus that receives a fistula can be aneurysmal, especially if it receives fistulas from two coronary arteries ( Fig. 19.6 ). A giant right coronary artery-to-superior vena caval fistula has been reported, as well as an aneurysmal coronary artery fistula in which the left main coronary connected to the right atrium. Bilateral coronary arterial fistulas usually drain into the pulmonary trunk. The relatively few that do not communicate with the right side of the heart drain into the left atrium (5%) ( Fig. 19.7 ), left ventricle (3%) ( Figs. 19.3 , 19.1 , and 19.8 ), ^, pulmonary veins, or into both ventricles. A coronary artery–to–left ventricular fistula is not the same as an aortic-to-left ventricular tunnel (see Chapter 7 ). ^, Small coronary arterial fistulas without clinical evidence of their presence have been unexpectedly discovered during routine echocardiography ( Fig. 19.9 ) and are incidental findings in 0.1% to 0.26% of patients undergoing routine coronary angiography. In a series of 14,708 coronary arteriograms, 19 congenital coronary arterial fistulas were found, and in a series of 11,000 coronary angiograms, 13 fistulas were identified. These incidentally found fistulas are characterized by one or more small channels that originate from the LAD coronary artery and form networks that communicate at sites in the pulmonary trunk (see Fig. 19.5 ).

The coronary artery that gives rise to the fistula is characteristically dilated, elongated, and tortuous (see Fig. 19.2 ), whereas the coronaries distal to the fistula are of normal caliber (see Fig. 19.2 ). A fistulous coronary artery may contain saccular aneurysms that reach an astonishing size and may rupture (see Figs. 19.2 and 19.5 ). ^,

An estimated 1% to 2% of coronary arterial fistulas close spontaneously in infants, children, and adults. ^, Occlusion of an atherosclerotic coronary artery proximal to the fistula was responsible for closure in a 44-year-old woman. Occasionally, there is calcification of the wall of the fistula, and thrombi with embolization.

The embryogenesis of coronary arterial fistulas is uncertain. Fistulas entering the right ventricle have been related to persistence of primitive intramyocardial sinusoids or to the development of a rectiform vascular network in the distal branches of the involved coronary artery. Fistulas entering the left ventricle are believed to result from direct flow through thebesian venous channels. Interestingly, the veins of Thebesius were cited as evidence of direct passage of blood from one side of the heart to the other before William Harvey discovered the circulation (see Chapter 29 ).

Of the six coronary anlagen in the embryo, three are in the developing aorta and three are in the developing pulmonary artery (see Chapter 18 ). These anlagen normally involute except for the two from the right and left aortic sinuses. The relatively high incidence of bilateral coronary arterial–to–pulmonary arterial fistulas is in accord with this observation. A coronary arterial–to–pulmonary artery fistula may result from persistence of one or more of the pulmonary arterial anlagen, hence the term accessory coronary artery, which is either a single large channel (see Fig. 19.5 ), one or more smaller channels, multiple tortuous channels, or a plexiform arrangement.

Physiological consequences

The physiological consequences of coronary arterial fistulas depend on the volume of blood flowing through them, the chamber or vascular bed into which they drain, and the myocardial ischemia that results from a coronary steal caused by low-resistance vascular channels. About 10% of blood from the aortic root normally enters the coronary circulation, but in the presence of a coronary arterial fistula, the volume is considerably larger. A fistula that drains into the right atrium, right ventricle or coronary sinus constitutes a left-to-right shunt. A coronary artery fistula with a left-to-right shunt may result in pulmonary hypertension and right ventricular dysfunction. If drainage is into the right ventricular outflow tract, pulmonary trunk (see Fig. 19.5 ), left atrium (see Fig. 19.1 ), or left ventricle (see Fig. 19.3 ), the hemodynamic burden is borne by the left ventricle alone. A fistulous coronary artery receives blood during systole when its stoma is large. ^, If the fistula drains into the inflow tract of the right ventricle, volume overload of the right ventricle coexists. If drainage is directly into the right atrium or indirectly through the coronary sinus (see Fig. 19.4 ), volume overload of right ventricle exists in addition to overload of the left side of the heart.

Pulmonary-to-systemic flow ratios are typically small, even negligible regardless of patient age. Shunts in excess of 2:1 are unusual, but an occasional neonate experiences congestive heart failure ( Fig. 19.10 , Videos 19.1 – 19.3 ) when an exceptionally large fistula drains into the left or right side of the heart. ^,

Myocardial ischemia is incurred when a coronary arterial fistula functions as a low-resistance pathway that constitutes a coronary steal . The coronary artery that gives rise to the fistula then assumes an important role because steal from a major branch of the left coronary artery is more significant than steal from a smaller right coronary artery. Acquired coronary artery stenosis distal to a congenital coronary arterial fistula aggravates the perfusion deficit because the fistula acts as a low-resistance alternative to the acquired obstruction.