THE MERCK VETERINARY MANUAL
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Anomalies of Derivatives of the Aortic Arches

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Embryonic aortic arches give rise to the carotid arteries (third pair of arches), the arch of the aorta (left fourth arch), and the pulmonary arteries and ductus arteriosus (sixth pair of arches). The other aortic arches regress, although the first aortic arches also become part of the maxillary arteries. Congenital defects may arise if development or dissolution of the aortic arches is disrupted.

Patent Ductus Arteriosus

In fetal life, oxygenated blood within the main pulmonary artery is shunted into the descending aorta through the ductus arteriosus, bypassing the nonfunctional lungs. At birth, several factors mediate closure of the ductus, which effects separation of the systemic and pulmonary circulatory systems. Inflation of the lungs allows the pulmonary circulation to function as a low-pressure system, and closure of the ductus prevents shunting of blood from the high-pressure systemic circulatory system into the pulmonary artery.

Persistence or patency of the ductus with an otherwise normal systemic and pulmonary circulatory system results in significant shunting of blood from left to right, ie, systemic to pulmonary. Because the systemic vascular resistance is always higher than that of the pulmonary circulation, shunting is continuous. The result is volume overload of the pulmonary arteries and veins, left atrium, and left ventricle. Left atrial and left ventricular dilatation may result in cardiac arrhythmias. Chronic volume overload and dilatation of the left-sided cardiac chambers usually result in signs of left-sided CHF. Therefore, most untreated cases develop refractory CHF. Animals with a small ductus may reach adulthood without signs of heart failure but are at an increased risk of infective endocarditis. In some animals with a large PDA, increased pulmonary blood flow may induce pulmonary vasoconstriction and development of pulmonary hypertension, which has several important implications: shunting through the ductus slows and reverses, which causes disappearance of the murmur and occurrence of caudal cyanosis (differential cyanosis); the right ventricle becomes dilated and hypertrophied as a result of pulmonary hypertension; and perfusion of the kidneys with deoxygenated blood causes excessive release of erythropoietin and subsequent polycythemia. Thus, if the ductus shunts right to left, clinical signs of polycythemia will predominate.

In animals with a PDA that shunts from left to right, a prominent, continuous, machinery-like murmur is present. The murmur is usually loudest at the time of the second heart sound, heard best over the aortic valve area, and is often associated with a precordial thrill. The diastolic component is softer and heard best over the pulmonic valve area, occasionally best at the axillary area (in some cases, the ductus remains open for several days after birth; therefore, a continuous murmur may be detected during examination of the neonate). Occasionally the diastolic component may be inaudible in late diastole. Femoral pulses are typically bounding. Most young animals do not demonstrate clinical signs. Those with a large shunt and older animals often have signs of left-sided CHF. Electrocardiography frequently demonstrates tall R waves in lead II, indicative of left ventricular enlargement. A spectrum of cardiac arrhythmias may also be seen, including both atrial and ventricular premature complexes. Radiographic abnormalities depend on the size of the ductus and, in left to right shunting PDA, may demonstrate left atrial and left ventricular enlargement, prominent pulmonary vessels, aortic and pulmonic aneurysmal dilatations, and variable degrees of pulmonary edema. Echocardiography is valuable in ruling out concurrent congenital cardiac defects as well as documenting presence of the PDA. Continuous turbulence in the main pulmonary artery is characteristic of a left to right shunting PDA. Left ventricular and left atrial dilatation are typically noted, and mild mitral regurgitation may be present.

Surgical ligation of the ductus in patients with left to right shunting PDA is usually curative and is almost always indicated. If present, CHF should be medically managed (with diuretics, vasodilators, etc) before anesthesia and surgery are performed. Interventional closure is an alternative to surgical ligation. This can be accomplished by transcatheter occlusion through placement of a device (eg, Gianturco helical coils, Gianturco-Grifka vascular occlusion device) in the PDA that results in clot formation or physical occlusion of the ductus.

In animals with a PDA that shunts from right to left, there is usually a history of lethargy, exercise intolerance, and collapse. Careful examination may reveal differential cyanosis. The second heart sound may be split, and there may be a soft diastolic murmur of pulmonic insufficiency. A continuous murmur is not present, and femoral pulses are not bounding. The finding of polycythemia in a young animal with the above clinical signs should prompt further diagnostic evaluation of the heart. Electrocardiography demonstrates severe right ventricular enlargement and occasional arrhythmias. In reversed PDA, right ventricular enlargement and aneurysmal dilatation of the descending aorta can be noted. Echocardiography is indicated in these cases and will demonstrate right ventricular dilatation and hypertrophy. The right ventricular outflow tract is enlarged. Contrast echocardiography can be used to confirm the diagnosis. After the injection of agitated saline into a peripheral vein, microbubbles will be seen within the abdominal aorta but not within the heart. Ligation of the ductus is contraindicated because this results in an increase in pulmonary hypertension (by causing an increase in flow through the already high and fixed pulmonary vascular resistance) and typically death. Therapy in theses cases involves control of polycythemia through periodic phlebotomies. Longterm prognosis is poor.

Persistent Right Aortic Arch

In this vascular ring anomaly, the right aortic arch persists, which causes obstruction of the esophagus at the level of the heart base. The esophagus is encircled by the persistent arch on the right, by the ligamentum arteriosum to the left and dorsally, and by the base of the heart ventrally.

Fig. 1

Persistent right aortic arch (PRAA) has been reported in cattle, horses, cats, and dogs (German Shepherds and Irish Setters in particular).

Other vascular ring anomalies have been reported and result in findings similar to PRAA. These congenital defects do not cause clinical signs referable to the cardiovascular system—signs of regurgitation and aspiration pneumonia predominate.

Last full review/revision April 2012 by Davin Borde, DVM, DACVIM

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