understood on the basis of this model LAA, Left aortic arch; LCCA, left common carotid artery; LPA, left pulmonary artery; LSA, left subclavian artery; MPA, main pulmonary artery; RAA, right aortic arch; RCCA, right common carotid artery; RPA, right pulmonary artery; RSA, right subclavian artery (Modified from Edwards JE Anomalies of the derivatives of the aortic arch system Med Clin N Am 1948;32:925–948; and Edwards JE Vascular rings and slings In: Moller JH, Neal AN, eds Fetal, Neonatal, and Infant Cardiac Disease Norwalk: Appleton & Lange; 1990:745–754.) With normal development, the left aortic arch and left-sided arterial duct persist, while the right aortic arch distal to the origin of the right subclavian artery, along with the right-sided arterial duct, regress (Fig 47.2A) As a result, the proximal part of the embryologic right aortic arch remains as the brachiocephalic artery, which bifurcates into the right common carotid and right subclavian arteries The brachiocephalic artery, of course, is also known as the innominate artery The left-sided aortic arch, in sequence, gives rise to the brachiocephalic, left common carotid, and left subclavian arteries (Fig 47.2B– C) Anomalies can be positional or reflect abnormal branching due to persistence of part or parts of the double arch that normally should have regressed In exceptional cases, nonetheless, it remains difficult to predict the embryologic mechanism, even using the concept of the double arch FIG 47.2 (A) Steps involved in normal formation of a left-sided aortic arch with a left arterial duct In the model of the hypothetical double arch, as shown at left, the red bars indicate the segments that regress In the normal left-sided aortic arch, shown at right, this results in disappearance of the right aortic arch (RAA) distal to the origin of the right subclavian artery (RSA), along with right-sided arterial duct regress In the fetal circulation, as shown in the middle diagram, the aortic arch (red arrow) and the left arterial duct (blue arrow) make a V-shaped confluence at the descending aorta In the postnatal circulation, the left arterial duct closes, becoming the arterial ligament or the ligamentous arterial duct (B) The fetal echocardiogram across the upper mediastinum shows the normal left aortic arch (LAA) and left-sided arterial duct, forming a V-shaped confluence at their union to form the descending aorta (C) The volumerendered magnetic resonance angiogram seen from the front (left) and the computed tomographic angiogram seen from above (right) show the normal arrangement of the left-sided aortic arch, which gives rise sequentially to the brachiocephalic, left common carotid artery (LCCA), and left subclavian artery (LSA) LV, Left ventricle; LPA, left pulmonary artery; MPA, main pulmonary artery; RA, right atrium; RV, right ventricle; RCCA, right common carotid artery; RIA, right innominate artery; RPA, right pulmonary artery; SCV, superior caval vein