vascular rings (see Chapter 48) Coarctation or interruption is also a frequent associated lesion when the ventriculoarterial connections are abnormal, including discordant ones, double-outlet right ventricle with subpulmonary defect, or the Taussig-Bing malformation,24 aortopulmonary window with intact septum,25,26 and congenitally corrected transposition.27 Collateral Circulation Although rarely present in infants, collateral circulation gradually develops throughout childhood in those with subcritical coarctation Such collateral arteries bypass the obstruction and augment perfusion to the lower body The most common pattern involves a large aberrant artery arising from the right subclavian artery, supplying the aorta below the coarctation, together with various branches of the left subclavian artery, including the thyrocervical trunk, and the left intercostal arteries via the left internal thoracic artery This leads to notching of the ribs and the anterior spinal artery through the left vertebral artery One particular vessel in this circulation has achieved recognition as the artery of Abbott This anomalous vessel (Fig 45.11) arises from the posterior aspect of the isthmus, passing medially behind the carotid artery and transverse arch.28 FIG 45.11 Intraoperative image through a left thoracotomy in a patient with aortic coarctation The Abbott artery arises from the posterior aspect of the aortic isthmus (Courtesy Dr Benson Wilcox, University of North Carolina.) Secondary Pathology Secondary pathology can be considered in terms of local effects, effects on the myocardium, and distant effects, with the last in general caused by hypertension The local changes are seen in older children and adults when fibrous intimal thickening is superimposed on the site of coarctation The thickened layer is composed of concentric layers of collagen with varying degrees of elastin and smooth muscle cells Depletion and disarray of elastic tissues seen with cystic medial necrosis has also been observed.29 The intimal proliferation together with superimposed thrombus can lead to near or complete obliteration of the lumen In such instances all distal perfusion becomes dependent on the collateral circulation The distal aortic wall often shows poststenotic dilation and is somewhat thinner than normal The abdominal aorta may be somewhat hypoplastic owing to diminished flow The combination of these local changes accounts for the occasional development of aortic dissection in unoperated patients with advanced disease The physiologic changes induced by pregnancy, particularly in the last trimester and peripartum, impose an increased hemodynamic strain on the aortic wall In this setting aortic dissection and even rupture can initially be misdiagnosed as preeclampsia Such complications can also follow an apparently successful repair Whether earlier surgical intervention decreases the occurrence of the changes in the aortic wall is not yet known The direct effect on the myocardium of obstruction to left ventricular ejection depends on the rapidity of the onset as well as the degree of the increase in afterload, the left ventricle having numerous compensatory mechanisms In the neonate undergoing rapid decompensation subsequent to ductal closure, left ventricular systolic and diastolic dysfunction rapidly leads to congestive heart failure Diastolic flow in the coronary arteries decreases as left ventricular wall stress increases This leads to ischemia, especially of the subendocardium The resultant decrease in cardiac output causes and perpetuates a metabolic acidosis, which further depresses left ventricular contractility In part, this is a consequence in infancy of the inability of the myocardium to mount the usual adaptive responses to increased impedance to outflow These features are discussed further on under Pathophysiology Unless intervention is performed, death can quickly ensue Of those who survive the initial insult, some develop marked subendocardial fibrosis If the onset of obstruction is less abrupt, compensatory adaptations can occur, primarily in the form of left ventricular hypertrophy Ischemic heart disease eventually occurs in many patients, even in the absence of proximal coronary arterial occlusion Distant complications include the well-recognized and classic “berry” or saccular aneurysm of the circle of Willis All the organs in the upper body can sustain pathology secondary to hypertension These changes are not entirely ameliorated by the initial relief of obstruction This is also discussed at greater length in the following sections Morphogenesis Three main aberrations in embryologic development have been suggested to explain abnormalities of the aortic arch The first, abnormal embryogenesis of the vessels of the arch, and the second, abnormal development of the arterial duct, are closely interlinked The third implicates changes in the ratio of flow between the pulmonary and systemic arterial pathways In the usual situation with a left-sided arch, it is the artery of the left fourth pharyngeal arch that becomes the definitive aortic arch The arterial duct is the persisting artery of the