left sixth pharyngeal arch, which connects to the dorsal aorta The left subclavian artery, in contrast, forms from the seventh segmental artery This must undergo a cephalad migration through differential growth before it assumes its definitive position proximal to the aortic isthmus It must cross many structures in its migration Derangements in this process are suggested to be of importance in the pathogenesis of coarctation The hypothesis implicating the arterial duct is based on the presence of the ductal sling around the entire circumference of the aortic isthmus in the setting of coarctation Unequivocal evidence of such a ductal sling (Fig 45.12) was provided initially by Wielenga and Dankmeijer30 and subsequently confirmed by others.31,32 FIG 45.12 Differing extent of ductal tissue (yellow) relative to the aortic arch In the normal situation, the ductal tissue is confined to the aortic mouth of the arterial duct In the setting of aortic coarctation, in contrast, the ductal tissue typically “lassos” the lumen of the isthmus (bracket; see also Fig 45.4) The third proposal is that the patterns of flow of blood in the fetal circulation influence embryogenesis, specifically that a reduction in the volume of blood passing through the ascending aorta in fetal life leads postnatally to the development of coarctation.33 Such a hypothesis is strongly supported by the common association of coarctation with other obstructive lesions in the left side of the heart along with those malformations that result in decreased flow in the fetal ascending aorta No single hypothesis can explain the morphogenesis of all obstructive lesions in the aortic arch It is most likely that there is interplay between the various mechanisms It is highly likely, for example, that decreased flow to the aorta in some way influences the distribution of ductal tissue in the aortic arch These basic mechanisms certainly help in clarifying the clinical presentation, early management, and even successful treatment of the various obstructive lesions to be described in the following text Presentation and Clinical Symptomatology Neonates and Infants Most infants with coarctation or interruption present with varying degrees of heart failure If coarctation presents itself immediately, this is typically manifested by hemodynamic collapse with the closing of the ductus arteriosus After the first weeks, poor feeding, sweating, breathlessness, and failure to thrive are the prevailing symptoms The onset of cardiac failure is commonly seen within the first 3 months of life, but a significant number of patients present within the first week of life; they uniformly will have critical narrowing of the aorta They present when the supplementary effect of blood flow through the duct from the right ventricle to the descending aorta is interrupted by its narrowing or closure, critically limiting the blood flow to the lower body This situation, therefore, is often described as a duct-dependent systemic circulation This process causes the neonate to become acutely unwell with metabolic acidosis, shock, renal failure, and necrotizing enterocolitis The secondary effects of acidosis on the myocardium lead to a global reduction in ventricular function and cardiac output The coarctation itself may be recognized only as the infant is being resuscitated With the introduction of prostaglandin E1 in the 1970s by Elliott,34 it was possible to temporarily maintain ductal patency; this revolutionized the management of these infants Similarly, interrupted aortic arch tends to present with cardiac heart failure of acute onset occurring simultaneously with closure of the arterial duct within the first few days of life In the most common situation, where interruption is associated with a patent arterial duct and ventricular septal defect, the infant will initially be well because pulmonary vascular resistance is high and blood will therefore pass through the arterial duct to the systemic circulation preferentially One of two events will precipitate collapse in these infants First, ductal constriction and closure will lead to a critical reduction in lower body perfusion and the rapid development of acidosis and shock Second, the presence of a nonrestrictive duct combined with falling pulmonary vascular resistance will lead to preferential flow of blood to the pulmonary circulation to the detriment of the systemic circulation Progressive tissue acidosis leading to hemodynamic collapse may also occur In those few infants with interruption occurring in isolation, there must be a collateral circulation, usually via the head and neck vessels, which can develop very rapidly.35 Physical Findings The signs on presentation in infancy include tachypnea with intercostal retractions If markedly low cardiac output is present, these patients will often show profound skin mottling, slow capillary refill, and peripheral cyanosis Central cyanosis will occur only in the presence of an associated cyanotic congenital cardiac lesion or when there is persistence of the fetal circulation The presence of palpable femoral pulses in the first day or two of life does not exclude the diagnosis of coarctation or interruption, since flow of blood to the lower body may be maintained through the persistently patent arterial duct Once symptoms occur, the femoral pulses are usually weaker or absent, with the maintenance of radial pulses This is not always the case if there is an aberrant right subclavian artery distal to the stenosis (Video 45.1) or the ostium of the left subclavian artery is involved in the ductal tissue If the patient has severe low output and no pulses are felt, resuscitation usually results in the return of pulses at least in the right arm The precordium is often active unless myocardial function is depressed On auscultation there is usually a summation gallop rhythm There is often a systolic murmur along the left sternal edge from the site of coarctation, and this may also be audible posteriorly Associated cardiac or central vascular defects, such as the persistently patent arterial duct, can produce additional murmurs An ejection systolic murmur may indicate an associated bifoliate aortic valve Signs of congestive heart failure, such as hepatomegaly and crepitations on auscultation, are commonly found The finding of pulsus paradoxus can also be found in neonates with interrupted aortic arch.36 Auscultation in patients with interrupted aortic arch is usually unhelpful Often a gallop rhythm is present, and the heart sounds are usually easily audible, the second being split An ejection click may indicate the presence of associated