Andersons pediatric cardiology 1078

arterial duct The right dorsal aorta is still present but is now significantly smaller that the left component, which will become the descending aorta as the right side continues to regress (Courtesy Dr Simon Bamforth, Newcastle University, United Kingdom.) When the developing distal part of the outflow tract is divided to form the intrapericardial components of the aorta and pulmonary trunk, the sixth arches, originating from the caudal part of the aortic sac, are placed in continuity with the pulmonary channel A significant event in the appropriate connection of the pulmonary arteries with the intrapericardial pulmonary trunk is the obliteration and disappearance of the right sixth arch On the left side, the artery of the sixth arch persists as the arterial duct, with the pulmonary arteries left in continuity with the channel from the pulmonary trunk to the left sixth arch (see Fig 41.2, right) All developmental anomalies of the aortic arch, including those associated with an abnormally situated duct, are well explained on the basis of the hypothetical double arch system devised by Edwards,18 irrespective of whether the duct itself is patent or represented by the arterial ligament The various possibilities are discussed in Chapter 47, including those in which there is persistence of both sixth arches, producing bilateral arterial ducts Such circumstances are rare and are always associated with intracardiac anomalies Typically, bilaterally persistently patent arterial ducts supply discontinuous pulmonary arteries in the setting of tetralogy of Fallot with pulmonary atresia, albeit that such bilateral ducts can also be found in association with isolation of a subclavian artery.20 However, the arterial duct does not always persist on one or other side Its absence was first described as a postmortem finding in 1671, being seen in a grossly malformed infant with an extrathoracic heart and tetralogy of Fallot described by Nicolas Steno.21 Absence of the duct is a typical finding in the syndrome of tetralogy of Fallot with so-called absent pulmonary valve and dilated pulmonary arteries.22 It was thought that the absence of the duct, and hence absence of any “overflow,” explained the dilated pulmonary arteries, but the pulmonary arteries can be dilated even when the duct is present and patent The duct is also absent in the majority of patients having a common arterial trunk Absence of significant flow through the duct in the presence of a larger aortopulmonary connection permits the duct to disappear early in fetal life However, in more complex varieties of common arterial trunk, such as those with so-called absence of one pulmonary artery or those patients with an associated interruption or atresia of the aortic arch, patency of the duct is essential to maintain both the systemic and pulmonary circulations (see Chapters 43, 45, 69) A number of teratogens are known to influence the development of the duct, including rubella, alcohol, amphetamines, the anticonvulsant hydantoin, and topiramate,23 with the duct being most sensitive from 18 to 60 days of gestation.24 Absence of the duct has been induced experimentally in chick embryos by the administration of agonists of β-adrenoceptors, leading to the suggestion that the teratogenesis is mediated by cyclic adenosine monophosphate.25 Anatomy Arterial Duct and Its Normal Closure Patency is maintained by the relatively low fetal oxygen tension26 and cyclooxygenase-mediated products of arachidonic acid metabolism, primarily prostaglandin and prostacyclin,27 whose levels are high because of production by the placenta and decreased metabolism in the fetal lungs Produced both locally, in ductal tissue, and circulating, these mediators cause vasodilation through interaction with prostanoid receptors In the fetus, and in the neonate prior to its closure, the arterial duct is a short and wide vessel of variable length It connects the pulmonary arteries to the lesser curve of the arch of the aorta, terminating at the point of transition from the isthmus to the descending aorta, distal to the origin of the left subclavian artery (Fig 41.3)