Andersons pediatric cardiology 1079

FIG 41.3 Arrangement of the arterial trunks at the time of birth The duct is the direct continuation of the pulmonary trunk and is larger than the right and left pulmonary arteries The duct joins with the isthmus of the aortic arch, with both pathways continuing as the descending aorta The extrapericardial component of the ascending aorta gives rise to the brachiocephalic artery, continuing as the arch, which gives rise to the left common carotid, and left subclavian arteries Prior to birth, the duct is very much the direct continuation of the pulmonary trunk, with the left and right pulmonary arteries taking origin as smaller branches from the trunk Posteriorly, the duct is related posteriorly to the left main bronchus, while anteriorly it is crossed by the vagus nerve This gives off the left recurrent laryngeal nerve, which encircles the duct before ascending behind the aortic arch into the neck (Fig 41.4) FIG 41.4 View through a left thoracotomy shows the relationship of the left recurrent laryngeal nerve to the persistently patent arterial duct (doubleheaded arrow) (Courtesy Benson Wilcox, University of North Carolina.) The pulmonary arterial end of the vessel is covered by a reflection of the pericardium In terms of its microscopic structure, the duct is a muscular artery endowed with an intima, media, and adventitia, differing markedly from the adjacent pulmonary trunk and aorta (Fig 41.5) FIG 41.5 (A) Microscopic trichrome-stained section showing the markedly different aspect of the muscular walls of the arterial duct compared with the fibroelastic walls of the aorta and pulmonary trunk (B) Note the intimal mounds within the ductal lumen (left), which can be appreciated in the opened closing duct (right) The media of the aorta is composed mainly of circumferentially arranged elastic fibers, whereas the media of the duct consists largely of spirally arranged smooth muscle cells, some with circular and others with longitudinal orientation,28 with an increased content of hyaluronic acid The intimal layers are thicker than those of the adjoining vessels and contain increased amounts of mucoid substance.29 In the newborn, the tissues are rather loosely arranged, with a well-defined internal elastic lamina that may be single or focally duplicated, with small interruptions encountered regularly No collagen is seen in the media by light microscopy, but abundant material that strains positively for acid mucopolysaccharides is observed between the muscle and the elastic laminas Electron microscopy reveals fine collagen fibrils lying between adjacent lamellas of smooth muscle cells and elastin During the second half of gestation, the smooth muscle cells show decreasing evidence of secretory activity, and increasing maturation of their contractile elements It is known that vessels cannot close by isolated contraction of circularly arranged muscle,30 so coincident shortening of the less abundant longitudinally arranged muscle fibers is critical to effective closure The duct is innervated mostly by adrenergic fibers, supplying largely the adventitia and outer media, with cholinergic fibers being extremely sparse or totally absent.31 Vessels are also found in its walls that may have a role in fueling contraction at birth.28