Andersons pediatric cardiology 1080

Some degree of hyperemia of these vessels is common in newborn infants There is controversy with regard to the structure of the intimal layers during fetal life Eccentrically placed intimal cushions, or mounds composed of smooth muscle and elastic tissue, have been described by many authors, with suggestions made that the formation of these mounds precedes normal ductal closure subsequent to birth However, it is questionable whether the intimal cushions are produced during normal fetal maturation When account is taken of the high flow of blood through the fetal duct, it is difficult to conceive that prominent protrusions of intima into the lumen could exist without inducing turbulent flow and a bruit No such bruit is heard in the undisturbed ducts of fetal lambs at term Furthermore, in many of the studies suggesting the presence of intimal cushions, the tissues have usually been subjected to one or more perturbations, such as relatively slow fixation, mechanical stimulation, or cessation of circulation with loss of intraluminal distending pressure On this basis, the existence of intimal cushions as prenatal structures has been challenged In a series of experiments,32–36 Hornblad and colleagues showed that, independent of the degree of closure, the lumen of the duct remained round, was without deformation, and showed no evidence of formation of mounds Wall thickness increased at the time of closure, while the internal elastic lamina became corrugated, especially in the midportion of the vessels A decrease in the lumen was associated with accumulation of endothelial cells within the lumen They concluded that closure was aided by passive central displacement of endothelial and inner medial cells but that no part of the medial layer was prepared prenatally for this process.32–36 These findings endorsed earlier studies in the human, which suggested that the cushions appeared as a normal reparative reaction to distending forces during fetal life.37 At birth, the vessel unequivocally constricts The intimal thickenings, or cushions, become irregular ridges protruding into the lumen, running mainly lengthwise By their extrusion, they exert traction on the media, causing disorganization and formation of mucoid lakes (see Fig 41.5) Anatomic obliteration follows functional closure The process begins with necrosis of the inner wall, followed by the formation of dense fibrous tissue The lumen is progressively obliterated by a process of fibrosis, probably representing organization of mural or occlusive thrombus Eventually, the duct becomes converted into a fibrous strand, the arterial ligament, which may become calcified Anatomic obliteration may take several weeks to complete Approximately two-thirds of ducts are normally obliterated by the age of 2 weeks38 and almost all by 1 year Persistently Patent Arterial Duct As stated earlier, in normal circumstances all ducts should be converted to an arterial ligament within the first year of life, with two-thirds closing in the initial 2 weeks However, some ducts never close These are the channels best described as showing persistent patency Gittenberger-de-Groot39 found the internal elastic lamina to be intact in some, but not all, of the persistently patent ducts studied histologically, along with a sparsity of intimal cushions (Fig 41.6) Bakker37 had noted similar findings, describing them in terms of “aortification.” FIG 41.6 Histologic section through the walls of a persistently patent arterial duct Note the intact internal elastic lamina and lack of intimal mounds (Courtesy Professor Siew Yen Ho, Imperial College, London.) The duct, if persisting as a patent structure, joins the pulmonary arteries to the descending aorta in the fashion seen in the neonate (Fig 41.7) The channel itself can vary markedly in its width (Fig 41.8) FIG 41.7 Dissection showing the course of a persistently patent arterial duct As expected, it follows the course of the channel seen at birth (see Fig 41.3)