THÔNG TIN TÀI LIỆU
Normal Fetal Circulation Approximately two-thirds of the fetal cardiac output originates from the right ventricle, with only 5% to 10% passing through the lungs.15,16 As such, the majority of right ventricular output passes through the arterial duct into the descending aorta, and its presence is essential for normal fetal development, permitting right ventricular output to be diverted away from the high-resistance pulmonary circulation Premature construction or closure may lead to right heart failure, resulting in fetal hydrops.17 Embryology and Pathogenesis During early fetal development, five arterial arches link the aortic sac with the paired dorsal aortas, although all arches are never present simultaneously The arches are numbered 1 to 6 However, to the best of our knowledge, there is but a single example identified of a fifth arch artery that occupies a discrete segment of pharyngeal mesenchyme It has become conventional to interpret several congenital malformations on the basis of persistence of this enigmatic entity These examples are discussed in Chapter 48 The initial symmetric arrangement of the pharyngeal arch arteries does provide the basis for the understanding the pattern of the hypothetical double arch proposed by Edwards18 to explain vascular rings and slings This bilaterally symmetric arrangement, with paired brachiocephalic arteries and arterial ducts (Fig 41.1, left), is usually transformed to the configuration seen in postnatal life by the disappearance of some arterial segments and realignment of others (see Fig 41.1, right).19 FIG 41.1 Left, Primitive system of double arches proposed by Edwards to explain vascular rings and slings The arches persist on both the right (R) and left (L) sides, uniting posteriorly to form a neutral descending (desc.) aorta Each arch gives rise to subclavian and common carotid (CA) arteries, along with an arterial duct, which joins the appropriate pulmonary artery The aortic sac and outflow tract have divided to form the aorta and the pulmonary trunk (PT) The double arch surrounds the tracheoesophageal pedicle (OE,T) Right, With normal development, the right arch and right duct regress, leaving a left arch feeding the descending aorta, an arterial duct on the left side, with the brachiocephalic artery (BCA) giving rise to the right common carotid and subclavian arteries LCA, Left carotid artery; LPA, left pulmonary artery; LSA, left subclavian artery; RCA, right carotid artery; RPA, right pulmonary artery; RSA, right subclavian artery Therefore the normal arterial duct develops from the dorsal portion of the left sixth arch From their inception, the sixth arches are associated with the developing lungs, with the arteries feeding the developing lungs developing within the anterior wall of the mediastinum The arteries take their origin of the floor of the aortic sac, which initially feeds also the bilateral arteries of the sixth arches (Fig 41.2, left) FIG 41.2 Reconstruction of the developing arteries of the pharyngeal arches in embryonic mice sacrificed during the 11th (left) and 12th (right) days of development The reconstructions are viewed from the front, with right-sided structures seen to the left At the early stage, the arteries of the second arch are beginning to regress, with the arteries of the third, fourth, and sixth arches formed in symmetric fashion The cavity of the distal outflow tract is a common structure at this stage The pulmonary arteries are beginning to develop within the ventral component of the pharyngeal mesenchyme By the 12th day, the right sixth arch artery is beginning to regress The distal outflow tract has now separated into the intrapericardial components of the aorta and pulmonary trunk The pulmonary arteries now arise from the pulmonary trunk, with the left sixth arch artery forming the
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