Andersons pediatric cardiology 160

Venous Duct The umbilical vein carries oxygenated blood, with an oxygen saturation of between 80% and 90%, from the placenta to the umbilical cord (Fig 6.3A) The umbilical vein enters the fetal abdomen, where it divides to form the portal sinus and the venous duct The portal sinus joins the portal vein, while the venous duct carries oxygenated blood to the inferior caval vein (Video 6.2) The origin and proximal part of the venous duct acts as a physiologic sphincter, which, during hypoxemia or hemorrhage, results in an increased proportion of oxygenated blood passing through the duct to the inferior caval vein and to the heart, with less exiting to the portal sinus and the liver.30 The oxygenated blood from the venous duct can be demonstrated coursing along the medial portion of the inferior caval vein after their confluence Flow in the inferior caval vein continues toward the inferior aspect of the right atrium, where a proportion of the oxygenated blood is slipstreamed by the lower border of the infolded atrial roof, also known as the dividing crest or “crista dividens.” This structure therefore acts as a baffle diverting blood into the atrium, a process that can readily be visualized during echocardiography (Video 6.3) FIG 6.3 (A) Simplified scheme of the fetal circulation The shading indicates the oxygen saturation of the blood, and the arrows show the course of the fetal circulation Three shunts permit most of the blood to bypass the liver and the lungs: the venous duct, the oval foramen, and the arterial duct (B) Power Doppler in the sagittal view of the human fetoplacental circulation demonstrates the iliac arteries (IA) arising from the descending aorta and continuing as the umbilical arteries (UA) in the umbilical cord and the umbilical vein (UV) returning to the fetus and connecting to the venous duct (DV) (A, Courtesy Kathryn Tyler, Pegasus Lectures, Inc.) Only a proportion of the oxygenated blood is diverted to the heart, and this proportion varies in different mammalian species and in health and disease states The remainder of the mainly desaturated blood mixes with the desaturated blood from the mesenteric, renal, iliac and right hepatic veins and with that from the coronary sinus and the brachiocephalic veins Oval Foramen Patency of the oval foramen is essential to enable filling of the left side of the heart in the fetus, as pulmonary venous return is low (Video 6.4) The proportion of oxygenated blood returning to the left side of the heart also varies between species This oxygenated blood mixes with the desaturated blood returning to the left atrium via pulmonary veins such that, after complete mixing in the left ventricle, the oxygen saturation is approximately 60%, compared with levels between 50% and 55% in the right ventricle Blood from the left ventricle is directed to the brachiocephalic circulation, thus supplying the most oxygenated and nutrient rich blood to the brain, which grows at a disproportionately greater rate than the rest of the body in the human fetus The majority of blood ejected into the ascending aorta by the left ventricle is directed cephalad to the head and upper limbs, and only approximately one-third of the left ventricular stroke volume crosses the aortic isthmus to reach the descending thoracic aorta and lower body (Video 6.5) Although the arterial saturation of oxygen is comparatively low, extraction of oxygen by the tissues is facilitated by the leftward displacement of the dissociation curve for oxygen of fetal hemoglobin compared with that of the adult Arterial Duct The systemic venous return has an oxygen saturation of approximately 40% This blood passes through the tricuspid valve into the right ventricle, where mixing with blood from the venous duct occurs before it enters the pulmonary trunk, where the oxygen saturation is between 50% and 55% The majority of blood in the pulmonary trunk passes through the arterial duct to the descending thoracic aorta, with only a small proportion continuing to the lungs via the right and left pulmonary arteries The arterial duct enters the descending aorta immediately distal to the isthmus and the origin of the left subclavian artery, and blood is directed to the descending thoracic aorta by the geometry of its insertion and also by a shelflike projection at its upper insertion (Video 6.6) The degree of patency of the arterial duct is regulated by the periductal smooth muscle cells, which produce prostaglandins Blood from the arterial duct mixes with that crossing the aortic isthmus from the aorta This produces a saturation of oxygen between 50% and 55% in the descending aorta, from which approximately 30% returns to the placenta via the umbilical arteries for reoxygenation (see Fig 6.3B) Aortic Isthmus The aortic isthmus is a watershed region of the aortic arch lying between the aortic arch, just proximal to the region where the arterial duct enters the descending aorta It is the only true shunt in the fetal circulation31 and allows communication of the left and right ventricular outflows (Fig 6.4) Only approximately one-third of the left ventricular stroke volume crosses the aortic isthmus and thus its caliber is a little less than that of the transverse arch and descending aorta This shunt is capable of responding to the different impedances of the placental and cerebral circulations, as well as reflecting differential ventricular performance and ejection volumes Thus blood flow is directed cephalad in growth-restricted fetuses to provide increased delivery of oxygen and nutrients (Video 6.7) and in disease states where there is left heart obstruction or cerebral vascular steal