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Atrial septal defect (ASD) is a congenital heart defect in which blood flows between the atria (upper chambers) of the heart. Normally, the atria are separated by a dividing wall, the interatrial septum. If this septum is defective or absent, then oxygen-rich blood can flow directly from the left side of the heart to mix with the oxygen-poor blood in the right side of the heart, or vice versa.〔(Atrial septal defect ) at Mount Sinai Hospital〕 This can lead to lower-than-normal oxygen levels in the arterial blood that supplies the brain, organs, and tissues. However, an ASD may not produce noticeable signs or symptoms, especially if the defect is small. A "shunt" is the presence of a net flow of blood through the defect, either from left to right or right to left. The amount of shunting present, if any, determines the hemodynamic significance of the ASD. A "right-to-left-shunt" typically poses the more dangerous scenario. During development of the fetus, the interatrial septum develops to separate the left and right atria. However, a hole in the septum called the ''foramen ovale'', allows blood from the right atrium to enter the left atrium during fetal development. This opening allows blood to bypass the nonfunctional fetal lungs while the fetus obtains its oxygen from the placenta. A layer of tissue called the septum primum acts as a valve over the foramen ovale during fetal development. After birth, the pressure in the right side of the heart drops as the lungs open and begin working, causing the foramen ovale to close entirely. In approximately 25% of adults, the foramen ovale does not entirely seal. In these cases, any elevation of the pressure in the pulmonary circulatory system (due to pulmonary hypertension, temporarily while coughing, etc.) can cause the foramen ovale to remain open. This is known as a patent foramen ovale (PFO), which is a type of atrial septal defect. ==Cause== * Down syndrome – patients with Down Syndrome have higher rates of ASDs, especially a particular type that involves the ventricular wall. As many as one half of Down Syndrome patients have some type of septal defect.〔 * Ebstein's anomaly - About 50% of individuals with Ebstein anomaly have an associated shunt between the right and left atria, either an atrial septal defect or a patent foramen ovale.〔(كتاب A Practical Approach to Clinical Echocardiography, p.119 ) Jaypee Brothers, First Edition 2014, ISBN 978-93-5152-140-2.〕 * Fetal alcohol syndrome – about one in four patients with fetal alcohol syndrome has either an ASD or a ventricular septal defect. * Holt–Oram syndrome – Both the osteium secundum and osteum primum types of ASD are associated with Holt–Oram syndrome * Lutembacher's syndrome – the presence of a congenital ASD along with acquired mitral stenosis.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atrial septal defect」の詳細全文を読む スポンサード リンク
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