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It involves a patent ostium secundum (that is, a patent foramen secundum). The secundum atrial septal defect usually arises from an enlarged foramen ovale, inadequate growth of the septum secundum, or excessive absorption of the septum primum. About 10 to 20% of individuals with ostium secundum ASDs also have mitral valve prolapse. [5]
Foramen secundum atrial septal defects are the most common atrial septal defects. This defect can arise as a result of defects of the septum primum and the septum secundum. For the septum primum, the problem can arise as a result of excess resorption of the septum during the process of apoptosis in order to form the foramen secundum.
The ostium primum atrial septal defect is a defect in the atrial septum at the level of the tricuspid and mitral valves. This is sometimes known as an endocardial cushion defect because it often involves the endocardial cushion, which is the portion of the heart where the atrial septum meets the ventricular septum and the mitral valve meets the tricuspid valve.
Now, we also have the septum secundum which grows downward just to the right of the septum primum, and covers the ostium secundum like a curtain, leaving a small opening called the foramen ovale, and essentially creating a makeshift valve that allows blood to go from right atrium to left atrium, but not the other way.
The gap below it is known as the ostium primum (from Latin 'first opening'), and becomes increasingly small. The septum primum eventually fuses with the endocardial cushion, closing the ostium primum off completely. Meanwhile, perforations appear in the superior part of the septum primum, forming the ostium secundum (from Latin 'second opening').
As the septum primum grows, the ostium primum narrows and eventually closes. Before it does so, bloodflow from the inferior vena cava wears down a portion of the septum primum, forming the ostium secundum. Some embryologists postulate that the ostium secundum may be formed through programmed cell death. [4]
The septum primum, a septum which grows down to separate the primitive atrium into the left atrium and right atrium, grows in size over the course of heart development.The primary interatrial foramen is the gap between the septum primum and the septum intermedium, which gets progressively smaller until it closes.
Before the ostium primum is completely occluded, a second opening called the ostium secundum begins to form in the septum primum. The ostium secundum allows continued shunting of blood from the right atrium to the left. To the right of the septum primum, the septum secundum begins to form. This thick, muscular structure initially takes on the ...