ASD's (Figure 4.03) come in several types and may also range from small to large in size (Table 4.13).
The defect may be of the secundum type resulting from abnormal resorption of the septum primum during formation of the foramen secundum and/or a large foramen ovale, of the type where the septum primum failed to fuse with the endocardial cushions leaving a patent foramen primum, of the type due to the incomplete absorption of the sinus venosus into the right atrium, or of the type where the interatrial septum is completely absent due to failure of the septum primum and septum secundum to develop.
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