What causes an atrial septal defect?
An ASD occurs during fetal life, when the septum between the upper chambers of the heart does not form properly. To understand how these come about, it is useful to understand how the atrial septum normally forms. This is described in Figure 2. In Panel A there is a first septum (primum) that comes down between the chambers. As it comes down, the gap left ahead of it is called the ostium primum. If this never closes off the hole between the upper chambers is called an ostium primum ASD. Normally, this closes as the septum primum completes its excursion to separate the atria. Next (Panel B), a hole or holes form in the middle of the septum primum. This new gap is called the ostium secundum. In Panel C, a second septum (septum secundum) now forms on the right side covering the ostium secundum hole. If it doesn’t cover this, then the hole between the atria is called an ostium secundum ASD. A large hole normally forms near the bottom of the septum secundum that is ovale in shape. It is cal
