Atrial septal defect

With atrial septal defects, “atrial” refers to the heart’s two upper chambers, called the atria; “septal” refers to the septum, which is the wall that separates them; while “defect” means there’s an opening that shouldn’t be there. So, an atrial septal defect is a congenital heart defect characterized by an abnormal gap in the wall separating the atria.

To understand how this opening forms, we need to review how the atrial septum forms during fetal development. Early during heart development, the atria start as a single chamber called the common atrium. Around the fourth week of development, a thin wall called the septum primum begins to grow downward from the roof of the common atrium, gradually dividing it into right and left atria.

As the septum primum grows, it leaves a small opening at the bottom called the ostium primum or “first opening”. But, as the septum primum continues to grow, the ostium primum becomes narrower. And before the septum primum reaches the endocardial cushions and completely closes the ostium primum, another opening called the ostium secundum, or “second opening”, appears in the upper area of the septum.

Then, just next to the septum primum, the heart grows a thicker wall called the septum secundum that grows downward and covers the ostium secundum like a curtain. This leaves a small gap called the foramen ovale. You can think of the septum primum and septum secundum as a pair of double doors that don’t quite line up perfectly, allowing blood to slip through the space between them. This arrangement creates a one-way passage, with the septum primum acting like a flap valve, allowing blood to flow from the right atrium to the left atrium, but not the other way around.

This atrial communication is essential during fetal development because the fetus isn’t using its lungs yet. In the fetus, the lungs are collapsed and filled with fluid, so the pulmonary vessels are squeezed tight. Because of this high resistance in the pulmonary vessels, the right ventricle is unable to pump blood into the lungs. Instead, the blood takes a detour through two fetal shunts. One of those is the foramen ovale, while the other one is the ductus arteriosus, which is a vessel that connects the pulmonary artery to the aorta. These shunts allow the blood from the right ventricle to bypass pulmonary circulation and enter systemic circulation. At the same time, the placenta provides a low-resistance pathway for blood leaving the left side of the heart, so the left ventricle doesn’t need to generate much more pressure than the right.

However, everything changes with the baby’s first breaths. As the lungs expand, the pulmonary vessels open, causing pulmonary resistance to drop. At the same time, clamping the umbilical cord takes the placenta out of the circulation, removing its low-resistance pathway and raising systemic resistance. At this point, the left ventricle starts pumping against a much higher pressure than the right ventricle. This pressure difference between the two sides of the heart pushes the septum primum against the septum secundum, sealing off the foramen ovale.

Now, when the walls that separate the right and left atria don’t form or fuse properly, a gap is left behind. This opening is what we call an atrial septal defect, or ASD. Based on the part of the septum that doesn’t close properly, ASDs are classified into several different types.

One of the most common types is secundum ASD, which accounts for around 90% of all cases. This type occurs when the septum secundum doesn’t cover the ostium secundum, or when the ostium secundum itself is too large. A less common type is primum ASD, which happens when the septum primum fails to fully grow downward and fuse with the endocardial cushions, leaving a persistent gap between the right and left atria. Although they are rare, ostium primum defects are found in around 25% of people with Down syndrome and are commonly associated with other congenital cardiac anomalies.

Finally, an ASD can also develop near the superior or inferior vena cava, or close to the coronary sinus, but these two types account for only 10% of all atrial septal defects.