Atrial septal defect

The atrial septum separates the right and left atrium of the heart. An atrial septal defect describes when there’s an opening in a baby’s heart between these two upper chambers. But how does this opening form? Well, let’s run through how the septum develops in the first place.

When the heart is first developing, a tissue called the septum primum between the left and right atria grows downward, slowly creating two separate chambers by closing this gap, or ostium primum, which means “first opening.” The septum primum then fuses with the endocardial cushion and closes the gap completely. Meanwhile, a hole appears in the upper area, called the ostium secundum, or “second opening.” 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, thus essentially creating a makeshift valve that allows blood to go from right atrium to left atrium, but not the other way.

The developing newborn gets oxygenated blood from the placenta, which goes from the umbilical vein over to the right atrium, which is different because after development, only deoxygenated blood goes to the right atrium, and then gets sent to the lungs to pick up some oxygen. In the developing fetus, though, it’s already oxygenated, so instead of going to the right ventricle and to the lungs and back to the left atrium, it just bypasses the right ventricle and lungs through the foramen ovale and goes into the left atrium. The oxygenated blood then goes to the left ventricle to be pumped to the body.

At birth, the septum secundum and septum primum slap shut, and then fuse and close off this foramen ovale so that we can rely on our own lungs for oxygen.

An atrial septal defect, or ASD, describes a condition where the septum between the right and left atrium doesn’t close up all the way and remains open even after birth. Most About 90% of ASD cases are due to the ostium secundum which can happen when the secundum septum doesn’t grow enough during development. This actually accounts for about 10-15% of all congenital heart defects and is the most common congenital heart defect in adults.

Fewer ASD cases are due to the ostium primum, where the “first opening” doesn’t make it all the way down, again leaving an opening between right and left atria. This primum type of congenital defect is found in around 25% of patients with Down syndrome. In general, atrial septal defects are also commonly associated with fetal alcohol syndrome.