The interatrial septum is the part of the larger septum, a wall in the heart dividing right from left side. This part of the wall divides the atria, the two upper chambers of the heart. Its formation in the fetus and function in the child and adult heart are interesting to explore.
In the fetus, the septal wall begins to form early and the interatrial septum undergoes many changes until it is fully formed. There are very important ways that this wall must develop in fetal growth because circulation is unlike what occurs once a child depends on the lungs to breathe. One distinction is that higher pressure exists in the right side of the heart because the lungs are not yet being used for breathing, which is opposite to the way pressure works after birth. To reduce this pressure, as the interatrial septum forms, it leaves several openings so blood can shunt from the right to left side of the heart.
Over the next few weeks, a single opening develops in the interatrial septum, called the foramen ovale, and this provides a passageway for right-sided blood to enter the left atrium. This passageway is absolutely necessary during life in the womb, but once a child is born, it is not needed or desirable. While it’s often said that children with heart defects are born with a hole in their hearts, this is inaccurate. Virtually all children are born with a hole in the hearts in the interatrial septum.
During the first few months of life, most children will have closure of the foramen ovale, as left-sided pressure in the heart increases. There isn’t continued need for blood from the right side of the heart to shunt into the left side. Occasionally, this hole remains open, and is referred to as a patent foramen ovale. It may close on its own over time, or might require surgical closure.
In the older infant, child, and adult heart, a principal function of the interatrial septum is to keep the two atria from passing blood back and forth to each other, but there are many instances where slight anomalies have occurred during septal formation. This can leave either the foramen ovale open, or there may be holes in the septum that maintain communication between the atria. These holes are called atrial septal defects, and they require monitoring or might necessitate surgical repair.
As children continue to develop, these communications can often close without intervention, but if there are many of them, if they’re particularly large, or if other heart defects are present, a child may benefit from early surgical intervention. Some people have holes that don’t close, don’t have surgery and are still able to have very complete lives, but there is risk of left-sided blood building abnormal pressure in the right atria or in the lungs. Those with any form of interatrial septal defects will usually be monitored carefully by pediatric and then adult cardiologists to look for arising complications.