Wikipedia:Osmosis/Atrial septal defect



Author: Tanner Marshall, MS

Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS

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

When the heart’s first developing, a tissue called the septum primum between left and right atria grows downward, slowly creating two separate chambers by closing this gap, or ostium primum, which means the “first opening”. This 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, and 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, which is then 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 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 we can rely on our own lungs for oxygen.

Most 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” or ostium primum, 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. Atrial septal defects, in general, are also commonly associated with fetal alcohol syndrome.

Okay, so there’s this opening in the septum, now what? Well you know how pressure’s higher on the left side than on the right? Well this pressure difference causes blood to be shunted from the left atrium to the right atrium, meaning more oxygenated blood gets over to the right side, which is usually deoxygenated blood, right? This type of defect is an acyanotic heart defect, which means “not blue”, since oxygenated blood is going into pulmonary circulation and essentially taking “an extra” trip to the lungs. In contrast, a cyanotic heart defect allows deoxygenated blood to bypass the lungs and make it out to the body, causing a blue or purplish discoloration of the skin—which is not the case here.

That being the case, you’ll see an increase in oxygen saturation of blood in the right atrium, right ventricle, and pulmonary artery. This extra blood volume passing by the pulmonic valve also causes a delay in the closure of the pulmonic valve relative to the aortic valve closure. This slight delay can be heard via auscultation as a splitting of the S2 sound, as well as a systolic murmur in some cases.

One potential complication for patients with an ASD is if the patient develops an embolus (let’s say a blood clot because that’s the most common type) that makes its way through the ASD. So if we take the example of a person with a deep vein thrombosis, part of that blood clot might break loose and become an embolus and usually this guy would travel into the right atrium, then right ventricle, then be pumped off to the lungs and lodge somewhere in the lung. It’s possible that the embolus crosses over to the left side, in which case it can enter the systemic circuit and potentially lodge in the brain. This situation is called paradoxical embolism, paradoxical because the embolus starts off in the right side, but crosses over into the left side.

Sometimes children with ASDs will be monitored for a little while, since smaller openings may close on their own. In other cases, though, especially for larger openings, surgery will be performed to prevent complications in the future, which typically involves plugging or patching the opening.

All right, as a quick recap… An atrial septal defect is a congenital heart defect where the septum between the right and left atrium doesn’t close up all the way and remains open after birth. This causes oxygenated blood to go into pulmonary circulation. One complication from this condition is a paradoxical embolus which crosses from the right to the left side via the ASD.