An atrial septal defect (ASD) is a hole in the part of the septum that separates the atria (upper chambers of the heart). This heart defect allows oxygen-rich blood from the left atrium to flow across the atrial septum into the right atrium instead of flowing down to the left ventricle as it should. This is inefficient because oxygen-rich blood gets pumped back to the lungs, where it has just been, instead of going to the body.

Figure A shows the normal structure and blood flow in the interior of the heart. Figure B shows a heart with an atrial septal defect, which allows oxygen-rich blood from the left atrium to mix with oxygen-poor blood from the right atrium.

ASD is the second most common congenital heart defect. More than 3,000 babies are born with ASD each year in the United States. ASD occurs twice as often in girls as in boys.

Half of all ASDs close without treatment. When treatment is necessary, it is usually successful. Once the defect has closed or has been repaired, most children don’t need additional treatment or medicine and can live normal, healthy lives.

Types

An ASD can be small or large. Small ASDs allow only a little blood to flow from one atrium to the other. Small ASDs don't affect the way the heart works and therefore don't need any special treatment. Many small ASDs close on their own as the heart grows during childhood.

Medium to large ASDs allow more blood to leak from one atrium to the other, and they are less likely to close on their own. Most children with ASDs have no symptoms, even if they have large ASDs.

There are three major types of ASD:

• Secundum. This defect is in the middle of the atrial septum. It’s the most common form of ASD. About 8 out of every 10 babies born with ASD have secundum defects. At least half of all secundum ASDs close on their own. This is less likely if the defect is large.
• Primum. This defect is in the lower part of the atrial septum. It often occurs along with abnormalities in the heart valves that connect the upper and lower heart chambers. Primum defects aren’t very common. This type of defect doesn’t close on its own.
• Sinus venosus. This defect is in the upper part of the atrial septum, near where a large vein (the superior vena cava) brings oxygen-poor blood from the upper body to the right atrium. Sinus venosus is a rare defect. Sinus venosus defects don’t close on their own.

Long-Term Effects

Over time, the extra blood flow to the right side of the heart and the lungs may cause problems for a heart that has an ASD. Usually, most of these problems don’t show up until adulthood, often around age 30 or later. They are rare in infants and children. These possible problems include:

• Right heart failure. The right side of the heart has to work harder to pump extra blood to the lungs. Over time, the heart may become tired from this extra work and not pump efficiently.
• Arrhythmias (irregular heartbeats). Extra blood flowing into the right atrium through an ASD can cause the atrium to stretch and enlarge. Over time, this can lead to problems with the heart’s rhythm. When this occurs, an arrhythmia can develop, with signs or symptoms such as palpitations (a feeling that your heart has skipped a beat or is beating too hard) or a rapid heartbeat.
• Stroke. Usually, the lungs filter out small clots that can form on the right side of the heart. Sometimes a blood clot formed on the right side of the heart can pass through an ASD to the left side and be pumped out to the body. A clot like this can travel to an artery in the brain, blocking blood flow through it and causing a stroke. This doesn’t occur in childhood.
• Pulmonary arterial hypertension (PAH). PAH is high blood pressure in the arteries in the lungs. Over time, high blood pressure in the lungs can damage the arteries and the small blood vessels in the lungs. They thicken and become stiff, making it harder for blood to flow through them.

These problems develop over many years and don’t occur in children. They also are rare in adults because most ASDs either close on their own or are repaired in early childhood.

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