An atrial septal defect (ASD) is a hole in the wall (septum) between the heart’s two upper chambers (atria). The hole may be as small as a pinpoint or as large as the space of the entire septum (i.e., the total absence of the septum). Although this is a form of congenital heart disease (heart defects that are present at birth), it is sometimes goes unnoticed until adulthood, when it begins to cause various symptoms. Most cases of ASD begin to show symptoms by age 30.

No matter when it is noticed, the sooner as ASD is corrected, the better for the patient. Small holes may close on their own in the first years of life without any symptoms or harmful effects. However, they could lead to the production of blood clots that may be carried to the brain and cause a stroke. Larger holes allow more blood to pass through, increasing the risk of symptoms and other harmful effects, including arrhythmias (abnormal heart rhythms).

When an ASD is present, some of the oxygen-rich blood that should travel to the left ventricle from the left atrium is diverted (shunted) through the ASD and into the right atrium, where it mixes with oxygen-poor blood from the rest of the body. The right atrium of the heart is thus receiving blood from two different places: from the veins and from the left atrium (through the ASD). This results in too much blood flow to the right side of the heart and lungs. The more blood that is diverted to the right side of the heart, the harder the lungs and right ventricle must work to compensate.

Eventually, the stress on the right side of the heart  may cause it to enlarge (atrial/ventricular enlargement).  Also, the lungs can become congested from receiving more blood than is needed, which may lead to high blood pressure in the lungs (pulmonary hypertension). This buildup of extra fluid may cause upper respiratory infections and difficulty breathing in the young patient.

Because the blood flows from the left side of the heart to the right, an ASD is known as a type of left-to-right shunt. Left-to-right shunts, which also include ventricular septal defect (VSD) and patent ductus arteriosus (PDA), account for over half of all congenital heart defects. ASD is the most common type of left-to-right shunt.

ASD occurs in three main forms:

• Primum ASDs are located in the lower portion of the atrial septum, and are almost always accompanied by an abnormality in the mitral valve and/or tricuspid valve. This defect produces at least some blood leakage (regurgitation) back though the valves. Primum ASDs rarely close on their own, and surgery is almost always necessary.
  
  
• Secundum ASDs are located more toward the middle of the septum. During fetal development, there is a normal hole in the atrial septum called the foramen ovale, which is covered by the septum primum. After birth, when pressure rises in the left atrium and the lungs start working, the septum primum should naturally seal the hole. Secundum ASDs occur when the septum primum is absent or somehow malformed.
  
  
• Sinus Venosus ASDs are located higher and usually involve the right upper pulmonary vein.

According to the National Institutes of Health (NIH), ASD occurs in four out of 100,000 people. It is about two to three times more common in girls than in boys. It may be present alone, or may exist with other defects, including:

• Ventricular septal defect (VSD). This defect refers to a hole in the septum between the lower chambers of the heart (ventricles). It is also considered a left–to–right shunt.
  
  
• Total anomalous pulmonary venous return (TAPVR). Instead of the pulmonary veins connecting with the left atrium, as they should, they connect to the right atrium.
  
  
• Mitral valve abnormalities. These valvular defects can affect the blood flow, often causing it to leak backward (regurgitation) instead of flowing forward.