An atrial septal defect (ASD) is a type of congenital heart defect in which a hole occurs between the heart’s two upper chambers (atria). Because the pressure is higher on the left of the heart than the right, the hole allows blood to seep from the left atrium to the right atrium. The amount of blood flow shunted from the left side of the heart to the right depends on the size of the hole.

An ASD results in excessive 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 for the problem.

Eventually, the stress on the right ventricle can cause it to enlarge (dilate) to make up for the increased workload. Also, the lungs can become congested upon receiving more blood than is needed, which may lead to high blood pressure in the lungs (pulmonary hypertension). Other problems associated with an ASD include regurgitation of blood into the right atrium, respiratory infections, the formation of blood clots (which could lead to a stroke) and the development of abnormal heart rhythms (arrhythmias). In some rare cases, an untreated ASD can lead to sudden cardiac death.

ASDs may be too small to cause any obvious symptoms, or they may cause symptoms that resemble heart failure in infancy. However treatment may be especially necessary if other defects are present. Non-surgical treatments include medication (e.g., diuretics to flush out excess fluids) or a catheter-based procedure to plug the hole. Surgeries that repair the defect include minimally invasive heart valve surgery and open-heart surgery.