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Atrial septal defect, or ASD, is a congenital heart defect in which blood flows abnormally between the two atria of the heart. Normally, the atria are separated by a wall called the interatrial septum. In ASD patients, this septum is defective allowing blood flow between the two chambers.
During fetal development, when the interatrial septum is being formed, a small passageway called the “foramen ovale” is left open to allow the blood to bypass the non-functional fetal lungs while the fetus obtains oxygen from the mother’s placenta. At birth, as the lungs become functional, the changes in pressures between the two sides of the heart force the opening to close. However, in as many as one fourth of all adults, this opening does not close properly resulting in ASDs.
The severity of a defect is measured by a net flow of blood, or a SHUNT. A shunt can be in either direction. Commonly, the condition starts as a “left-to-right-shunt” due to significantly higher blood pressure in the left side of the heart. This is because the left side has to pump blood all over the body while the right side only needs to send it to the lungs. Fortunately, in a majority of people the defect is relatively small; the shunt is negligible and does not cause any symptoms. When the defect is large, a clinically significant left-to-right blood flow may overload the right side of the heart, resulting in its enlargement and eventually right side heart failure.
Without treatment, other complications may also occur. As the right ventricle continuously pumps more blood to the lungs, the entire pulmonary vasculature may be overloaded and pulmonary hypertension may result. Pulmonary hypertension, in turn, would force the right ventricle to generate even higher pressures to overcome the high pressure in the lungs. In some cases, this vicious cycle may cause the blood pressure in the right side of the heart to increase to a GREATER level than that of the left side. If this happens, the shunt will reverse its direction; a “right-to-left shunt” will result; the oxygen-poor blood will flow from the right atrium to the left atrium and will be sent to all tissues of the body. Fortunately, this complication, called the Eisenmenger syndrome, usually develops over many years and occurs only in a small percentage of people with large atrial septal defects.
Small ASDs do not cause any symptoms and may not require treatment. In fact, many small defects close on their own during early childhood. Large atrial septal defects that cause clinically significant symptoms usually require surgical closure. As a general rule, the earlier in life the surgery is performed, the higher the success rate and the lesser complications presented.
