Atrial Septal Defect (ASD), Animation.


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.