An atrial septal defect (ASD), a congenital defect, is an opening between two chambers (atria) of your heart. They can cause "shunting"- excessive flow of blood through the lungs. In turn, this can cause cyanosis, where inadequately oxygenated blood can cause early fatigue and subsequently, congestive heart failure.
Small ASDs can often be followed without surgery if they cause minimal shunting. Larger ASDs are closed surgically by simply sewing them closed, or placing a patch of your own tissue or a synthetic material (such as Dacron) over it. They can also be fixed in the cardiac catheterization laboratory using a Dacron patch.