What is an atrial septal defect?

An atrial septal defect is an opening in the atrial septum, or dividing wall between the two upper chambers of the heart known as the right and left atria.

ASD is a congenital (present at birth) heart defect. As the fetus is growing, something occurs to affect heart development during the first eight weeks of pregnancy, resulting in an ASD. Normally, oxygen-poor (blue) blood returns to the right atrium from the body, travels to the right ventricle, then is pumped into the lungs where it receives oxygen. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle, and then is pumped out to the body through the aorta.An atrial septal defect allows oxygen-rich (red) blood to pass from the left atrium, through the opening in the septum, and then mix with oxygen-poor (blue) blood in the right atrium.

Atrial septal defects occur in 6 to 8 percent of all children born with congenital heart disease. For unknown reasons, girls have atrial septal defects twice as often as boys.