An atrial septal defect (pronounced EY-tree-uhl SEP-tuhl DEE-fekt) is a birth defect of the heart in which there is a hole in the wall (septum) that divides the upper chambers (atria) of the heart. What is Atrial Septal Defect? An atrial septal defect is a birth defect of the heart in which there is a hole in the wall (septum) that divides the upper chambers (atria) of the heart. A hole can vary in size and may close on its own or may require surgery. An atrial septal defect is one type of congenital heart defect. Congenital means present at birth. As a baby’s heart develops during pregnancy, there are normally several openings in the wall dividing the upper chambers of the heart (atria). These usually close during pregnancy or shortly after birth. If one of these openings does not close, a hole is left, and it is called an atrial septal defect. The hole increases the amount of blood that flows through the lungs and over time, it may cause damage to the blood vessels in the lungs. Dama...