Atrial Septal Defect Introduction Welcome to the atrial septal defect page. On this page we provide a definition, sample heart sounds and links to the training lessons available on this site. What is atrial septal defect? Atrial septal defect is a congenital heart abnormality where the septum (wall between the atria) has a hole. Atrial Septal Defect is a congenital condition associated with abnormal blood flow between the left atrium and the right atrium. Before birth there is a large connection between right and left atria. During development of the fetus the connection gradually disappears. However, in some cases the opening persists and is known as an atrial septal defect.