Atrial septal defect (ASD) represents approximately 10% of all congenital heart diseases and is the third most common form of congenital heart defect. The incidence of congenital heart disease in Belgium is 1%, with ASD accounting for 25% of the cases (Data from the database of congenital heart diseases UZ Leuven.) Characterized by a free communication between the left and the right atrium, it may take the form of an ostium secundum defect (in the region of the fossa ovalis); an ostium primum defect (in the lower part of the atrial septum and associated with mitral regurgitation) or a sinus venosus defect (in the upper atrial septum and associated with anomalous drainage of one or more pulmonary veins)