**5.1 Atrial septal defect**

There are three major types of atrial septal defects (ASDs) and these include ostium secundum, ostium primum and sinus venosus defects. The clinical features are essentially similar but I will mainly concentrate on ostium secundum ASDs. Atrial septal defects constitute 8% to 13% of all CHDs. Pathologically, there is deficiency of the septal tissue in the region of fossa ovalis. These may be small to large. Most of the time, these are single defects, although, occasionally multiple defects and fenestrated defects can also be seen. Because of left-to-right shunting across the defects, the right atrium and right ventricle are dilated and somewhat hypertrophied. Similarly, main and branch pulmonary arteries are also dilated. Pulmonary vascular obstructive changes are not usually seen until adulthood.
