It is a hole or deficiency in the septum or partition between the two receiving chambers. As a result of that there is abnormal flow between (L) atrium and (R) atrium leading to increased volume overload of (R) side of heart. More blood goes to the lung. These patients may remain asymptomatic in the childhood. But mostly they have recurrent respiratory tract infection due to increased flow to the lung. Sometimes particularly in aged patients they may present with recurrent palpitation.

Diagnosis is mostly by echocardiography. Sometimes if ASD is complicated by other complex anomaly. Cardiac Catheterization is necessary. 

(a) Ostium secundum  type, where defect is in the place of natural connection before birth (b) Ostium primum type, where it is associated with abnormality of atrioventricular valve and sometimes with ventricular septal defect . Abnormalities of pulmonary venous return may be associated with ASD.

About 40% of ASD defects will close by themselves before the child is 2 yrs old. After that, natural closure is rare and therefore surgery is recommended. The defect is closed by open heart surgery. The relative risk of this operation is very less and can be done with 99 % success rate.

These days ASD (ostium secundum) can be closed with  device mounted on a catheter inserted through the groin. But only small ostium secundum defects with good margins can be treated by this method.