Atrial septal defects can be several types, including:
Secundum. This is the most common type of ASD, and occurs in the middle of the wall between the atria (atrial septum).
Primum. This defect occurs in the lower part of the atrial septum, and may occur with other congenital heart problems.
Sinus venosus. This rare defect occurs in the upper part of the atrial septum.
Coronary sinus. In this rare defect, part of the wall between the coronary sinus — which is part of the vein system of the heart — and the left atrium is missing.