AP view

The pressure in the left atrium is higher than the right atrium. therefore, a defect in the atrial septum will result in left to right shunting of blood. The increased blood volume flowing through the right heart and pulmonary vasculature will cause prominant pulmonary vascular markings on chest X-Ray, and in moderate to severe cases there will also be cardiomegaly due to right atrial enlargement causing prominance of the right heart border.

Prominant pulmonary vasculature is noted, particularly in the hilar region. In severe cases the right ventricle is dilated, this will be manifested by fullness of the anterior border of the heart in the lateral views resulting in obliteration of lung tissue normally noted in between the heart and sternum.