A congenital (present from birth) heart defect which is an opening in the septum (wall) between the left and right atrium of the heart.  Since the blood pressure is higher in the left atrium, this allows blood to shunt from the left to the right atrium.  This can lead to overload and enlargement of the right atrium and ventricle, dysrhythmias, ventricular dysfunction, and damage to the blood vessels in the lungs.  Surgical correction before age 4 is recommended.