Heart with myocardial infarct (heart attack) and atrioseptal defect. An atrioseptal defect is a congenital heart defect characterized by an opening or hole in the septum; the wall that separates the left and right atria of the heart. The defect can cause venous (oxygen-low) and arterial (oxygen-rich) blood to mix. People with the defect my not show symptoms, though it makes the heart inefficient as oxygen-rich blood is sent back to lungs to be oxygenated.