An atrial septal defect allows oxygen-rich (red) blood to pass from the left atrium, through the opening in the septum, and then mix with oxygen-poor (blue) blood in the right atrium. An ostium secundum is an opening in the middle of the atrial septum which is the most common type of ASD. Atrial septal defects occur in 5 to 10 percent of all children born with congenital heart disease. For unknown reasons, girls have atrial septal defects twice as often as boys.
Atrial septal defect causes left to right shunt due to the high compliance of the right atrium and the difference in pressure between the two atria. Secondary to this mechanism, the pressure in the pulmonary circulation is increased.