How it works? Transcatheter closure of Atrial Septal Defect (ASD) An atrial septal defect (ASD) is a congenital heart defect that involves a hole in the atrial septum. This is the muscular wall that separates the two upper chambers (atria) of the heart. Every child is born with an opening between the upper heart chambers. It's a normal fetal opening that allows blood to detour away from the lungs before birth. After birth, the opening is no longer needed and usually closes or becomes very small