PFO & ASD Closure - Atrial septal defect (ASD) is a form of congenital heart defect that enables blood flow between the left and right atria via the interatrial septum. The interatrial septum is the tissue that divides the right and left atria. Without this septum, or if there is a defect in this septum, it is possible for blood to travel from the left side of the heart to the right side of the heart, or vice versa. This results in the mixing of arterial and venous blood, which may or may not be clinically significant. This mixture of blood may or may not result in what is known as a "shunt". The amount of shunting present, if any, dictates hemodynamic significance (see Pathophysiology below). A "right-to-left-shunt" typically poses the more dangerous scenario.