The most common type of congenital heart defect is the ventricular septal defect or VSD. This is what many people call the "hole-in-the-heart." The ventricular septum is the wall that separates the right and left sides of the heart. A hole in the wall between the two ventricles is called a ventricular septal defect.

Usually, blood entering the right side of the heart stays on the right side (low oxygen blood), and blood on the left side stays on the left side (oxygenated blood) which is then pumped to the rest of the body. When a defect or "hole" is present between the ventricles, blood from the left side of the heart is forced through the defect to the right side every time the heart beats. It then goes back to the lungs even though it is already rich in oxygen. Because of this, blood that is not oxygenated cannot get to the lungs.