A specially trained doctor called a pediatric interventional cardiologist inserts a balloon-tipped catheter (a thin, flexible tube) into a vein, guides it into the heart’s upper right chamber (the right atrium) and then through the natural hole in the wall between the chambers of the heart (called the septum), and into the left atrium. Once inside the left atrium, the balloon is inflated. The interventional cardiologist then pulls the catheter back into the right atrium - with the balloon still inflated on the catheter tip. This enlarges the hole in the wall between the two heart chambers. When this happens, more blood can flow through the chambers to allow blood to get to the pumping chambers to get out of the heart.
This initial management involves making sure that just the right amount of blood can go to the lungs, since this circulation can sometimes send too much blood to the lungs. This is accomplished with surgery. Some babies are born with a natural restriction that allows a better balance of blood flow to the body and lungs, and will not require an operation shortly after birth.