ToF occurs when the part of the wall between the pumping chambers close to the valves is too far towards the right, leading to a narrowed valve between the right chamber and the lung arteries, a hole between the pumping chambers (VSD) and the aorta overriding this defect. The muscle of the right chamber can be quite thick.
Depending on the size of the child, surgery involves one or two steps. In some cases an artificial vessel (called a BT-shunt), is needed first to provide enough blood flow into the lungs to allow the arteries to grow. Later, corrective surgery is carried out. In most cases the shunt is not necessary.