(Note: It will be helpful to review normal heart function before reading this.)

The tetralogy of Fallot has four components. The first major one is a ventricular septal defect. This is a large hole between the two ventricles that lets venous (bluish) blood pass from the right ventricle to the left one. From there it goes to the aorta and on to the body without passing through the lungs to be refreshed with oxygen.

The second major component of tetralogy of Fallot is a stenosis (narrowing) at, or just beneath, the pulmonary valve. The narrowing partly blocks the flow of venous blood into the lungs. This varies in severity from child to child.

The last two components of tetralogy of Fallot are: (1) the right ventricle is more muscular than normal; (2) and the aorta lies directly over the ventricular septal defect. This results in blueness (cyanosis), which may appear soon after birth, in infancy or later in childhood. These "blue babies" may have sudden episodes of severe cyanosis with rapid breathing. They may even become unconscious. During exercise, older children may become short of breath and have fainting spells. These symptoms happen because not enough blood flows to the lungs to supply the child's body with oxygen. Some infants with severe tetralogy of Fallot may need an operation that will give temporary relief by increasing blood flow to the lungs with a shunt. This procedure is done by making a connection between the aorta and the pulmonary artery. Thus, some blood from the aorta flows into the lungs to get more oxygen. This reduces the cyanosis and allows the child to grow and develop until the repair can be done when the child is older.

Most children with tetralogy of Fallot have open-heart surgery before school age. The operation involves closing the ventricular septal defect and removing the obstructing muscle. If the pulmonary valve is narrow, it's opened; and if it's small, a graft or patch may be needed to finish the repair. After surgery the long-term outlook varies a great deal. Usually it's quite good, but it depends largely on how severe the defects were before surgery -- especially the amount of pulmonary narrowing.

Lifelong follow-up is needed to be sure that any remaining defects or problems are treated properly. Children with tetralogy of Fallot risk an infection of the heart's walls or valves (endocarditis) before and after surgery. Your child should get antibiotics such as amoxicillin before dental work and certain surgeries to help prevent endocarditis. Good dental hygiene also lowers the risk of endocarditis. For more information about dental hygiene and preventing endocarditis, ask your pediatric cardiologist.

Copied from the American Heart Association Site