Tetralogy of Fallot (TOF)
Tetralogy of Fallot (TOF) is a combination of four different heart problems. The term "tetralogy" comes from the Greek word for "four." The four problems are: pulmonary stenosis (PS), ventricular septal defect (VSD), overriding of the aorta and right ventricular hypertrophy. It is helpful to first understand each of the four defects separately and then look at the effect of their combination on the heart.
Pulmonary Stenosis
When the pulmonary artery, the artery that carries blue blood from the right ventricle to the lungs, is blocked, the condition is called pulmonary stenosis. The blockage may be the result of too much muscle below the pulmonary valve or that the valve itself is too small or unable to open all the way. The blood vessel, the main pulmonary artery, also may be too narrow above the valve. In severe cases of TOF, the entire pulmonary arterial system can be underdeveloped with all branches being smaller than normal. This condition is called hypoplastic pulmonary arteries.
Ventricular Septal Defect
A ventricular septal defect (VSD) is a hole between the two lower pumping chambers (ventricles) of the heart. This is the most common of all congenital heart defects. In TOF, the hole is very large and situated under the aorta. The pressure in the two ventricles is equally high due to the large hole.
Overriding Aorta
Normally the aorta comes from the left lower chamber of the heart, the left ventricle. The aorta is the largest blood vessel in the heart and carries pink blood from the heart to all parts of the body. When the aorta is described as overriding, it means that the vessel is inappropriately positioned and straddles both the right and left ventricle just above the ventricular septal defect.
Right Ventricular Hypertrophy
The right ventricle is the lower chamber of the heart that pumps blue blood to the pulmonary arteries. Normally, the RV muscle is thin. Hypertrophy means the muscle wall of the chamber has become thickened. This occurs in TOF because of the extra work the muscle must do to pump blood past the blocked pulmonary artery as well as pump blood to the high-pressure aorta.
The combination of these four heart defects comprises the condition called Tetralogy of Fallot. The result is that blue blood from the right ventricle is partially blocked from getting to the lungs. Instead, some blood can go through the VSD and out the overriding aorta to the body. The more severe the blockage in the pulmonary arteries, the more blue blood will go out the aorta instead. When blue blood goes out to the body, the child will not appear as pink as normal; his or her skin, lips and nails will appear to have a bluish color. This is called cyanosis.
The severity of pulmonary stenosis in TOF can vary quite a bit. It may be very mild in the newborn because very little blue blood will go out the aorta. The pulmonary stenosis, however, tends to become more severe as the baby grows. The baby’s oxygen level will slowly decrease, until surgical correction of the TOF will become necessary. If the branches of the pulmonary artery are close to normal size, this operation can be done in one step.
If, however, the pulmonary arteries of the baby are hypoplastic, that is, too small, an additional procedure may be needed first to help these blood vessels grow to a more normal size. This operation, usually done in the first months of life, involves creating a shunt to force more blood into the pulmonary arteries, causing them to grow over time while bringing more blood to the lungs and raising the baby’s oxygen level.
Complete repair of TOF is usually done in the first year of life. Complete repair of TOF consists of patch closure of the VSD so that the aorta arises solely from the LV, as well as enlarging the RV outflow tract, pulmonary valve, and branch PAs as much as possible. A severe event called a "Tet spell" can be seen in children with severe Tetralogy of Fallot, especially if the child has a great deal of extra muscle in the right ventricle blocking the area below the pulmonary artery. In a "Tet spell," the extra muscle in the heart below the pulmonary valve may squeeze down tightly, allowing very little blood to get to the lungs. The baby will become irritable and very blue, breath very fast and may even pass out. If this happens, bring the baby’s knees all the way up to his chest, call 911 and alert the doctor immediately. A "Tet spell" can be very dangerous for the baby and may require urgent surgery. There are also medications that can be used to treat a “Tet spell,” or prevent it from occurring again.
DIAGRAMS OF NORMAL & TOF HEARTS
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| NORMAL HEART |
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| TOF HEART |
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