What is tetralogy of Fallot?
Tetralogy of Fallot (TOF or "TET") is a complex condition of several congenital (present at birth) defects that occur due to abnormal development of the fetal heart during the first eight weeks of pregnancy. These problems include the following:
- Ventricular septal defect (VSD- An opening in the ventricular septum, or dividing wall between the two lower chambers of the heart known as the right and left ventricles.
- Pulmonary (or right ventricular outflow tract) obstruction- A muscular obstruction in the right ventricle, just below the pulmonary valve, that decreases the normal flow of blood. The pulmonary valve may also be small.
- Overriding aorta- The aorta is shifted towards the right side of the heart so that it sits over the ventricular septal defect.
"Tetralogy" refers to four heart problems. The fourth problem is that the right ventricle becomes enlarged as it tries to pump blood past the obstruction into the pulmonary artery.
Normally, oxygen-poor (blue) blood returns to the right atrium from the body, travels to the right ventricle, then is pumped through the pulmonary artery into the lungs where it receives oxygen. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle, and then is pumped through the aorta out to the body.
In tetralogy of Fallot, blood flow within the heart varies, and is largely dependent on the size of the ventricular septal defect, and how severe the obstruction in the right ventricle is.
- With mild right ventricle obstruction, the pressure in the right ventricle can be slightly higher than the left. Some of the oxygen-poor (blue) blood in the right ventricle will pass through the VSD to the left ventricle, mix with the oxygen-rich (red) blood there, and then flow into the aorta. The rest of the oxygen-poor (blue) blood will go its normal route to the lungs. These children may have slightly lower oxygen levels than usual, but may not appear blue.
- With more serious obstruction in the right ventricle, it is harder for oxygen-poor (blue) blood to flow into the pulmonary artery, so more of it passes through the VSD into the left ventricle, mixing with oxygen-rich (red) blood, and then moving on out to the body. These children will have lower than normal oxygen levels in the bloodstream, and may appear blue, especially whenever the pressure in the right ventricle is very high and large amounts of oxygen-poor (blue) blood passes through the VSD to the left side of the heart.
According to the National Heart, Lung, and Blood Institute (NHLBI), tetrology of Fallot affects about five of every 10,000 babies and occurs equally in boys and in girls.