What is tetralogy of Fallot?
Tetralogy of Fallot is the most common form of cyanotic congenital heart disease. Cyanosis is the abnormal bluish discoloration of the skin that occurs because of low levels of circulating oxygen in the blood. Tetralogy of Fallot consists of the combination of four different heart defects: a ventricular septal defect (VSD); obstructed outflow of blood from the right
ventricle to the lungs (pulmonary stenosis); a displaced aorta, which causes blood to flow into the aorta from both the right and left ventricles (dextroposition or overriding aorta); and abnormal enlargement of the right ventricle (right ventricular hypertrophy). The severity of the symptoms is related to the degree of blood flow obstruction from the right ventricle.
The normal heart has four chambers. The two upper chambers, known as atria, are separated from each other by a fibrous partition known as the atrial septum. The two lower chambers are known as ventricles and are separated from each other by the ventricular septum. Valves connect the atria (left and right) to their respective ventricles. The valves allow for blood to be pumped through the chambers. Blood travels from the right ventricle through the pulmonary artery to the lungs where it receives oxygen. The blood returns to the heart through pulmonary veins and enters the left ventricle. The left ventricle sends the now oxygen-filled blood into the main artery of the body (aorta). The aorta sends the blood throughout the body.
Tetralogy of Fallot is actually four defects in combination. First, the septum that divides the two ventricles is incomplete (so there is a ventricular septal defect), and oxygen-poor blood is thus allowed to mix with oxygen-rich blood. Second, the passageway from the right ventricle to the lungs is markedly narrowed. Third, the origin of the aorta is shifted toward the right side of the heart from the left. Fourth, the muscle in the wall of the right ventricle is thickened and stiffened. Only the first two of these defects cause significant trouble or require an operation. Tetralogy of Fallot constitutes about 10 percent of all congenital heart disease.
"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.