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Causes
By Mayo Clinic staff
Tetralogy of Fallot involves four ("tetra") defects within the heart structures of infants and young children. Together these defects cause reduced blood flow to the lungs and mixing of blood from both sides of the heart. This results in oxygen-poor blood flowing out of the heart and into the body — usually resulting in blue-tinged skin (cyanosis) in infants and children with tetralogy of Fallot.
A problem during fetal growth
Tetralogy of Fallot occurs during fetal growth, when the baby's heart is developing. While factors such as poor maternal nutrition, viral illness or genetic disorders may increase the risk of this condition, in the majority of cases the cause of tetralogy of Fallot is unknown.
Tetralogy of Fallot is named after a doctor who described four heart abnormalities common to several of his patients in 1888. This group of abnormalities includes:
- Pulmonary valve stenosis. This is a narrowing of the pulmonary valve, the flap that separates the right ventricle of the heart from the pulmonary artery, the main blood vessel leading to the lungs. Constriction of the pulmonary valve reduces blood flow to the lungs.
- Ventricular septal defect. This is a hole in the wall that separates the two lower chambers (ventricles) of the heart. The hole allows deoxygenated blood in the right ventricle — blood that has circulated through the body and is en route to the lungs to replenish its oxygen supply — to flow into the left ventricle and mix with oxygenated blood fresh from the lungs. Blood from the left ventricle also flows back to the right ventricle in an inefficient manner. This ability for blood to flow through the ventricular septal defect dilutes the supply of oxygenated blood to the body and eventually can weaken the heart.
- Overriding aorta. Normally the aorta, the main artery leading out to the body, branches off the left ventricle. In tetralogy of Fallot, the aorta is shifted slightly to the right and lies directly above the ventricular septal defect. In this position the aorta receives blood from both the right and left ventricles, mixing the oxygen-poor blood from the right ventricle with the oxygen-rich blood from the left ventricle.
- Right ventricular hypertrophy. When the heart's pumping action is overworked, it causes the muscular wall of the right ventricle to enlarge and thicken. Over time this may cause the heart to stiffen, become weak and eventually fail.
The combined result of these abnormalities is that less blood reaches the lungs to pick up oxygen, more oxygen-poor blood is diverted into the left ventricle and the body fails to receive enough oxygenated blood.
