Named for the French physician who first described it in 1888, tetralogy of Fallot is a type of heart defect that is present at birth (congenital heart disease). According to the American Heart Association, it occurs in less than 5 out of 10,000 babies. It involves a tetralogy (a complex of four conditions) that includes: Ventricular septal defect (VSD). A hole in the wall (septum) between the heart’s two lower chambers (the ventricles). The VSD is the prime defect with tetralogy of Fallot and leads to all the other conditions seen. This particular VSD however is different than most other VSDs in that its position is shiftCongenital heart disease is any heart abnormality, defect or malformation present from birth.ed in such a way that there is a tendency for much of the blood from the right ventricle to be shunted toward the left ventricle. This tendency to have the blood criss-cross at the VSD results in a significant amount of oxygen poor blood being pumped out to the body without going to the lungs first. Hypertrophy (enlargement and thickening) of the right ventricle. An enlargement of the muscle tissue of the right ventricle due to overexertion, usually as a result of increased blood flow to the right side of the heart (caused by the VSD) and by blockage of the blood being pumped out to the lungs. Much of the hypertrophy is secondary to the unusual location of the VSD causing some of the muscle bundles in the right ventricle to abnormally thicken. This will block the channel through which the right ventricle pumps blood out to the lungs. Pulmonary stenosis. A narrowing (stenosis) of the pulmonary valve or the channel in the right ventricle leading to the pulmonary valve. This narrowing decreases the amount of oxygen-poor blood from the right ventricle that can travel through the pulmonary artery to the lungs. Thus, there is a decreased blood flow to the lungs. Displaced, deviated or overriding aorta. This is another effect of the location of the VSD. Instead of opening into the left ventricle, which pumps oxygen-rich blood into the aorta, the main artery out to the body opens into both the right and left ventricle. This allows excess blood in the right ventricle (usually as a result of pulmonary stenosis) to be pumped out to the body. Because the body is receiving so much oxygen-poor blood, the skin of the child often has a bluish tint (cyanosis, or blue baby when occurring in infants). Prenatal heart circulation is different than adults and the heart continues to evolve after birth. The combination of these four heart defects lead to reduced blood flow to the lungs because less oxygen-poor blood can squeeze through the pulmonary valve to get to the lungs, and more oxygen-poor blood is pumped to the tissues of the body.