Tetralogy of Fallot / TOF is a cardiac anomaly that refers to a combination of four related heart defects that commonly occur together. The four defects are:
- Ventricular septal defect / VSD
- Overriding aorta − the aortic valve is enlarged and appears to arise from both the left and right ventricles instead of the left ventricle as in normal hearts
- Pulmonary stenosis − narrowing of the pulmonary valve and outflow tract or area below the valve that creates an obstruction (blockage) of blood flow from the right ventricle to the pulmonary artery
- Right ventricular hypertrophy − thickening of the muscular walls of the right ventricle, which occurs because the right ventricle is pumping at high pressure
A small percentage of children with tetralogy of Fallot may also have additional ventricular septal defects, an atrial septal defect / ASD or abnormalities in the branching pattern of their coronary arteries. Some patients with tetralogy of Fallot have complete obstruction to flow from the right ventricle, or pulmonary atresia. Tetralogy of Fallot may be associated with chromosomal abnormalities, such as 22q11 deletion syndrome.
The pulmonary stenosis and right ventricular outflow tract obstruction seen with tetralogy of Fallot usually limits blood flow to the lungs. When blood flow to the lungs is restricted, the combination of the ventricular septal defect and overriding aorta allows oxygen-poor blood ("blue") returning to the right atrium and right ventricle to be pumped out the aorta to the body.
This "shunting" of oxygen-poor blood from the right ventricle to the body results in a reduction in the arterial oxygen saturation so that babies appear cyanotic, or blue. The cyanosis occurs because oxygen-poor blood is darker and has a blue color, so that the lips and skin appear blue.
The extent of cyanosis is dependent on the amount of narrowing of the pulmonary valve and right ventricular outflow tract. A narrower outflow tract from the right ventricle is more restrictive to blood flow to the lungs, which in turn lowers the arterial oxygen level since more oxygen-poor blood is shunted from the right ventricle to the aorta.