Tetralogy of Fallot is a condition in which a child is born with the following four different heart defects:

• Overriding aorta. Normally the large blood vessel that carries blood to the body (aorta) receives only oxygen-rich blood from the left side of the heart. With an overriding aorta, the aorta gets blood from both lower chambers of the heart. This lets oxygen-poor blood mix with oxygen-rich blood, allowing oxygen-poor blood to flow to the body.
• Ventricular septal defect (VSD). A ventricular septal defect is an opening in the heart wall (septum). In tetralogy of Fallot, there is a very large opening in the wall between the lower heart chambers (ventricles). This lets oxygen-poor blood mix with oxygen-rich blood, allowing oxygen-poor blood to flow to the body.
• Pulmonary stenosis. In tetralogy of Fallot, there is also a narrowing (stenosis) of the pulmonary valve between the lower right heart chamber and the pulmonary artery, which carries blood to the lungs. The narrow valve lets less blood flow through the pulmonary artery to the lungs.
• Thickened right lower chamber of the heart. Because the pulmonary valve is narrowed, it is more difficult for blood to be pumped out of the lower right chamber of the heart. This makes the heart chamber thicker.

Tetralogy of Fallot is treated surgically. If surgery is done on a small baby, another surgery is sometimes needed later. Complete surgical repair of the condition is often done when a child is bigger and stronger, such as between ages 1 year and 5 years.

People who have had tetralogy of Fallot surgically repaired can usually do most normal activities. But competitive sports and strenuous exercise may need to be restricted. The person needs to be closely monitored by a doctor to detect and treat any problems right away.