Tetralogy of Fallot refers to a type of congenital heart defect. Congenital means present at birth.

Tetralogy of Fallot is classified as a cyanotic heart defect because the condition causes low oxygen levels in the blood. This leads to cyanosis (a bluish-purple color to the skin).

The classic form of tetralogy includes four related defects of the heart and its major blood vessels:

• Ventricular septal defect (hole between the right and left ventricles)
• Narrowing of the pulmonary outflow tract (the valve and artery that connect the heart with the lungs)
• Overriding aorta (the artery that carries oxygen-rich blood to the body) that is shifted over the right ventricle and ventricular septal defect, instead of coming out only from the left ventricle
• A thickened muscular wall of the right ventricle (right ventricular hypertrophy)

At birth, infants may not show signs of cyanosis. However, later they may develop sudden episodes (called "Tet spells") of bluish skin from crying or feeding.

Tetralogy of Fallot is rare, but it is the most common form of cyanotic congenital heart disease. Patients with tetraology of Fallot have a higher incidence of major non-heart congenital defects.

The cause of most congenital heart defects is unknown. Many factors seem to be involved.

Factors that increase the risk for this condition during pregnancy include:

• Alcoholism in the mother
• Diabetes
• Mother who is over 40 years old
• Poor nutrition during pregnancy
• Rubella or other viral illnesses during pregnancy

There is a high incidence of chromosomal disorders in children with tetralogy of Fallot, such as Down syndrome and DiGeorge syndrome (a condition that causes heart defects, low calcium levels, and immune deficiency).