DefinitionTetralogy of Fallot refers to a type of congenital heart defect. Congenital means present at birth.Alternative NamesTet; TOFCauses, incidence, and risk factorsTetralogy 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 t