“In most children, the cause of tetralogy of Fallot isn’t known. It’s a common type of heart defect. It may be seen more commonly in children with Down syndrome or DiGeorge syndrome. Some children can have other heart defects along with tetralogy of Fallot.  Infants and young children with unrepaired tetralogy of Fallot are often blue (cyanotic-not enough oxygen getting to their tissues).”

AHA American Heart Association (www.heart.org/en/health-topics/congenital-heart-defects/about-congenital-heart-defects)