A blue baby is an infant with cyanosis, a bluish coloration of the skin resulting from incomplete oxygenation of the blood
in the arteries. The cyanosis commonly occurs in a congenital disease called patent, or open, ductus arteriosus.
The ductus arteriosus is a shunt between the pulmonary artery and the aorta. In the fetus it diverts venous blood away from the nonfunctioning lungs to the aorta and eventually into the placental circulation, where gas
exchange occurs between fetal and maternal blood. After birth, the shunt normally closes, causing venous blood to be carried through the baby's pulmonary artery to the lungs for gas exchange. In some babies, however, the duct fails to close, causing oxygen-deficient venous blood to mix with
arterial blood in the aorta and be sent throughout the body. The result is a chronic deficiency of oxygen in the blood.
Cyanosis is also seen in a congenital condition called the tetralogy of Fallot, in which a hole in the wall between the right and left ventricles of the heart allows venous and arterial blood to mix. Surgical correction of these circulatory defects is necessary and is highly successful. Without treatment, the average blue baby lives only until the age of 12 years.