A blue baby is an infant born with a type of congenital heart disease that causes the infant’s skin to have a bluish tint (cyanosis). Central cyanosis is a condition in which there is too much oxygen–poor blood traveling through the arteries, which normally carry only oxygen–rich blood to nourish the body. In technical terms, cyanosis is diagnosed when the level of reduced (desaturated) hemoglobin exceeds 3  grams per deciliter (g/dl) of blood in the arteries.

Not all cyanosis is caused by congenital heart defects. Peripheral cyanosis, or a slight bluish tinge in the extremities, may occur in healthy, fair–skinned babies. This type of cyanosis is normal. 

Cyanosis may also be caused by lung abnormalities and blood conditions that somehow impede the delivery of oxygen to the tissues and organs of the body.

In general, physicians have identified the five cardiac–related “Ts” that may cause cyanosis in newborns:

A sixth “T” is sometimes called “tons of other diseases,” which might include double outlet right ventricle, pulmonary atresia, hypoplastic left heart syndrome and many others.

In general, the defining feature that unites these various conditions and the underlying cause of blue baby is a cardiac defect known as right–to–left shunt. This describes a condition in which oxygen–poor blood from the body may be “shunted” across an opening in the heart. This allows it to pass from the right side of the heart, where there is normally only oxygen–poor blood, to the left side, where there is oxygen–rich blood. Other conditions may deliver the oxygen-poor blood directly out to the body. The common feature is oxygen-poor blood does not have the opportunity to go to the lungs prior to going out to the body. As a result, some oxygen–poor blood is pumped back out to the body, thus depriving the tissues and organs of necessary oxygen.

The right–to–left shunt may be part of the fundamental defect, or it may be an associated defect that is actually keeping the baby alive by allowing some oxygen–rich blood to be pumped from the heart. In some cases, physicians will create a temporary right–to–left shunt in babies with no cross–circulation between the left side of the heart and the right side in an attempt to keep the baby alive until open-heart surgery can be performed.