Blue baby syndrome can refer to a number of conditions that affect oxygen transportation in the blood, resulting in blueness of the skin in babies. Historically, the term "blue baby syndrome" has referred to babies with one of two conditions:Cyanotic heart disease, which is a category of congenital heart defect that results in low levels of oxygen in the blood. This can be caused by either reduced blood flow to the lungs or mixing of oxygenated and deoxygenated blood. Methemoglobinemia, which is a disease defined by high levels of methemoglobin in the blood. Increased levels of methemoglobin prevent oxygen from being released into the tissues and result in hypoxemia.
Both of these conditions cause cyanosis, or a bluish discoloration of skin or mucous membranes. Normally, oxygenated blood appears red and deoxygenated blood has more of a blue appearance. In babies with low levels of oxygen or mixing of oxygenated and deoxygenated blood, the blood can have a blue or purple color, causing cyanosis.