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By Noah Lechtzin, MD, MHS, Associate Professor of Medicine and Director, Adult Cystic Fibrosis Program, Johns Hopkins University School of Medicine

Cyanosis is a bluish discoloration of the skin resulting from an inadequate amount of oxygen in the blood.

Cyanosis occurs when oxygen-depleted (deoxygenated) blood, which is bluish rather than red, circulates through the skin. Cyanosis can be caused by many types of severe lung or heart disease that cause levels of oxygen in the blood to be low. It can also result from certain blood vessel and heart malformations that allow blood to flow directly to the heart without ever flowing past the air sacs of the lung (alveoli) where oxygen is extracted from the air. This abnormal blood flow is called a shunt.

In a shunt, blood from veins in the body, which is oxygen-depleted, may flow directly into blood vessels returning blood from the lungs to the left side of the heart or directly into the left side of the heart itself. The oxygen-depleted blood then is pumped out to the body, to circulate through the skin and other tissues.

The amount of oxygen in the blood can be estimated by pulse oximetry, in which a sensor is attached to a finger or an earlobe, or it can be measured directly by arterial blood gas analysis. Chest x-rays, echocardiography, cardiac catheterization, pulmonary function tests, and sometimes other tests may be needed to determine the cause of decreased oxygen in the blood and the resulting cyanosis.

Oxygen therapy is often the first treatment given, in a similar fashion as for other conditions in which the level of oxygen in the blood is low. Many malformations that cause shunts can be treated with surgery or other procedures.