What is Cyanosis? Blue to Purple Skin Discoloration (Low Oxygen)

Cyanosis is the medical term for a blue to purple discoloration of the skin which arises from deoxygenated hemoglobin in the blood stream. It may also be seen on the mucous membranes, especially of the mouth, tongue and eyes. It is an indication of poor oxygen saturation – low availability of oxygen – in the arterial blood.

The red blood cells carry oxygen through the blood stream by binding it to the molecule hemoglobin. It gives the blood its characteristic red color. The higher the blood oxygen saturation, the redder the color of blood. Hemoglobin molecules that do no contain oxygen (deoxygenated hemoglobin) has a dark red to purple color. This is usually not evident since the circulating red blood cells are quickly oxygenated again as it travels through the blood vessels of the lung.

However, under certain conditions, the blood may not be oxygenated fast enough and the deoxygenated hemoglobin builds up. Cyanosis is evident when the blood in the arteries contain more than 5 grams of deoxygenated hemoglobin per 100 milliliters of blood (> 5g/dL or > 50g/L). This is hypoxemia which is the decrease in partial pressure of oxygen in the blood, or simply, insufficient blood oxygenation with an oxygen saturation (SaO2) of less than 90%. Oxygen saturation is the measure of oxygen transported in the blood stream expressed as a percentage relative to the maximum oxygen-carrying capacity.

Cyanosis can be central where the bluish discoloration of the skin is evident on the face, particularly the mouth and tongue, or it may be peripheral where it is only evident in the arms and legs, particularly the fingertips and toes, and even the ears.

What does cyanosis mean?

Cyanosis is an indication of one of two disturbances – lung oxygenation is compromised or blood circulation is inadequate. The respiratory and cardiovascular components may sometimes co-exist. Cyanosis associated with hypoxia will lead to other symptoms like breathlessness, dizziness, and swelling. There are certain blood disorders that will also contribute to cyanosis although respiratory and cardiovascular functioning is intact.

With conditions like methemoglobinemia, oxygen cannot bind to the methemoglobin (oxidized form of hemoglobin) in red blood cells and due to a higher than normal concentration of methemoglobin, the oxygen saturation concentration of arterial blood is significantly reduced. In other conditions like sulfhemoglobinemia, sulfur binds to hemoglobin and prevents oxygen from bindng to it. Pseudocyanosis is a bluish tinge of the skin and mucous membranes that ocurs with the exposure to certain metals and drugs and is not an indication of low oxygen saturation.

Cyanosis with Polycythemia and Anemia

It is the high concentration of deoxygenated blood in the skin’s blood vessels, particularly the arteries,  coupled with the properties of human skin that contributes to the blue-purple skin color (cyanosis). However, with certain red blood cell disorder, cyanosis may not be present at the levels mentioned above. This applies mainly to polycythemia and anemia which are discussed under too many and too few red blood cells.

With polycythemia, the excess of red blood cells and therefore hemoglobin in circulation means that even a slight decrease in blood oxygen levels will lead to cyanosis. Patients with polycythemia can therefore become slightly cyanosed even with normal oxygen saturation. In anemia, the low hemoglobin levels means that there is insufficient hemoglobin to become deoxygenated and therefore cyanosis is rarely ever seen.

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