Skin coloration is determined by the amount of pigment in the skin and the blood flowing through it. Blood that is saturated with oxygen is bright red. Blood that has lost its oxygen is dark bluish-red. People who have a large quantity of blood deficient in oxygen tend to take on a bluish discoloration called cyanosis.
Lack of oxygen (such as in suffocation or cyanotic heart disease), abnormal hemoglobin (such as methemoglobinemia) and toxins (such as cyanide) can all produce cyanosis. Most cyanosis occurs as a result of heart disease such as congestive heart failure, lung disease, or as a terminal event such as cardiopulmonary arrest.
Mild cyanosis is difficult to detect. There needs to be more than 5 grams of deoxygenated hemoglobin in a person's bloodstream to see cyanosis. Usually the oxygen saturation of the blood has to drop below 90% before this occurs.
Cyanosis is more obvious in the mucous membranes and nail beds, particularly for dark-skinned people. It may also appear on the feet, nose, and ears. Many people get brief, bluish-purplish, painful color changes in their fingers (called Raynaud's syndrome) resulting from spasm of blood vessels in the hands, often in response to the cold. Some people with Raynaud's syndrome also have a collagen-vascular disease called scleroderma.
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