Cyanosis is the condition where the skin turns to a blue or purple hue.The red blood cells bring oxygen to the different body tissues and in return, take up the carbon dioxide and other wastes to be expelled from the body. It usually begins in the lips, tongue or fingernails, and most visible in the same areas. It is usually indicative of decreased oxygen in the bloodstream. It may denote an underlying lung illness but it may also connote of a heart or major blood vessel defect.

When oxygen levels drop by only a small amount, cyanosis may be hard to detect, especially in dark-skinned people. For dark-skinned people, it may be easier to detect cyanosis in the lips, gums, around the eyes and other mucus membranes.

Causes of peripheral cyanosis
-- All causes of central cyanosis cause peripheral cyanosis.
-- Reduced cardiac output - eg, heart failure, shock.
-- Peripheral arterial disease - eg, thrombosis, atheroma or embolism.

Raynaud's phenomenon.
-- Acrocyanosis: benign, caused by spasm of smaller skin arteries and arterioles, causing hands and feet to be cold and mottled.
-- Erythrocyanosis: usually affects young women; blotches of cyanosis occur in the lower legs.
Beta-blocker drugs.

Venous obstruction (eg, lower limb deep vein thrombosis) can occasionally produce a painful blue leg (phlegmasia cerulea dolens). Obstruction of the superior vena cava can cause cyanosis, venous engorgement and oedema affecting the face.

Symptoms
-- Cyanosis due to congenital heart disease causing anatomical right-to-left shunts may have been present since birth or the first few years of life.
-- Acute onset of cyanosis may be due to pulmonary emboli, cardiac failure, Pneumonia or Asthma.
-- Patients with COPD develop cyanosis over many years and associated polycythaemia may exacerbate the degree of cyanosis.
-- The description may be typical of Raynaud's phenomenon.
-- Past history: cyanosis can result from any lung disease of sufficient severity.
-- Drug history: certain drugs may cause methaemoglobinaemia (eg, nitrates, dapsone) or sulfhaemoglobinaemia (eg, metoclopramide).