Blue colouring of the skin and mucous membranes. The condition is the result of oxygen shortage in the tissue, and occurs whenever capillary oxygen content falls below a certain level. There are two kinds of cyanosis, central and peripheral. The central type is caused by too little oxygen reaching the blood from the lungs, leading to oxygen shortage in all tissues and often originating in a lung condition such as ephysema, severe pneumonia or interstitial fibrosis. It is also caused whenever too little blood reaches the lungs, e.g. in congenital heart abnormalities (e.g. Fallot’s tetralogy). Central cyanosis can also occur as a result of heart failure, when too little blood reaches the heart, or when there is a shortage of oxygen in the air, in the mountains, for example, or in an unventilated room. In peripheral cyanosis the blood may well contain sufficient oxygen but certain parts of the body are not properly supplied because the arteries serving them are constricted by Raynaud’s disease or acrocyanosis. This also occurs when the heart is not strong enough to pump sufficient blood to the fingers and toes. In both conditions treatment is entirely dependent on the cause.