cyanosis this, like any word with the prefix
cyan, derives from the Greek for dark blue. It refers to a blue tinge seen on the surface of the whole or part of the body, due to lack of
oxygen in the blood. The apparent colour of the skin depends on the state of oxygenation of the blood in the microscopic vessels below the surface. Blood in the arteries is normally bright red, the colour of red blood cells when the
haemoglobin they contain is carrying its full quota of oxygen. In conditions of
hypoxia due to
altitude, lung disease, heart defects, or
heart failure, the blood leaves the lungs without being fully oxygenated, and the arterial blood is less red. The degree of desaturation of haemoglobin at which such ‘central cyanosis’ is detectable varies between observers as well as between patients. Detection also depends on the superficial blood vessels being well-filled; if they are largely ‘shut down’ the skin is simply pale whatever the colour of the blood. Undoubtedly, however, if blueness is evident, there is significant hypoxia.
When arterial oxygen saturation is normal, the extent to which the blood becomes desaturated as it flows through the skin depends on the rate of blood flow. If blood flow is sluggish, a larger fraction of the oxygen is removed than if it is florid. Thus when cheeks are flushed, increased blood flow brings bright red blood near the surface; the oxygen supply is far in excess of need, with very little being removed. But when hands and feet are cold, the reflex constriction of blood vessels — to conserve heat as part of body
temperature regulation — reduces the flow, so a higher proportion of the oxygen is removed to supply the skin tissue, and the blood becomes bluer before it moves on. Hence we can become ‘blue with cold’ — but only superficially. The arterial blood itself remains bright red, if everything else is normal. For similar reasons of diminished blood flow, cyanosis is seen locally in a part of the body — say a leg or a big toe — when the circulation in that part is compromised by arterial disease.
Sheila Jennett
See also
breathing;
haemoglobin;
hypoxia;
lungs;
oxygen.
