cyanosis
Cyanosis is a bluish or purplish tinge of the skin and mucous membranes that appears when the blood carries too little oxygen. Oxygen-rich blood is bright red; oxygen-poor blood is dark, almost bluish, and when enough of it shows through thin tissues — the lips, tongue, nail beds, or fingertips — the skin takes on the dusky color the term describes.
It is divided into two patterns. Central cyanosis affects warm, well-perfused areas like the lips and tongue and reflects genuinely low oxygen in the arterial blood, from heart or lung disease — for example, congenital defects that let blue blood bypass the lungs and mix into the body's circulation. Peripheral cyanosis affects the hands and feet alone and usually reflects sluggish flow or cold, where tissues extract extra oxygen from blood that is moving too slowly, without the arterial blood itself being abnormal.
The amount of poorly oxygenated hemoglobin needed to make cyanosis visible means it can be hard to spot in anemia and easier to see when the blood count is high; lighting and skin tone also affect detection. It is therefore a useful but imperfect clinical sign, best confirmed by measuring blood oxygen directly.
A newborn with a cyanotic congenital heart defect has persistently blue lips and tongue that do not pink up with extra oxygen, signaling that blue blood is bypassing the lungs.
Blue lips and tongue that fail to improve with oxygen point to central cyanosis from a right-to-left shunt.
Central cyanosis is the more serious pattern because it indicates low oxygen in the arterial blood reaching the whole body, whereas isolated peripheral cyanosis is often due simply to cold or poor local circulation.