cyanosis

Cyanosis is a bluish or purplish tint of the skin, lips or nail beds that appears when the blood is carrying too little oxygen. The color comes from hemoglobin, the oxygen-carrying pigment in red cells: oxygen-rich hemoglobin is bright red, while hemoglobin without oxygen is dark and bluish, and when enough of the latter accumulates the tissues take on that hue.

Clinicians divide it into two kinds. Central cyanosis affects warm, well-supplied areas such as the lips and tongue and reflects a genuine fall in blood oxygen from lung or heart disease. Peripheral cyanosis affects the fingers, toes or earlobes and can occur simply because cold or poor circulation lets the tissues extract more oxygen from slowly flowing blood, without the central blood being abnormal.

Cyanosis is a relatively late and imperfect sign. It depends on the absolute amount of deoxygenated hemoglobin, so a very anemic person may be dangerously short of oxygen yet not look blue, while someone with extra red cells may look blue at milder degrees of low oxygen. Skin pigmentation and lighting also make it harder to judge in some patients. For these reasons it is confirmed and quantified by pulse oximetry and blood gases rather than relied on by eye alone.