Cyanosis

Cyanosis is a bluish or purplish discoloration of skin or mucous membranes caused by increased deoxygenated hemoglobin or abnormal hemoglobin in blood.

Deoxyhemoglobin: Hemoglobin not bound to oxygen, which gives blood a darker red appearance. Its increased concentration in superficial blood can make skin or mucosa appear blue.
Tetralogy of Fallot: A congenital heart defect combining four structural abnormalities and often causing a right-to-left shunt. Severe cases cause central cyanosis by allowing oxygen-poor blood to enter systemic circulation.
Hemoglobin: The iron-containing protein in red blood cells that transports oxygen and helps carry carbon dioxide. Cyanosis reflects the color or quantity of hemoglobin in superficial blood.
Pallor: Unusual paleness of skin or mucous membranes, often associated with reduced blood flow or anemia. Unlike cyanosis, pallor lacks the blue-purple hue associated with deoxygenated or abnormal hemoglobin.
Oxyhemoglobin: Hemoglobin bound to oxygen, giving arterial blood its brighter red color. Its proportion relative to deoxyhemoglobin helps determine whether cyanosis is visible.
Pulmonary embolism: A blockage of a pulmonary artery, usually caused by a blood clot traveling from the veins. A large embolism can impair oxygenation and cause acute cyanosis.
Red blood cell: A blood cell that transports oxygen using hemoglobin and circulates through the body. The amount of hemoglobin carried in these cells influences whether cyanosis becomes visible.
Ecchymosis: A flat area of skin discoloration caused by blood leaking into surrounding tissue, commonly called a bruise. Bruising can look blue-purple but reflects extravasated blood, not circulating hemoglobin.
Oxygen saturation: The percentage of hemoglobin binding sites occupied by oxygen. Low saturation can increase deoxyhemoglobin, but cyanosis is an unreliable measure of it.
Chronic obstructive pulmonary disease: A chronic lung disease characterized by persistent airflow limitation, most often from emphysema or chronic bronchitis. Advanced disease can reduce arterial oxygen enough to produce central cyanosis.

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