Cyanosis is a bluish or purplish discoloration of the skin, lips, tongue, or nail beds caused by an increased amount of deoxygenated hemoglobin in the blood.

Types

Central cyanosis
Bluish discoloration of the lips, tongue, and mucous membranes.
Usually due to low oxygen levels in arterial blood.

Causes:
Severe lung diseases (e.g., Pneumonia, Chronic Obstructive Pulmonary Disease)
Congenital heart diseases with right-to-left shunts
High-altitude hypoxia
Peripheral cyanosis
Bluish discoloration mainly of fingers, toes, ears, and nose.
Due to reduced blood flow or increased oxygen extraction by tissues.
Causes:
Cold exposure
Shock
Heart failure
Peripheral vascular disease.

Clinical significance
Cyanosis is a sign of inadequate oxygenation or circulation and may indicate a serious underlying condition.

When to seek urgent medical attention
Seek emergency care if cyanosis is accompanied by:
Difficulty breathing
Chest pain
Altered consciousness
Sudden onset of bluish lips or face.