cyanose

Cyanosis is a clinical sign characterized by a bluish discoloration of the skin and mucous membranes resulting from an excess of deoxygenated hemoglobin in arterial blood, or from abnormal hemoglobins. It is usually most evident in the lips, tongue, and nail beds and is considered an indicator of hypoxemia or abnormal hemoglobin.

There are two main forms: central cyanosis and peripheral cyanosis. Central cyanosis reflects a systemic reduction in arterial oxygen saturation and is seen with significant hypoxemia from lung disease, heart disease with right-to-left shunting, or compromised oxygen carrying capacity. Peripheral cyanosis arises when blood flow to the extremities is reduced or cold, and can occur without marked systemic hypoxemia; it may improve with warming.

Common causes include cardiopulmonary disorders such as congenital or acquired heart disease with shunting, severe chronic obstructive pulmonary disease, interstitial lung disease, pneumonia, pulmonary embolism, or acute respiratory distress syndrome. Abnormalities in hemoglobin—such as methemoglobinemia or sulfhemoglobinemia—and rare hemoglobin variants can produce cyanosis even with near-normal PaO2.

Evaluation relies on clinical assessment and laboratory testing. Pulse oximetry estimates SpO2 but can be misleading in methemoglobinemia. Arterial blood gas with co-oximetry measures PaO2 and the saturation of various hemoglobins. Cyanosis typically becomes apparent when deoxygenated hemoglobin concentration exceeds about 5 g/dL. The distribution (central vs peripheral) helps localize the cause.

Management focuses on treating the underlying cause and correcting hypoxemia when appropriate. Supplemental oxygen is often given, but specific therapies depend on the etiology: for methemoglobinemia, antidotes such as methylene blue; for severe abnormal hemoglobins or life-threatening hypoxemia, exchange transfusion or other supportive measures may be required.