Definition and PathogenesisTop
Cyanosis is characterized by abnormal bluish discoloration of the skin and mucous membranes. It is caused by increased concentration of deoxygenated hemoglobin in the capillary blood (>3.1 mmol/L [50 g/L]) or presence of an abnormal hemoglobin (most frequently methemoglobin >0.31 mmol/L or [5 g/L]).
1. Central cyanosis: Generalized, visible on the mucous membranes (mainly lips) and skin, which is usually warm. As cyanosis may not be clearly apparent in patients with darker skin pigmentation, close evaluation of the nail beds, tongue, and mucous membranes is critical. Causes:
1) Hypoxemia (usually hemoglobin oxygen saturation (SaO2) <85%, partial pressure of oxygen (PaO2) <60 mm Hg): Ventilation-perfusion mismatch (obstructive lung disease, pulmonary embolism), impaired diffusion (pulmonary fibrosis), shunt (alveolar collapse, congenital heart disease [right to left shunting], pulmonary arteriovenous malformation), decreased oxygen partial pressure in inhaled air (at high altitudes).
2) Presence of a pathologic hemoglobin: Methemoglobinemia (congenital or acquired), sulfhemoglobinemia (in such cases PaO2 is normal).
2. Peripheral cyanosis: Visible only over the skin of distal body parts, where blood flow is less rapid. The skin is usually cold. When present over the pinna, it disappears with pressure. Peripheral cyanosis is a manifestation of excessive deoxygenation (increased oxygen extraction) of hemoglobin in peripheral tissues. Normal arterial oxygen saturation (SaO2) is usually observed. Causes:
1) Significant hypothermia (physiologic vasoconstriction).
2) Decreased cardiac output (eg, cardiogenic shock, advanced heart failure, mitral or aortic stenosis).
3) Local abnormalities of arterial circulation (eg, atheroma, arterial emboli, Buerger disease, diabetic angiopathy).
4) Vasomotor disturbances (anxiety-related, Raynaud phenomenon, acrocyanosis).
5) Impaired venous drainage (thrombosis, postthrombotic syndrome, superficial vein phlebitis).
6) Increased blood viscosity (polycythemia vera, cryoglobulinemia, gammopathy).
3. Pseudocyanosis: As opposed to central and peripheral cyanosis, this does not disappear with the application of pressure to the skin. It is a rare phenomenon. Causes: abnormal pigmentation of the skin (drugs: chlorpromazine, amiodarone, minocycline; exposure to heavy metals, eg, to silver [argyria] or gold [chrysiasis]).
1. Assess stability (mental status, airway patency, respiratory distress) and vital signs (respiratory rate, pulse, blood pressure, body temperature, pulse oximetry).
2. Take a focused history (timing of cyanosis, exposures) and perform physical examination (central or peripheral cyanosis; exclude pseudocyanosis; assess respiratory and cardiovascular systems).
3. Co-oximetry provides a more accurate assessment of oxygen saturation as it measures absorption of light at 4 different wavelengths, including oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin.
4. Perform arterial blood gas analysis (cyanosis is not a reliable sign of hypoxemia) and measurement of carboxyhemoglobin and methemoglobin levels.
5. Assess response to oxygen therapy. Response is poor in patients with cyanotic congenital heart disease, in cases of significant pulmonary shunts of deoxygenated blood, and in patients with a pathologic hemoglobin.
6. Perform a complete blood count (CBC) (true cyanosis [central or peripheral] is masked in individuals with severe anemia and manifests earlier in those with polycythemia vera), chest radiography, and, depending on the suspected cause, other cardiovascular (electrocardiography [ECG], echocardiography, contrast-enhanced echocardiography) or respiratory (pulmonary function tests, computed tomography [CT] of the thorax) studies as well as measurement of pathologic hemoglobin levels (eg, congenital or acquired methemoglobinemia).