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Cyanosis, or bluish discoloration of the skin, is derived from the Greek word kuaneos, meaning “dark blue.” Cyanosis is caused by the presence of deoxygenated hemoglobin in the blood vessels that is most visible on the surface of the skin and mucosa. The mechanisms of cyanosis are outlined in Table 51-1 and Figure 51-1. In general, cyanosis occurs because (1) the binding of oxygen to hemoglobin is abnormal, so that blood does not carry much oxygen despite having a normal partial pressure of oxygen, or PO2 (eg, methemoglobin or carboxyhemoglobin); (2) the perfusion of the skin is poor, such that the venous and capillary blood are very deoxygenated even though the arterial blood may be well oxygenated (eg, cold environment or circulatory shock); or (3) the arterial, and therefore the capillary and venous blood, is poorly oxygenated (eg, a right-to-left shunt with congenital cardiac disease, parenchymal pulmonary disease, or hypoventilation).

Figure 51-1

Mechanisms of cyanosis in newborn infants. Decreased inspired oxygen concentration, airway abnormalities, lung disease (by ventilation-perfusion mismatch), abnormalities of hemoglobin, poor perfusion, polycythemia, heart disease (reduced pulmonary blood flow or right-to-left shunting or poor perfusion), and hypoventilation and fatigue can result in cyanosis. PVR, pulmonary vascular resistance; RV, right ventricle. (Used with permission from Satyan Lakshminrusimha and Robin Steinhorn.)


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