Cyanosis is a condition caused by an abnormally high concentration of deoxygenated hemoglobin in the arterial blood, usually 4 or more g/100 ml (ie. g/dL) regardless of the total hemoglobin concentration (Table 4.09). Arterial oxygen saturation is below 85%.
Symptoms include a bluish discoloration of the skin, mucous membranes, and nail beds, and dyspnea upon exertion. There may also be polycythemia, clubbing of the fingers and toes, and cerebral thrombosis.
Cyanosis is of two types, central or shunt and peripheral. Central or shunt cyanosis is caused by some forms of congenital heart defects such as tetralogy of Fallot (Table 4.08), and by right and left heart failure. Peripheral cyanosis is caused by extensive oxygen extraction at the capillary level, or by extreme peripheral vasoconstriction as in Raynaud's disease.
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