Clinically, the presence of cyanosis suggests the possibility of tissue hypoxia, and possible causes for hypoxia should be considered. Unexplained cyanosis, particularly in association with normal arterial oxygen tension (Pa o2), suggests the possibility of abnormal hemoglobin, such as methemoglobin. However, the absence of cyanosis does indicate adequate tissue oxygenation; severe states of tissue hypoxia are possible without the presence of cyanosis
Cyanosis is traditionally divided into two categories: central and peripheral. The central type is seen under conditions with unsaturated arterial blood or abnormal hemoglobin. The mucous membranes and skin are both affected. In contrast, peripheral cyanosis is due to the slowing of blood flow to an area and an abnormally great extraction of oxygen from normally saturated arterial blood. Congestive failure, peripheral vascular disease, shock states, and cold exposure all create states of vasoconstriction and decreased peripheral blood flow. The differentiation between central and peripheral cyanosis may not be possible in conditions where there may be an admixture of mechanisms (Table.,5.8:§.).
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