Standard teaching has been that cyanosis is usually present when there is 5 g or more of reduced hemoglobin in 100 mL of capillary blood. However, this figure has been questioned recently when it was demonstrated that central cyanosis can be detected when the deoxyhemoglobin concentration is as low as 1.5 g/100 mL. 28 The increase in the amount of reduced hemoglobin in the cutaneous vessels can result from an increase in the quantity of venous blood in the skin, dilatation of the venules, or a decrease in the oxygen saturation in the capillary blood. In some instances, cyanosis can be detected when the arterial saturation has fallen to 85 percent; in others, it may not be detected until saturation is 75 percent. The absolute, rather than the relative, amount of reduced hemoglobin produces cyanosis. 29

Various physiologic, anatomic, and physical factors other than the amount of reduced hemoglobin may influence the appearance of cyanosis, making an accurate clinical detection of the degree or even the presence of cyanosis difficult. Physiologic factors include the oxygen content of the blood, level of tissue oxygenation, degree of oxygen extraction, and oxyhemoglobin dissociation curve. Anatomic factors include the status of the cutaneous microcirculation, pigmentation, and thickness of the skin. Physical factors include the lighting under which the patient is examined and the skill of the physician. The tongue is considered one of the most sensitive sites for observing central cyanosis, and the earlobes, conjunctivae, and nail beds are considered much less reliable.

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