Cyanosis is commonly detected by a family member or friend. The subtle bluish discoloration may go completely unnoticed by the patient. Central cyanosis occurs with inadequate gas exchange in the lungs that results in a significant reduction in arterial oxygenation. Primary pulmonary problems or diseases that cause mixed venous blood to bypass the lungs (e.g., intracardiac shunt) are frequently the causative factors. The bluish discoloration is best seen in the mucous membranes of the mouth (e.g., the frenulum) and lips. Peripheral cyanosis results from an excessive extraction of oxygen at the periphery. It is limited to the extremities (e.g., the fingers, toes, nose). Ask the following questions:

Where is the cyanosis present?''

How long has the cyanosis been present?''

Are you aware of any lung problem? heart problem? blood problem?'' What makes the cyanosis worse?'' ' 'Is there associated shortness of breath? cough? bleeding?'' ''What types of work have you performed?'' Is there anyone else in your family who has cyanosis?''

Cyanosis from birth is associated with congenital heart lesions. The acute development of cyanosis can occur in severe respiratory disease, especially acute airway obstruction. Peripheral cyanosis results from increased oxygen extraction in states of low cardiac output and is seen in cooler areas of the body such as the nail beds and the outer surfaces of the lips. Peripheral cyanosis disappears as the area is warmed. Cyanosis of the nails and warmth in the hands suggest that the cyanosis is central. Central cyanosis occurs only after the oxygen saturation has fallen below 80%. Central cyanosis diffusely involves the skin and mucous membranes and does not disappear with warming of the area. At least 2 to 3 g of unsaturated hemoglobin per 100 mL of blood must be present for the patient to manifest central cyanosis. Exercise worsens central cyanosis because the exercising muscles require an increased extraction of oxygen from the blood. In patients with severe anemia, in whom hemoglobin levels are markedly decreased, cyanosis may not be seen. Clubbing of the nails is seen in association with central cyanosis and significant cardiopulmonary disorders. See Figure 8-12.

Some industrial workers, such as arc welders, inhale toxic levels of nitrous gases that can produce cyanosis by methemoglobinemia. Hereditary methemoglobinemia is a primary hemoglobin abnormality causing congenital cyanosis.

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