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Cyanosis is a bluish discoloration of the skin, mucous membranes, tongue, lips, or nail beds and is due to an increased concentration of reduced hemoglobin (Hb) in the circulation.1 Clinically evident cyanosis typically occurs at an oxygen saturation of 85% or less.2 It is easier to identify under natural lighting and is typically more difficult to detect in patients with mild cyanosis, dark skin pigmentation, or anemia. Long-term complications of chronic cyanosis include clubbing, polycythemia, stroke, brain abscess, platelet abnormalities, lower-than-expected IQ, scoliosis, and hyperuricemia.1


Normally, there is ~ 2 g/dL of reduced (or deoxygenated) Hb in the circulation, and clinically evident cyanosis occurs when the concentration of reduced Hb reaches 5 g/dL. Therefore, the total amount of Hb is critical to the development of cyanosis. For instance, a hypoxic child with severe anemia may not have enough reduced Hb in the circulation to produce clinical cyanosis. In contrast, a hypoxic child with polycythemia may have enough reduced Hb to produce clinical cyanosis at a higher oxygen saturation level.3

Hypoxia results from one or a combination of the following mechanisms:

  1. Decreased inspired oxygen content (FIO2)

  2. Decreased respiratory rate or apnea

  3. Increased right-to-left shunt

  4. Increased ventilation–perfusion mismatch

  5. Diffusion abnormality

  6. Decreased affinity of hemoglobin for oxygen

Methemoglobin is formed when the iron in Hb is oxidized to the ferric state (Fe3+).4 Methemoglobin is unable to carry oxygen and normally constitutes less than 2% of circulating Hb. With congenital or acquired causes of methemoglobinemia, as well as with certain abnormal Hb variants, such as the Hb M group, elevated levels of methemoglobin cause central cyanosis and give the blood a chocolate brown appearance.5


Cyanosis: Bluish discoloration of the skin or mucus membranes due to reduced fraction of oxygenated hemoglobin.

Hypoxemia: A relative or absolute deficiency of oxygen in the blood.

Hypoxia: A relative or absolute deficiency of oxygen.


A careful and complete history and physical examination are essential in the evaluation of cyanosis (Tables 21-1 and 21-2). The history should focus on a detailed description of the cyanosis including areas of the body involved, timing, and severity, as well as associated symptoms. Changes in cyanosis with crying, choking, vomiting, or postural changes may help identify the underlying cause.

TABLE 21-1History

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