What is the role of cyanosis in the pathophysiology of methemoglobinemia?

Updated: Dec 09, 2018
  • Author: Mary Denshaw-Burke, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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Organs with high oxygen demands (eg, the central nervous system [CNS] and the cardiovascular system) usually are the first systems to manifest toxicity. Oxygenated blood is bright red, deoxygenated blood is dark red, and blood containing methemoglobin is dark reddish brown (see the image below). This dark hue is responsible for clinical cyanosis.

Note chocolate brown color of methemoglobinemia. I Note chocolate brown color of methemoglobinemia. In tubes 1 and 2, methemoglobin fraction is 70%; in tube 3, 20%; and in tube 4, normal.

Clinical evidence of cyanosis depends on the level of methemoglobin. Skin discoloration can occur in patients who are not anemic when as little as 1.5 g/dL (about 10%) of hemoglobin is in the form of methemoglobin. By comparison, a deoxyhemoglobin level of 5 g/dL is required to produce clinical cyanosis. When methemoglobin levels are relatively low, cyanosis may be observed without cardiopulmonary symptoms.

In methemoglobinemia, cyanosis is usually the first presenting symptom. In other conditions associated with cyanosis resulting from hypoxemia, it is a much later finding.

In patients with severe anemia, a higher percentage of methemoglobin is required for cyanosis to be obvious. These patients are more likely to exhibit signs of hypoxemia and have less cyanosis than is seen in patients who do not have anemia.

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