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Sodium bicarbonate
8%
32/388
Pralidoxime
2%
9/388
Deferoxamine
23%
88/388
Dimercaprol
9%
34/388
Methylene blue
53%
206/388
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In a patient with cyanosis, shortness of breath, and chocolate-colored blood on arterial blood draw, suspect methemoglobinemia. The treatment for methemoglobinemia is methylene blue. Methemoglobinemia occurs when a significant proportion of the body's hemoglobin is oxidized to the ferric (Fe3+) state and thus has a decreased affinity for oxygen resulting in cyanosis. The ferric state of hemoglobin is also responsible for the chocolate-colored appearance to the blood. Methemoglobinemia can be congenital (secondary to a deficiency in methemoglobinemia reductase) or acquired when this reduction pathway overwhelmed (Illustration B). Acquired methemoglobinemia can be cause by antibiotics, local anesthetics, exposure to aniline dyes (as in the vignette), or nitrates. Symptoms may be more pronounced in individuals with co-morbinities such as anemia. Methylene blue treats the condition by directly reducing the ferric hemoglobin to the ferrous (Fe2+) state. Zoorob and Campbell discuss the approach to acute shortness of breath in the primary care setting. They recommend that any patient with severe dyspnea, new at-rest dyspnea, or dyspnea in association with sudden chest pain go directly to the ED. Patients with congestive heart failure or COPD and mild, progressive symptoms can be seen in clinic for evaluation. Cortazzo and Lichtman review management of methemoglobinemia in the critical care setting. They remind physicians to not be fooled by erroneous pulse-oxymetry readings in the patient with methemoglobinemia. Because methemoglobin absorbs both wavelengths of light used by the pulse-oxymeter to compare levels of deoxygenated and oxygenated hemoglobin, anyone with a methemoglobin level over a certain threshold will read as 85% oxygen saturation, regardless of the actual oxygen saturation. The authors recommend the use of multiwavelength CO-oxymetry instead. Illustration A shows an infant with cyanosis secondary to methemoglobinemia. Illustration B shows the endogenous mechanism of reducing methemoglobin. Incorrect Answers: Answer 1: Sodium bicarbonate is used to treat salicylate and tricyclic antidepressant overdoses. Answer 2: Pralidoxime is used to treat organophosphate poisoning. Answer 3: Deferoxamine is used to treat iron overdose. Answer 4: Dimercaprol is used to treat mercury, arsenic, and gold poisoning.
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