This patient has methemoglobinemia, which results from a relative imbalance between hemoglobin and methemoglobin, with iron being present more in its ferric state rather than its normal ferrous state. Methemoglobinemia can be produced because of abnormal hemoglobin M, enzyme deficiency (eg, hemoglobin reductase), or drug or toxin ingestion (eg, benzocaine, dapsone, sulfonamides) but often may be idiopathic (in 70% of patients). Benzocaine is present in teething gel. Abnormal vital signs such as tachycardia and tachypnea are thought to be caused by tissue hypoxia related to the functional anemia of methemoglobinemia. Methemoglobin absorbs light at the same wavelengths as both oxygenated and nonoxygenated hemoglobin, which essentially confuses the oximeter into reporting that that both oxygenated and nonoxygenated hemoglobin levels are increased. Such oximeter values are inaccurate; because of the absorption characteristics of methemoglobin, the saturation reported by the pulse oximeter will plateau at 85%, regardless of any further increase in methemoglobin level above 40%. Methylene blue is the treatment of choice for this patient. Exchange transfusion or hyperbaric oxygen may be beneficial when methylene blue is ineffective.
- Administer methylene blue.
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