Respiratory · Year 1 · from Respiratory
Case 3: Methemoglobinemia from Dapsone
Case Presentation
A 55-year-old woman with Pneumocystis jirovecii pneumonia prophylaxis on dapsone presents with progressive dyspnea, fatigue, and cyanosis over 3 days. Her HIV viral load is undetectable on antiretroviral therapy, and her CD4 count is 180 cells/microliter. On examination, she has central cyanosis with brownish discoloration of her nail beds, lips, and mucous membranes. Vital signs show respiratory rate 22/min, heart rate 100 bpm, blood pressure 118/72 mmHg, and pulse oximetry reading 85% that does not improve with supplemental oxygen.
The arterial blood gas on 100% FiO2 shows pH 7.38, PaCO2 38 mmHg, PaO2 380 mmHg, calculated SaO2 99%. This striking discrepancy between pulse oximetry (85%) and calculated saturation (99%) with an elevated PaO2 is characteristic of methemoglobinemia. Co-oximetry confirms methemoglobin level of 28% (normal <1%).
Methemoglobin forms when hemoglobin iron is oxidized from the ferrous (Fe2+) to ferric (Fe3+) state, which cannot bind oxygen. Dapsone is a known oxidizing agent that can cause methemoglobinemia. Like carboxyhemoglobin, methemoglobin also shifts the oxygen-hemoglobin dissociation curve leftward, impairing oxygen release from the remaining functional hemoglobin. Pulse oximetry characteristically reads approximately 85% regardless of true saturation because methemoglobin absorbs light equally at both wavelengths used by pulse oximeters.
The patient is treated with intravenous methylene blue 1 mg/kg, which acts as an electron donor to reduce methemoglobin back to functional hemoglobin. Within 30 minutes, her cyanosis resolves and pulse oximetry improves to 98%. Dapsone is discontinued and alternative PCP prophylaxis is prescribed. G6PD deficiency is ruled out, as methylene blue is contraindicated in G6PD-deficient patients.
Key Learning Points
- Methemoglobin contains oxidized iron (Fe3+) that cannot bind oxygen
- Methemoglobin shifts the O2-Hgb dissociation curve leftward, impairing O2 unloading to tissues
- Pulse oximetry characteristically reads approximately 85% in methemoglobinemia, regardless of true saturation
- Co-oximetry is required for diagnosis; PaO2 is typically normal or elevated
- Methylene blue is the treatment of choice (contraindicated in G6PD deficiency)