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Subtle desaturation and perioperative methemoglobinemia. The need for continued vigilance

Duke University Medical Center, Durham, USA, E-mail: habib001{at}mc.duke.edu

To the Editor:

A 32-yr old 60 kg diabetic patient presented for exploratory laparotomy. Her past medical history included renal and pancreatic transplantation, gastro-paresis, peripheral neuropathy, retinopathy, necrobiosis diabeticorum, and hypertension. She was receiving cyclosporine, prednisone, aspirin, ondansetron, dapsone, paroxetine, amitriptyline, nortriptyline, gabapentin, sirolimus, clotrimazole, metoclopramide, lansoprazole, and alendronate. Preoperative SpO₂ was 92 to 96% on air. Following iv clindamycin 600 mg and midazolam 2 mg, anesthesia was induced with fentanyl 150 µg, lidocaine 100 mg, propofol 100 mg and cisatracurium 10 mg iv. Her SpO₂ dropped to 84–88% despite effective hand ventilation on 100% oxygen. An endotracheal tube was placed and secured at 20 cm to the lips. There was minimal improvement in SpO₂ to 89%. The oximeter probe was checked for proper placement. Breath sounds were equal with no tracheal secretions on suctioning. Fibreoptic bronchoscopy revealed no obvious obstruction. The patient’s oral mucosa appeared cyanotic. Arterial blood gas sample revealed pH 7.42, PaCO₂ 29 mmHg, PaO₂ 466 mmHg (FIO₂ 100%), BE -5, and SaO₂ 91% (pulse oximeter reading 85%). Methemoglobin (MHb) was measured using a multi-wavelength spectrometer (co-oximeter), was 6.5% (normal 0.4–1.5%). With hemodynamic stability, and improved SpO₂ towards the end of surgery, methylene blue treatment was withheld. The SpO₂ continued to improve in the recovery room (91–98% on 10 L·min^–1O₂). Twelve hours after surgery, MHb level was 3.4%. The medical records revealed a two-year treatment on dapsone for pneumocystis carinii prophylaxis, and mildly elevated MHb (1.9%) during a previous anesthetic involving 60 mg lidocaine.

This case highlights the subtle presentation of methemoglobinemia as a cause of low SpO₂. Its diagnosis was established by the presence of three oxidizing agents (dapsone, metoclopramide and lidocaine) and a discrepancy between arterial oxygen saturation and pulse oximetry.¹ Methemoglobinemia commonly results from exposure to oxidizing agents (Table).² Normal levels of MHb are maintained by cytochrome-b5-MHb reductase responsible for converting MHb to hemoglobin.³ Long-standing dapsone use probably caused elevated MHb levels preoperatively. Lidocaine likely added further oxidative stress causing the cytochrome-b5-MHb reductase system to be overwhelmed.

References

1 Watcha MF, Conner MD, Hing AV. Pulse oximetry in methemoglobinemia. Am J Dis Child 1989; 143: 845–67.[Abstract]

2 Wright RO, Lewander WJ, Woolf AD. Methemoglobinemia: etiology, pharmacology, and clinical management. Ann Emerg Med 1999; 34: 646–56.[Medline]

3 Jaffe ER, Hultquist DE. Cytochrome b5 reductase deficiency and enzymopenic hereditary methemoglobinemia. In: Scriver CR, Beaudet AL, Sly WS, Valle D (Eds). The Metabolic and Molecular Bases of Inherited Disease, 7th ed. New York: McGraw-Hill; 1995: 3399–415.

4 Ferguson AJ, Lavery GG. Deliberate self-poisoning with dapsone. A case report and summary of relevant pharmacology and treatment. Anaesthesia 1997; 52: 359–63.[Medline]

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