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Methemoglobinaemia — FRCA Final MCQ

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HardDay SurgeryMethemoglobinaemiaFRCA Final

A 25-year-old man undergoes wisdom-tooth extraction under general anaesthesia as a planned day case. Before nasal intubation, his nasal and oropharyngeal mucosa are topicalised with a benzocaine-containing spray. In recovery he is awake, with a respiratory rate of 16 breaths per minute, a normal capnogram, an end-tidal carbon dioxide concentration of 4.8 kPa and a train-of-four ratio of 0.95. His SpO2 remains 88% despite oxygen at 15 L/min through a non-rebreather mask. An arterial sample obtained simultaneously shows PaO2 42 kPa, PaCO2 4.5 kPa and an analyser-calculated SaO2 greater than 99%. Air entry is equal with no added sounds. What is the most likely diagnosis?

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Correct answer: B — Acquired methaemoglobinaemia

The answer is B, acquired methaemoglobinaemia caused by the topical local anaesthetic. The key feature is the saturation gap: SpO2 remains near 85–88% despite a very high PaO2 and adequate ventilation. PaO2 reflects oxygen dissolved in plasma, whereas the blood-gas analyser's calculated SaO2 assumes normal haemoglobin species. Ferric methaemoglobin cannot bind oxygen and alters two-wavelength pulse-oximeter absorption, driving the displayed saturation towards 85%. Diagnosis is confirmed by multiwavelength co-oximetry. Pulmonary embolism, atelectasis and bronchospasm would reduce PaO2 and usually have supporting respiratory findings. Opioid-induced hypoventilation would produce reduced respiratory effort and an increased PaCO2. Symptomatic cases are treated with methylthioninium chloride, usually 1–2 mg/kg intravenously over five minutes, after checking relevant contraindications and interactions.

Reference: Haymond S, Cariappa R, Eby CS, Scott MG. Laboratory assessment of oxygenation in methemoglobinemia. Clinical Chemistry. 2005;51(2):434–444. https://pubmed.ncbi.nlm.nih.gov/15514101/