Complications and emergencies: methaemoglobinaemia treatment: practice question

A Anaesthesia question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Anaesthesia · Complications and emergencies: methaemoglobinaemia treatment

Sample item

After benzocaine spray for awake intubation, a patient is cyanosed with a persistent SpO2 of 85 percent on 100 percent oxygen. PaO2 is normal and the arterial blood looks chocolate brown. Which drug is the specific treatment?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Intravenous methylene blue

Benzocaine and prilocaine can oxidise haemoglobin to methaemoglobin, which cannot carry oxygen. Methylene blue, 1 to 2 mg/kg intravenously, is reduced to leucomethylene blue and speeds the conversion of methaemoglobin back to haemoglobin.

Option by option

  • A. Intravenous methylene blue (correct)

    Correct. Methylene blue is the specific reducing agent used for symptomatic methaemoglobinaemia.

  • B. Intravenous hydroxocobalamin

    Hydroxocobalamin is an antidote for cyanide poisoning and does not reduce methaemoglobin.

  • C. Intravenous sodium thiosulphate

    Sodium thiosulphate is also used for cyanide toxicity, and it would not correct this problem.

  • D. Intravenous N-acetylcysteine

    N-acetylcysteine is the antidote for paracetamol poisoning and has no role in methaemoglobinaemia.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is A, Intravenous methylene blue. Benzocaine and prilocaine can oxidise haemoglobin to methaemoglobin, which cannot carry oxygen.

Why is "Intravenous hydroxocobalamin" not the answer?

Hydroxocobalamin is an antidote for cyanide poisoning and does not reduce methaemoglobin.

Why is "Intravenous sodium thiosulphate" not the answer?

Sodium thiosulphate is also used for cyanide toxicity, and it would not correct this problem.

Why is "Intravenous N-acetylcysteine" not the answer?

N-acetylcysteine is the antidote for paracetamol poisoning and has no role in methaemoglobinaemia.

What concept does this question test?

It tests Complications and emergencies: methaemoglobinaemia treatment in Anaesthesia. Methylene blue, 1 to 2 mg/kg intravenously, is reduced to leucomethylene blue and speeds the conversion of methaemoglobin back to haemoglobin.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Complications and emergencies: methaemoglobinaemia treatment?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.