Intravenous and inhalational agents: methylene blue and pulse oximetry: 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 · Intravenous and inhalational agents: methylene blue and pulse oximetry

Sample item

After a methylene blue infusion for refractory vasoplegia, the pulse oximeter reading drops suddenly from 98 to 82 percent, yet the patient is pink and stable and an arterial blood gas shows a PaO2 of 280 mmHg. What explains the reading?

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

The answer and the reasoning

D. Methylene blue absorbs light at the oximeter wavelengths, giving a spuriously low SpO2

Intravenous dyes such as methylene blue, indocyanine green and indigo carmine absorb light at the oximeter wavelengths and cause a transient falsely low saturation. A normal PaO2 on blood gas analysis confirms the reading is artefactual.

Option by option

  • A. Interference by fetal haemoglobin

    Fetal haemoglobin is irrelevant in an adult.

  • B. Severe intrapulmonary shunting not detected by the blood gas

    A shunt large enough to produce this fall would be reflected in a low PaO2.

  • C. Carboxyhaemoglobin formation that reads as deoxygenated haemoglobin

    Carboxyhaemoglobin is read as oxyhaemoglobin, and it was not formed here.

  • D. Methylene blue absorbs light at the oximeter wavelengths, giving a spuriously low SpO2 (correct)

    Correct. The dye interferes with the optical measurement, and the blood gas excludes real hypoxaemia.

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 D, Methylene blue absorbs light at the oximeter wavelengths, giving a spuriously low SpO2. Intravenous dyes such as methylene blue, indocyanine green and indigo carmine absorb light at the oximeter wavelengths and cause a transient falsely low saturation.

Why is "Interference by fetal haemoglobin" not the answer?

Fetal haemoglobin is irrelevant in an adult.

Why is "Severe intrapulmonary shunting not detected by the blood gas" not the answer?

A shunt large enough to produce this fall would be reflected in a low PaO2.

Why is "Carboxyhaemoglobin formation that reads as deoxygenated haemoglobin" not the answer?

Carboxyhaemoglobin is read as oxyhaemoglobin, and it was not formed here.

What concept does this question test?

It tests Intravenous and inhalational agents: methylene blue and pulse oximetry in Anaesthesia. A normal PaO2 on blood gas analysis confirms the reading is artefactual.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Intravenous and inhalational agents: methylene blue and pulse oximetry?

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.