General anaesthetics: nitrous oxide and closed air spaces: practice question

A Pharmacology 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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Pharmacology · General anaesthetics: nitrous oxide and closed air spaces

Sample item

A patient with a small traumatic pneumothorax awaiting a chest drain needs a short general anaesthetic for fracture fixation. Which agent should be avoided because it can enlarge the pneumothorax?

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

The answer and the reasoning

C. Nitrous oxide

Nitrous oxide is about 30 times more soluble in blood than nitrogen, so it enters air-filled cavities faster than nitrogen leaves. Pneumothorax, bowel obstruction, pneumocephalus and middle ear cavities may therefore expand under nitrous oxide. Volatile agents, intravenous induction drugs and opioids do not diffuse into closed spaces to the same extent.

Option by option

  • A. Propofol

    Propofol is an intravenous agent and has no effect on gas volumes.

  • B. Sevoflurane

    Sevoflurane is poorly soluble in blood and does not diffuse into gas pockets in this way.

  • C. Nitrous oxide (correct)

    Rapid inflow of nitrous oxide into closed gas spaces enlarges them.

  • D. Fentanyl

    Fentanyl is an opioid with no effect on gas-filled cavities.

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.

More Pharmacology questions

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is C, Nitrous oxide. Nitrous oxide is about 30 times more soluble in blood than nitrogen, so it enters air-filled cavities faster than nitrogen leaves.

Why is "Propofol" not the answer?

Propofol is an intravenous agent and has no effect on gas volumes.

Why is "Sevoflurane" not the answer?

Sevoflurane is poorly soluble in blood and does not diffuse into gas pockets in this way.

Why is "Fentanyl" not the answer?

Fentanyl is an opioid with no effect on gas-filled cavities.

What concept does this question test?

It tests General anaesthetics: nitrous oxide and closed air spaces in Pharmacology. Pneumothorax, bowel obstruction, pneumocephalus and middle ear cavities may therefore expand under nitrous oxide. Volatile agents, intravenous induction drugs and opioids do not diffuse into closed spaces to the same extent.

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 General anaesthetics: nitrous oxide and closed air spaces?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.