Retina: nitrous oxide and intraocular gas: practice question

A Ophthalmology 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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Ophthalmology · Retina: nitrous oxide and intraocular gas

Sample item

A patient had a vitrectomy with intraocular sulphur hexafluoride gas tamponade a week ago and now needs another operation under general anaesthesia. Which agent must be avoided?

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

The answer and the reasoning

D. Nitrous oxide

Nitrous oxide is far more soluble in blood than nitrogen, so it diffuses into the gas bubble faster than nitrogen diffuses out. The bubble expands, intraocular pressure rises, and the retinal circulation can be occluded. Nitrous oxide is avoided until the gas has gone.

Option by option

  • A. Propofol

    Propofol is an intravenous agent that does not diffuse into a gas bubble.

  • B. Sevoflurane

    Sevoflurane is an inhalational agent without the diffusion problem of nitrous oxide.

  • C. Ketamine

    Ketamine is an intravenous agent and does not expand an intraocular gas bubble.

  • D. Nitrous oxide (correct)

    Correct. Nitrous oxide enters the bubble quickly and causes dangerous expansion.

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 Ophthalmology questions

Questions about this Ophthalmology question

What is the answer to this Ophthalmology question?

The best answer is D, Nitrous oxide. Nitrous oxide is far more soluble in blood than nitrogen, so it diffuses into the gas bubble faster than nitrogen diffuses out.

Why is "Propofol" not the answer?

Propofol is an intravenous agent that does not diffuse into a gas bubble.

Why is "Sevoflurane" not the answer?

Sevoflurane is an inhalational agent without the diffusion problem of nitrous oxide.

Why is "Ketamine" not the answer?

Ketamine is an intravenous agent and does not expand an intraocular gas bubble.

What concept does this question test?

It tests Retina: nitrous oxide and intraocular gas in Ophthalmology. The bubble expands, intraocular pressure rises, and the retinal circulation can be occluded. Nitrous oxide is avoided until the gas has gone.

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 Retina: nitrous oxide and intraocular gas?

Group the red eye by what to examine and what the pupil does. Then use the flashcards and the other Ophthalmology 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.