Intravenous and inhalational agents: additive MAC of nitrous oxide: 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: additive MAC of nitrous oxide

Sample item

Adding 60 percent nitrous oxide to the anaesthetic allows a lower sevoflurane concentration for the same depth. Why?

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

The answer and the reasoning

C. The MAC fractions of inhaled agents are additive

The MAC contributions of inhalational agents add together, so 60 percent nitrous oxide, which is about 0.6 MAC, allows a corresponding reduction in the volatile agent. Nitrous oxide is a weak anaesthetic with a MAC of about 104 percent.

Option by option

  • A. Nitrous oxide is more potent than sevoflurane and replaces it

    Nitrous oxide is much less potent than sevoflurane.

  • B. Nitrous oxide inhibits the metabolism of sevoflurane

    Little sevoflurane is metabolised, and the effect is not metabolic.

  • C. The MAC fractions of inhaled agents are additive (correct)

    Correct. The fractions of MAC add up to the same total depth.

  • D. Nitrous oxide raises the vapour pressure of sevoflurane

    Gas mixing does not change the vapour pressure in the vaporiser.

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 C, The MAC fractions of inhaled agents are additive. The MAC contributions of inhalational agents add together, so 60 percent nitrous oxide, which is about 0.

Why is "Nitrous oxide is more potent than sevoflurane and replaces it" not the answer?

Nitrous oxide is much less potent than sevoflurane.

Why is "Nitrous oxide inhibits the metabolism of sevoflurane" not the answer?

Little sevoflurane is metabolised, and the effect is not metabolic.

Why is "Nitrous oxide raises the vapour pressure of sevoflurane" not the answer?

Gas mixing does not change the vapour pressure in the vaporiser.

What concept does this question test?

It tests Intravenous and inhalational agents: additive MAC of nitrous oxide in Anaesthesia. 6 MAC, allows a corresponding reduction in the volatile agent. Nitrous oxide is a weak anaesthetic with a MAC of about 104 percent.

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 Intravenous and inhalational agents: additive MAC of nitrous oxide?

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.