Intravenous and inhalational agents: effect of acidosis on thiopentone: 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: effect of acidosis on thiopentone

Sample item

Thiopentone is a weak acid with a pKa of about 7.6. How does a metabolic acidosis alter its action?

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

The answer and the reasoning

C. Raises the unionised fraction, so more crosses into the brain and the effect is greater

For a weak acid, lowering the pH shifts the balance towards the unionised form, which is lipid-soluble and crosses the blood-brain barrier. Acidosis therefore increases the effect of thiopentone, and doses are reduced in sick patients.

Option by option

  • A. Leaves the action unchanged, since ionisation is independent of pH

    Ionisation of a weak acid depends strongly on pH, so the action changes.

  • B. Lowers the unionised fraction, so less reaches the brain and the effect is weaker

    This reverses the effect; acidosis increases the unionised fraction of an acidic drug.

  • C. Raises the unionised fraction, so more crosses into the brain and the effect is greater (correct)

    Correct. A weak acid is more unionised at low pH, so brain uptake and effect rise.

  • D. Converts it to a stable water-soluble salt that is inactive

    The salt form is the vial preparation; acidosis does not convert the drug into an inactive salt.

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, Raises the unionised fraction, so more crosses into the brain and the effect is greater. For a weak acid, lowering the pH shifts the balance towards the unionised form, which is lipid-soluble and crosses the blood-brain barrier.

Why is "Leaves the action unchanged, since ionisation is independent of pH" not the answer?

Ionisation of a weak acid depends strongly on pH, so the action changes.

Why is "Lowers the unionised fraction, so less reaches the brain and the effe…" not the answer?

This reverses the effect; acidosis increases the unionised fraction of an acidic drug.

Why is "Converts it to a stable water-soluble salt that is inactive" not the answer?

The salt form is the vial preparation; acidosis does not convert the drug into an inactive salt.

What concept does this question test?

It tests Intravenous and inhalational agents: effect of acidosis on thiopentone in Anaesthesia. Acidosis therefore increases the effect of thiopentone, and doses are reduced in sick patients.

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: effect of acidosis on thiopentone?

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.