Local anaesthetics: bicarbonate: 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 · Local anaesthetics: bicarbonate

Sample item

Why is sodium bicarbonate sometimes added to lignocaine for infiltration?

AIt prolongs the block by many hoursBIt acts as a vasoconstrictorCIt raises pH, increasing the unionised fraction and reducing injection painDIt prevents toxicity

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

The answer and the reasoning

C. It raises pH, increasing the unionised fraction and reducing injection pain

Alkalinisation of the solution increases the proportion of unionised base, speeding onset and making injection less painful. A typical mix is 1 mL of 8.4% bicarbonate per 10 mL of lignocaine.

Option by option

A. It prolongs the block by many hours
Bicarbonate does not prolong the block by hours.
B. It acts as a vasoconstrictor
It has no vasoconstrictor effect.
C. It raises pH, increasing the unionised fraction and reducing injection pain (correct)
Correct. Alkalinisation hastens onset and reduces pain.
D. It prevents toxicity
It does not prevent toxicity.

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.

Questions about this Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is C, It raises pH, increasing the unionised fraction and reducing injection pain. Alkalinisation of the solution increases the proportion of unionised base, speeding onset and making injection less painful.

Why is "It prolongs the block by many hours" not the answer?

Bicarbonate does not prolong the block by hours.

Why is "It acts as a vasoconstrictor" not the answer?

It has no vasoconstrictor effect.

Why is "It prevents toxicity" not the answer?

It does not prevent toxicity.

What concept does this question test?

It tests Local anaesthetics: bicarbonate in Anaesthesia. A typical mix is 1 mL of 8.4% bicarbonate per 10 mL of lignocaine.

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 Local anaesthetics: bicarbonate?

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.