Physiology: cerebral blood flow and PaCO2: 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 · Physiology: cerebral blood flow and PaCO2

Sample item

Within the physiological range, how does cerebral blood flow respond to changes in PaCO2?

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

The answer and the reasoning

C. It changes in direct proportion, with vasoconstriction in hypocapnia and vasodilation in hypercapnia

Cerebral blood flow changes by about 1 to 2 mL per 100 g per minute for each mmHg change in PaCO2 between about 20 and 80 mmHg. Hyperventilation therefore lowers intracranial pressure briefly in raised ICP, but prolonged hypocapnia risks ischaemia. Autoregulation is maintained for mean pressures of about 50 to 150 mmHg.

Option by option

  • A. It is independent of PaCO2

    Flow is strongly dependent on PaCO2.

  • B. It falls in hypercapnia

    Hypercapnia increases flow.

  • C. It changes in direct proportion, with vasoconstriction in hypocapnia and vasodilation in hypercapnia (correct)

    Correct. CO2 is a potent cerebral vasodilator.

  • D. It rises in hypocapnia

    Hypocapnia reduces flow.

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, It changes in direct proportion, with vasoconstriction in hypocapnia and vasodilation in hypercapnia. Cerebral blood flow changes by about 1 to 2 mL per 100 g per minute for each mmHg change in PaCO2 between about 20 and 80 mmHg.

Why is "It is independent of PaCO2" not the answer?

Flow is strongly dependent on PaCO2.

Why is "It falls in hypercapnia" not the answer?

Hypercapnia increases flow.

Why is "It rises in hypocapnia" not the answer?

Hypocapnia reduces flow.

What concept does this question test?

It tests Physiology: cerebral blood flow and PaCO2 in Anaesthesia. Hyperventilation therefore lowers intracranial pressure briefly in raised ICP, but prolonged hypocapnia risks ischaemia. Autoregulation is maintained for mean pressures of about 50 to 150 mmHg.

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 Physiology: cerebral blood flow and PaCO2?

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.