Respiratory system: oxygen therapy in chronic hypercapnia: practice question

A Physiology 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.

Try it

Physiology · Respiratory system: oxygen therapy in chronic hypercapnia

Sample item

A 66-year-old man with severe COPD and chronic CO2 retention is given high-flow oxygen and becomes drowsy; his PaCO2 rises from 60 to 85 mmHg. Besides a reduction in hypoxic ventilatory drive, which mechanism contributes importantly?

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

The answer and the reasoning

C. Loss of hypoxic vasoconstriction in poorly ventilated units, worsening V/Q matching

In poorly ventilated alveoli, oxygen relieves hypoxic pulmonary vasoconstriction, so blood is redirected to underventilated units and physiological dead space rises. The Haldane effect also means oxygenated haemoglobin binds less CO2. Together with some loss of hypoxic drive, these raise PaCO2, so oxygen should be titrated to a target saturation.

Option by option

  • A. Increased CO2 production by the respiratory muscles

    Oxygen does not meaningfully raise respiratory muscle CO2 production.

  • B. Direct depression of the pre-Botzinger complex by oxygen

    Oxygen does not directly depress the medullary rhythm generator; the effect is on gas exchange and chemoreceptor drive.

  • C. Loss of hypoxic vasoconstriction in poorly ventilated units, worsening V/Q matching (correct)

    Correct. Release of hypoxic vasoconstriction diverts perfusion to poorly ventilated areas.

  • D. Failure of red cell carbonic anhydrase at high PO2

    Carbonic anhydrase does not fail at high PO2.

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 Physiology questions

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, Loss of hypoxic vasoconstriction in poorly ventilated units, worsening V/Q matching. In poorly ventilated alveoli, oxygen relieves hypoxic pulmonary vasoconstriction, so blood is redirected to underventilated units and physiological dead space rises.

Why is "Increased CO2 production by the respiratory muscles" not the answer?

Oxygen does not meaningfully raise respiratory muscle CO2 production.

Why is "Direct depression of the pre-Botzinger complex by oxygen" not the answer?

Oxygen does not directly depress the medullary rhythm generator; the effect is on gas exchange and chemoreceptor drive.

Why is "Failure of red cell carbonic anhydrase at high PO2" not the answer?

Carbonic anhydrase does not fail at high PO2.

What concept does this question test?

It tests Respiratory system: oxygen therapy in chronic hypercapnia in Physiology. The Haldane effect also means oxygenated haemoglobin binds less CO2. Together with some loss of hypoxic drive, these raise PaCO2, so oxygen should be titrated to a target saturation.

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 Respiratory system: oxygen therapy in chronic hypercapnia?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.