Respiratory system: alveolar-arterial gradient: 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.

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Physiology · Respiratory system: alveolar-arterial gradient

Sample item

A 25-year-old man is found drowsy with slow, shallow breathing after an opioid overdose. On room air (FiO2 0.21, barometric pressure 760 mmHg, water vapour 47 mmHg, R = 0.8) his PaO2 is 55 mmHg and PaCO2 is 70 mmHg. What is his A-a gradient and what does it show?

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

The answer and the reasoning

A. About 7 mmHg, which is normal, so hypoventilation alone explains the hypoxaemia

PAO2 = FiO2 x (PB - PH2O) - PaCO2/R = 0.21 x 713 - 70/0.8 = 149.7 - 87.5 = about 62 mmHg. The A-a gradient is 62 - 55 = about 7 mmHg, which is normal for a young adult. A normal gradient with raised PaCO2 means the lungs transfer oxygen normally and the hypoxaemia is from reduced alveolar ventilation.

Option by option

  • A. About 7 mmHg, which is normal, so hypoventilation alone explains the hypoxaemia (correct)

    The gradient is about 7 mmHg, so the lungs are normal and the fault is central hypoventilation.

  • B. About 45 mmHg, which is raised, so a diffusion defect explains the hypoxaemia

    45 mmHg arises only if the raised PaCO2 is ignored and 40 is assumed, which overstates the gradient.

  • C. About 95 mmHg, which is raised, so a right-to-left shunt explains the hypoxaemia

    95 mmHg arises only if the CO2 term is left out completely.

  • D. About 62 mmHg, which is raised, so ventilation-perfusion mismatch explains the hypoxaemia

    62 mmHg is the calculated alveolar PO2, not the gradient.

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 Physiology question

What is the answer to this Physiology question?

The best answer is A, About 7 mmHg, which is normal, so hypoventilation alone explains the hypoxaemia. PAO2 = FiO2 x (PB - PH2O) - PaCO2/R = 0.

Why is "About 45 mmHg, which is raised, so a diffusion defect explains the hy…" not the answer?

45 mmHg arises only if the raised PaCO2 is ignored and 40 is assumed, which overstates the gradient.

Why is "About 95 mmHg, which is raised, so a right-to-left shunt explains the…" not the answer?

95 mmHg arises only if the CO2 term is left out completely.

Why is "About 62 mmHg, which is raised, so ventilation-perfusion mismatch exp…" not the answer?

62 mmHg is the calculated alveolar PO2, not the gradient.

What concept does this question test?

It tests Respiratory system: alveolar-arterial gradient in Physiology. 21 x 713 - 70/0.8 = 149.7 - 87.5 = about 62 mmHg. The A-a gradient is 62 - 55 = about 7 mmHg, which is normal for a young adult. A normal gradient with raised PaCO2 means the lungs transfer oxygen normally and the hypoxaemia is from reduced alveolar ventilation.

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: alveolar-arterial gradient?

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.