Respiratory medicine: alveolar-arterial gradient: practice question

A General Medicine 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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General Medicine · Respiratory medicine: alveolar-arterial gradient

Sample item

A 45-year-old man is found drowsy after taking a large dose of an opioid. On room air at sea level (FiO2 0.21, water vapour pressure 47 mmHg, respiratory quotient 0.8), arterial blood gas shows pH 7.25, PaCO2 70 mmHg, HCO3 30 mmol/L and PaO2 62 mmHg. What does the alveolar-arterial oxygen gradient indicate?

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

The answer and the reasoning

D. A normal gradient, since hypoxaemia is fully explained by alveolar hypoventilation

Alveolar PO2 = FiO2 x (760 minus 47) minus PaCO2/0.8 = 0.21 x 713 minus 70/0.8 = 149.7 minus 87.5 = 62.2 mmHg. The A-a gradient is 62.2 minus 62, about 0 mmHg, and the expected normal at this age is about 15 mmHg. A normal gradient with hypercapnia means pure hypoventilation (opioids, neuromuscular weakness), whereas lung parenchymal causes of hypoxaemia widen the gradient.

Option by option

  • A. A raised gradient from a diffusion barrier defect

    A diffusion defect widens the gradient, which is not the case with a calculated gradient near zero.

  • B. A raised gradient from ventilation-perfusion mismatch

    Ventilation-perfusion mismatch widens the gradient, but the calculated value is about 0 mmHg.

  • C. A raised gradient from a right-to-left shunt

    A true shunt widens the gradient and responds poorly to oxygen; the arithmetic here does not show a widened gradient.

  • D. A normal gradient, since hypoxaemia is fully explained by alveolar hypoventilation (correct)

    Correct. A gradient near zero shows the lungs are exchanging gas normally and the low PaO2 comes from reduced ventilation.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is D, A normal gradient, since hypoxaemia is fully explained by alveolar hypoventilation. Alveolar PO2 = FiO2 x (760 minus 47) minus PaCO2/0.

Why is "A raised gradient from a diffusion barrier defect" not the answer?

A diffusion defect widens the gradient, which is not the case with a calculated gradient near zero.

Why is "A raised gradient from ventilation-perfusion mismatch" not the answer?

Ventilation-perfusion mismatch widens the gradient, but the calculated value is about 0 mmHg.

Why is "A raised gradient from a right-to-left shunt" not the answer?

A true shunt widens the gradient and responds poorly to oxygen; the arithmetic here does not show a widened gradient.

What concept does this question test?

It tests Respiratory medicine: alveolar-arterial gradient in General Medicine. 8 = 0.21 x 713 minus 70/0.8 = 149.7 minus 87.5 = 62.2 mmHg. The A-a gradient is 62.2 minus 62, about 0 mmHg, and the expected normal at this age is about 15 mmHg. A normal gradient with hypercapnia means pure hypoventilation (opioids, neuromuscular weakness), whereas lung parenchymal causes of hypoxaemia widen the gradient.

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 medicine: alveolar-arterial gradient?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.