Acid-base: chronic respiratory acidosis: 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 · Acid-base: chronic respiratory acidosis

Sample item

A patient with COPD has had stable hypercapnia with PaCO2 70 mmHg for several weeks. Which serum bicarbonate indicates appropriate renal compensation?

AAbout 18 mmol/LBAbout 35 mmol/LCAbout 24 mmol/LDAbout 27 mmol/L

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

The answer and the reasoning

B. About 35 mmol/L

In chronic respiratory acidosis, bicarbonate rises by roughly 3.5 to 4 mmol/L for every 10 mmHg rise in PaCO2 above 40, through renal ammonium excretion and bicarbonate retention. The rise is 30 mmHg here, so bicarbonate is about 24 + (3 x 3.5 to 4) = 34.5 to 36. In acute respiratory acidosis the rise is only about 1 per 10 mmHg.

Option by option

A. About 18 mmol/L
A bicarbonate of 18 would indicate an added metabolic acidosis.
B. About 35 mmol/L (correct)
Correct. Chronic renal compensation raises bicarbonate by about 3.5 to 4 per 10 mmHg.
C. About 24 mmol/L
A normal bicarbonate suggests no compensation, which is not seen in weeks of hypercapnia.
D. About 27 mmol/L
About 27 mmol/L is expected in acute hypercapnia, before renal compensation develops.

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 Physiology question

What is the answer to this Physiology question?

The best answer is B, About 35 mmol/L. In chronic respiratory acidosis, bicarbonate rises by roughly 3.

Why is "About 18 mmol/L" not the answer?

A bicarbonate of 18 would indicate an added metabolic acidosis.

Why is "About 24 mmol/L" not the answer?

A normal bicarbonate suggests no compensation, which is not seen in weeks of hypercapnia.

Why is "About 27 mmol/L" not the answer?

About 27 mmol/L is expected in acute hypercapnia, before renal compensation develops.

What concept does this question test?

It tests Acid-base: chronic respiratory acidosis in Physiology. 5 to 4 mmol/L for every 10 mmHg rise in PaCO2 above 40, through renal ammonium excretion and bicarbonate retention. The rise is 30 mmHg here, so bicarbonate is about 24 + (3 x 3.5 to 4) = 34.5 to 36. In acute respiratory acidosis the rise is only about 1 per 10 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 Acid-base: chronic respiratory acidosis?

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.