Emergency and critical care: combined acid-base disorder: 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.

Try it

General Medicine · Emergency and critical care: combined acid-base disorder

Sample item

A patient with diarrhoea and drowsiness has arterial pH 7.17, PaCO2 40 mmHg and bicarbonate 14 mmol/L. Using Winter's formula, expected PaCO2 is 1.5 x bicarbonate + 8, give or take 2. What is the interpretation?

AMetabolic acidosis with a concurrent respiratory acidosisBAcute respiratory acidosis aloneCMetabolic acidosis with a concurrent respiratory alkalosisDMetabolic acidosis with appropriate respiratory compensation

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

The answer and the reasoning

A. Metabolic acidosis with a concurrent respiratory acidosis

The expected PaCO2 is 1.5 x 14 + 8, which is 29, with a range of 27 to 31. The measured value of 40 is higher than predicted, so ventilation is inadequate and there is a second respiratory acidosis, which signals impending respiratory failure.

Option by option

A. Metabolic acidosis with a concurrent respiratory acidosis (correct)
Correct. PaCO2 is above the predicted range, so there is a superimposed respiratory acidosis.
B. Acute respiratory acidosis alone
Bicarbonate is low, which excludes an isolated respiratory acidosis.
C. Metabolic acidosis with a concurrent respiratory alkalosis
A PaCO2 below 27 would indicate a coexisting respiratory alkalosis.
D. Metabolic acidosis with appropriate respiratory compensation
Appropriate compensation would give a PaCO2 between 27 and 31.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is A, Metabolic acidosis with a concurrent respiratory acidosis. The expected PaCO2 is 1.

Why is "Acute respiratory acidosis alone" not the answer?

Bicarbonate is low, which excludes an isolated respiratory acidosis.

Why is "Metabolic acidosis with a concurrent respiratory alkalosis" not the answer?

A PaCO2 below 27 would indicate a coexisting respiratory alkalosis.

Why is "Metabolic acidosis with appropriate respiratory compensation" not the answer?

Appropriate compensation would give a PaCO2 between 27 and 31.

What concept does this question test?

It tests Emergency and critical care: combined acid-base disorder in General Medicine. 5 x 14 + 8, which is 29, with a range of 27 to 31. The measured value of 40 is higher than predicted, so ventilation is inadequate and there is a second respiratory acidosis, which signals impending respiratory failure.

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 Emergency and critical care: combined acid-base disorder?

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.