Monitoring: practice question

A Anaesthesia 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

Anaesthesia · Monitoring

Sample item

A patient rescued from a house fire is breathless and has a headache, but pulse oximetry reads 99% on room air. Why is the reading unreliable?

APulse oximeters detect only plasma dissolved oxygenBCarboxyhaemoglobin is read as oxyhaemoglobin, so SpO2 is falsely normalCCarboxyhaemoglobin is read as deoxyhaemoglobin, so SpO2 is falsely lowDSkin pigmentation always reduces readings by half

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

The answer and the reasoning

B. Carboxyhaemoglobin is read as oxyhaemoglobin, so SpO2 is falsely normal

Standard two-wavelength pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, since both absorb red light similarly. SpO2 therefore overestimates true oxygen saturation in carbon monoxide poisoning. Co-oximetry on an arterial blood gas sample gives the true value.

Option by option

A. Pulse oximeters detect only plasma dissolved oxygen
Pulse oximeters measure haemoglobin saturation, not dissolved oxygen.
B. Carboxyhaemoglobin is read as oxyhaemoglobin, so SpO2 is falsely normal (correct)
Correct. The device misreads carboxyhaemoglobin as oxygenated haemoglobin.
C. Carboxyhaemoglobin is read as deoxyhaemoglobin, so SpO2 is falsely low
Carboxyhaemoglobin does not read as deoxyhaemoglobin, so the error is a falsely high, not low, value.
D. Skin pigmentation always reduces readings by half
Skin pigmentation can cause small inaccuracies, not a halving of the value.

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 Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is B, Carboxyhaemoglobin is read as oxyhaemoglobin, so SpO2 is falsely normal. Standard two-wavelength pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, since both absorb red light similarly.

Why is "Pulse oximeters detect only plasma dissolved oxygen" not the answer?

Pulse oximeters measure haemoglobin saturation, not dissolved oxygen.

Why is "Carboxyhaemoglobin is read as deoxyhaemoglobin, so SpO2 is falsely low" not the answer?

Carboxyhaemoglobin does not read as deoxyhaemoglobin, so the error is a falsely high, not low, value.

Why is "Skin pigmentation always reduces readings by half" not the answer?

Skin pigmentation can cause small inaccuracies, not a halving of the value.

What concept does this question test?

It tests Monitoring in Anaesthesia. SpO2 therefore overestimates true oxygen saturation in carbon monoxide poisoning. Co-oximetry on an arterial blood gas sample gives the true value.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Monitoring?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia 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.