skip to main content

Methaemoglobin pulse oximetry — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

HardPhysics and Clinical MeasurementMethaemoglobin pulse oximetryFRCA Primary

After topical local anaesthetic for bronchoscopy, a patient appears cyanosed with PaO2 30 kPa and pulse oximetry around 85%. Co-oximetry is pending. Which measurement artefact is most likely?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: B — Methaemoglobin causes similar red and infrared absorption

Methaemoglobin absorbs red and infrared light more equally, causing pulse oximetry to trend toward approximately 85% despite high PaO2. Carboxyhaemoglobin more often gives falsely high saturation. Low perfusion affects signal quality but does not explain high PaO2 with cyanosis. Co-oximetry is required to differentiate dys-haemoglobins.

Reference: Dorsch and Dorsch Understanding Anesthesia Equipment; Primary FRCA physics texts