Pulse Oximetry Principles — FRCA Primary MCQ
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Correct answer: C — It uses red light at approximately 660 nm and infrared light at approximately 940 nm
A conventional pulse oximeter uses red light at about 660 nm and infrared light at about 940 nm. Oxyhaemoglobin and deoxyhaemoglobin have different absorption at these wavelengths. The device analyses the ratio of the pulsatile changes in absorbance, thereby estimating arterial haemoglobin oxygen saturation (SpO2). It does not directly measure PaO2 or dissolved oxygen; these require arterial blood-gas analysis using an oxygen electrode. Conventional two-wavelength devices cannot distinguish oxyhaemoglobin reliably from carboxyhaemoglobin, so carbon monoxide poisoning may produce a falsely reassuring SpO2. Ultraviolet light is not used. Therefore, D is the single correct answer.
Reference: Burns SL et al. The impact of skin tone on performance of pulse oximeters used by NHS England COVID Oximetry @home scheme: measurement and diagnostic accuracy study. BMJ. 2025;392:e085535. https://www.bmj.com/content/392/bmj-2025-085535