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Pulse oximetry signal detection — FRCA Primary MCQ

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ModeratePhysics and Clinical MeasurementPulse oximetry signal detectionFRCA Primary

A 63-year-old patient has arterial oxygen saturation measured by pulse oximetry during profound peripheral vasoconstriction. The trace is poor and values fluctuate. Which feature of pulse oximetry is most relevant?

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

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Correct answer: C — It measures venous absorption at a single wavelength

Pulse oximetry estimates saturation from pulsatile arterial absorbance at red and infrared wavelengths. Low perfusion reduces the pulsatile signal, making readings unreliable. It does not measure oxygen content and does not accurately quantify carboxyhaemoglobin with standard two-wavelength technology. The pearl is to evaluate the plethysmographic waveform before trusting the number.

Reference: RCoA Primary FRCA Syllabus 2026; Davis and Kenny Basic Physics and Measurement in Anaesthesia