Oxygen Monitoring — SpO2–SaO2 Discordance

Current · V5 (2025) → C9.i Historical · V4 (2023) → F12.i 1 exam appearance

2023B Q01

Exam question

Explain why the oxygen-haemoglobin saturation value derived by a pulse oximeter (SpO2) could be different from the measured arterial value (SaO2).

CICMWrecks answer

Master answer

Blood gas analyzers measure arterial SaO2 by co-oximetry. (gold standard of SaO2 measurement)
measures concentrations of oxygenated hemoglobin (oxyHb), deoxygenated hemoglobin (deoxyHb or reduced Hb), carboxyhemoglobin (COHb), and methemoglobin (MetHb) as a percentage of the total hemoglobin concentration in the blood sample
Accurate measure of oxygen saturation: low readings indicate true hypoxia, and high readings always represent true hypoxia
Not confused by ambient light, absence of pulsatile flow, tricuspid regurgitation, methylene blue dye.
Uses Beer-Lambert law to detect different Hb species.
- “Incidence of light is inversely proportional to the path distance and concentration of light absorbing particles within the path”
multi-wavelength spectrophotometry (measures the absorption of light passing through blood from several dozens of wavelengths)

Light emitter
- Produces infra-red light at 660nm and 950nm
Light detector
- Detects light at 660nm and 950nm
Processor
- Calculates SpO₂ using the ratio of absorbance at 660nm and 950nm
- Light detected (incidence) represents light passing through both pulsatile (arterial) blood and non-pulsatile elements (venous blood and other tissues)
- Processor differentiates light incidence during maxima and minima of pulse wave and calculates SpO₂ from pulsatile blood only giving SaO₂

PATIENT FACTORS
Low or High SpO2	Low SpO2	Normal or High SpO2
Met-Hb Sulph-Hb	Poor perfusion of finger Movement artifact Venous pulsations Fingernail polish Intravenous pigmented dyes Haemoglobinopathy Anaemia with co-existing hypoxia	Carbon Monoxide poisoning
EQUIPMENT FACTORS
– Ambient light interference – Poorly fitting probe – Assay calibrated using healthy volunteers only down to SpO₂ 80%. Unknown significance if tested SpO₂ less than 80%
PHYSIOLOGICAL FACTORS
– Due to O2 dissociation curve, insensitive to changes above PaO2 80mmHg – Does not measure tissue oxygenation

Past papers

1 appearance

Exam	Exact exam wording	Candidate success
2023B Q01	Explain why the oxygen-haemoglobin saturation value derived by a pulse oximeter (SpO2) could be different from the measured arterial value (SaO2).	32%