Oxygen Monitoring — Pulse Oximetry

Current · V5 (2025) → C9.i Historical · V4 (2023) → F12.i 5 exam appearances

2025A Q18

Exam question

(a) Describe how arterial haemoglobin oxygen saturation is measured using a pulse oximeter including the underlying scientific principles (60% of marks). (b) Outline the limitations and sources of error of this method (40% of marks).

CICMWrecks answer

Master answer

Principles

Beer-Lambert Law

Incidence of light is inversely proportional to the path distance and concentration of light absorbing particles within the path.

Hb absorbance at 660nm and 940nm utilized

660nm maximally absorbed by deoxyhaemoglobin
- Ratio of absorbance at 660nm and 940nm utilized to calculate SpO₂

R \; = \; { {Pulsatile_{660} \; / \; Non-Pulsatile_{660} } \over { Pulsatile_{940} \; / \; Non-Pulsatile_{940} } }

using healthy volunteers derived R values
Sats 100% R = 0.4
Sats 85% R = 1.0
Sats 50% R = 2
Sats 0% R = 3.4

Diagram used in Resp – Oximetry — Master Answer — Image: Adrian Curtin · CC BY-SA 3.0

Pulse oximeter

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

Limitations

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

Absorption spectra confounded by:

Carboxyhaemoglobin
- Absorbs 660nm, not at 940nm
- R closer to 0.4
- causes the pulse oximeter to read artificially high
Methemoglobin
- though it absorbs 660nm light, it also absorbs 940nm light to a greater degree
- R closer to 1
- causes the SpO2 to trend towards 85%

Sakurai 2016

Reusable content

Formulae used in this answer

Principles

CWF-0069

R \; = \; { {Pulsatile_{660} \; / \; Non-Pulsatile_{660} } \over { Pulsatile_{940} \; / \; Non-Pulsatile_{940} } }

Answer by Sakurai

Past papers

Exam appearances

5 appearances

Exam	Exact exam wording	Candidate success
2025A Q18	(a) Describe how arterial haemoglobin oxygen saturation is measured using a pulse oximeter including the underlying scientific principles (60% of marks). (b) Outline the limitations and sources of error of this method (40% of marks).	76%
2021A Q17	Describe the principles of measurement of arterial haemoglobin O2 saturation using a pulse oximeter (60% marks). Outline the limitations of this technique (40% marks).	74%
2014A Q07	Outline the principles underlying pulse oximetry. ( 80% of marks) Briefly describe the effect of an elevated level of the following upon pulse oximetry values. (20% of marks) a. Carboxyhaemoglobin b. Methaemoglobin	34%
2013B Q05	Describe the principles of measurement of arterial haemoglobin O2 saturation using a pulse oximeter. (60% of marks) Outline the limitations of this technique. (40% of marks)	—
2008A Q03	Describe the principles of measurement of arterial haemoglobin oxygen saturation using a pulse oximeter. Outline the limitations of this technique.	—