What is pulse oximetry, and what does it measure?

Pulse oximetry is a non-invasive and continuous method of assessing arterial oxygen saturation (SaO2) and pulse rate.

Note that it is not a measure of the total oxygen content of the blood nor the PaO2. It does not assess ventilation, which requires a measure of the PaCO2.

By which principle does pulse oximetry work?

Pulse oximetry works on the principles of spectrophotometry. It contains a probe emitting light at the red (660 nm) and infra-red (940 nm) wavelengths, and a photodetector.

It relies on the differing amount of light absorbed by the saturated and unsaturated Hb molecules. The percentage oxygen saturation of the blood is calculated from the ratio of these two forms of the molecule.

What are its disadvantages and sources of error?

Problems encountered include

Diminished accuracy below a saturation of ∼70%

There is a delay of ∼20 s between actual and displayed values, limiting its use in the emergency setting

Poor peripheral perfusion leads to a poor signal quality

So does ambient light pollution

Abnormal pigments affect the results. External pigments include nail varnish. Internal pigments include bilirubin of the jaundiced patient, methaemoglobin, and carboxyhaemoglobin. Jaundice underestimates the true SaO2 and carbon monoxide poisoning overestimates the true SaO2

Abnormal pulsations, such as cardiac arrhythmia or venous pulsations of right heart valve defects may interfere with the signal

Note, however, that there is no interference from polycythemia or fetal haemoglobin

What is methaemoglobin?

This is a haemoglobin molecule that contains iron in the ferric (Fe3+) state within its haem portion, as opposed to the normal ferrous (Fe2+) state.