33. Pulse Oximetry

Question 1

What is a pulse of oximeter and how does it work?

Answer 1

A pulse oximeter is a piece of equipment
used to measure the percentage
of arterial haemoglobin in the blood,
which is saturated with oxygen (HbO2 sats).

The equipment consists of an electronic processor,
two light-emitting diodes (LEDs)
and a photodiode.

The LEDs and the photodiode
are usually arranged on either
end of an adhesive strip, or on a ‘clip’,

that is placed around a 
thin part of the patient’s anatomy, 
typically a finger,
an ear lobe or the 
forefoot of an infant. 

The light from the LEDs
shines through the patient
and is detected by the photodiode.

Each LED emits light at a different frequency:

one at 660 nm (red light)
and

the other at 940 nm (infrared light).

Oxyhaemoglobin and
deoxyhaemoglobin absorb
these wavelengths of light differently;

this is why arterial blood appears brighter red
than venous blood to the human eye.

• Oxyhaemoglobin absorbs more infrared light
(940 nm) and allows more red light (660 nm) to pass through.

• Deoxyhaemoglobin absorbs more 
red light (660 nm) and allows more
infrared light (940 nm) to pass through.

Question 2

Whose law is used in this calculation?

Answer 2

Beer-Lambert law,

which relates the attenuation of
light to the properties of the
material through which
the light is travelling.

It is used to calculate the absorbance of a solution.

The law states that the absorbance
of a solution depends on:

• The concentration of that solution,
i.e. the more molecules of a lightabsorbing
compound there are in the sample,
the more light will be absorbed.

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• The path-length of light travelling through the solution, i.e. the longer
the length of the sample container, the more light will be absorbed
because the light will come into contact with more molecules.

Question 3

Whose law is used in this calculation?

Answer 3

Beer-Lambert law,

which relates the attenuation of
light to the properties of the
material through which
the light is travelling.

It is used to calculate the absorbance of a solution.

The law states that the absorbance
of a solution depends on:

• The concentration of that solution,
i.e. the more molecules of a light absorbing
compound there are in the sample,
the more light will be absorbed.

• The path-length of light travelling
through the solution,

i.e. the longer the length of the 
sample container, the more light 
will be absorbed
because the light will come into 
contact with more molecules.

Question 4

Expression of Beer Lambert

Answer 4

This is most simply expressed below:

A = εlc

Where A is absorbance of the solution

and A = log10/ I0 /I.

I is the light intensity of the wavelength
being passed through the solution.

If I is less than I0,

then a proportion of the original light
must have been absorbed by the solution.

ε is the molar absorption coefficient

(L mol–1 cm–1).

It compensates for variance in
concentration and the path-length
to allow comparison between solutions.

l is the length of solution that
the light passes through.
c is the concentration of the
compound in solution, expressed in mol L–1

Question 5

Can you draw a graph
comparing the absorbance of
light by oxyhaemoglobin with
deoxyhaemoglobin?

Answer 5

Fig. 82.1 Absorption spectra of oxy and deoxyhaemoglobin for red and
infrared light

Question 6

What factors may decrease the accuracy of pulse oximetry?

Answer 6

The following may cause erroneously low readings:

• Poor perfusion: hypotension/vasoconstriction
• Movement artefact
• Electrical interference from diathermy
• Highly calloused skin
• Nail varnish/artificial nails
• Severe anaemia
• Cardiac arrhythmias

• Methaemoglobinaemia –
characteristically cause saturations to be
measured at around 85%

• Increased venous pulsation, e.g. severe tricuspid regurgitation

• Intravenous administration of methylene blue dye because it absorbs
light in the 660–670 nm range.

Question 7

The following may cause high readings:

Answer 7

• Carbon monoxide poisoning –
CO irreversibly binds to haemoglobin

• Cyanide poisoning – this is not inaccurate.
Cyanide prevents oxygen
being utilised in respiration and so its extraction from the blood falls,
meaning saturations are high.

Question 8

Miscellaneous factors:

Answer 8

• The human volunteers used to construct empirical saturation tables
did not have their oxygen saturations dropped below approximately
85%; hence readings below this number are extrapolated, not
validated.
Of note, fetal haemoglobin and HbS (sickle) do not affect readings.