SpO
2
measurement
The light absorption properties of oxygenated arterial
blood (oxyhaemoglobin HbO
2
) differ from those of
unsaturated venous blood (reduced haemoglobin Hb).
O
2
saturation is a logarithmic function of the irradiated
light intensity (Lambert-Beer's law).
The effect of such dyshaemoglobins as carbon monoxide
haemoglobin HbCO and methaemoglobin MetHb is
normally negligible.
The sensor comprises two light-emitting diodes which
alternately emit infrared and red light at typical wave-
lengths of 920 nm and 660 nm respectively. The radiation
intensity is measured by a photodetector opposite the
diodes. The sensor is positioned on a limb in which the
arterial blood vessels can be irradiated, such as a finger,
toe or the nose.
These two wavelengths – 920 nm and 660 nm – are used
because meaningful absorption values are still obtained
for oxygenated and reduced blood, even in the presence
of slight perfusion, and because they differ significantly.
The light alternately emitted by the diodes is completely
absorbed by the pulsating arterial blood, the skin, finger
nails, muscular tissue, bones and venous blood.
Except for the pulsating arterial blood, the amount of light
absorbed by the other components remains constant as
regards the quantity and optical density over a defined
unit of time.
The arterial blood pulsating with every beat of the heart,
however, produces a change of volume synchronous
with the pulse in the irradiated tissue. In other words,
absorption of the irradiated light also changes in time with
the pulse.
The light absorbed when there is no pulsating blood
(during the diastole) is determined first. This yields the
amount of light absorbed by tissue and non-pulsating
blood.
The absorption value does not normally change during
the pulse phase and provides a reference value for the
pulsating part of the absorption.
Following the next beat of the heart, the absorption is
measured again when the pulsating blood enters the
tissue. The absorption of light changes for both wave-
lengths, due to the pulsating arterial blood.
131
Descriptions
Principles of measurement
Light-emitting diodes
Infrared light
920 nm
Red light
660 nm
Photodetector
20 000
10 000
5 000
1 000
500
100
50
10
500
650
750
850
950
660 nm
Red
920 nm
Infrared
Reduced haemoglobin Hb
Oxygenated haemoglobin HbO
2
Wavelength nm
Absorption coefficient
Absorption
Time (s)
Absorption by
arterial blood
Absorption by
venous blood
Absorption by
other tissues
(incl. skin)
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