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5. SPO2 MONITORING
5.1 Measurement Principle
SPO2 plethysmogram measurement is employed to determine the oxygen saturation of
haemoglobin in arterial blood. The SPO2/PLETH parameter can also provide a pulse rate
signal, pulse strength and a plethysmogram wave.
How the SPO2/ PLETH Parameter Works
Arterial oxygen saturation is measured by a method called pulse oximetry. It is a continuous,
non-invasive method based on the different spectra absorption of haemoglobin and
oxyhaemoglobin (the spectro-photometer principle). It measures how much light, sent from
one side of the sensor, is transmitted through the patient’s tissue (such as a finger or a toe),
to a receiver on the other side.
The sensor measurement wavelengths are nominally 660nm for the Red LED and 940nm for
Infrared LED. Maximum optical power output for LED is 4mW.
The amount of light transmitted depends on many factors, most of which are constant.
However, one of these factors, the blood flow in the arteries, varies with time, because it is
pulsating. By measuring the light absorption during a pulsation, it is possible to derive the
oxygen saturation of the arterial blood. Detecting the pulsation gives a PLETH waveform,
pulse rate signal and pulse strength.
The SPO2 value, PR value, pulse strength and the PLETH waveform can be displayed on
the main screen.
SPO2 is a non-invasive measurement of the functional oxygen saturation.