Wavelengths and SATs values
Pulse oximetry is nothing more than a
specialized version of spectrometry / absorption spectroscopy. In spectrometry
if the wrong wavelengths are used the results will not be accurate. This is no
different for pulse oximetry.
Disposable and re-usable sensors work on
the same principle. Both types of sensor are equally susceptible to wavelength
errors and can be tested on The Lightman. We have heard it said that it is not
worth testing disposable sensors for accuracy; we do not agree with this because
the data from a disposable sensor is as important as the data from a re-usable
sensor. If a sensor is discolored in any way The Lightman will predict the
effect of this discoloration on the SATs vales.
At lower oxygen
saturations the error introduced into pulse oximetry by wavelength errors is
greater as the absorption of reduced haemoglobin is much more sensitive to
wavelength errors. So just when it is needed, a pulse oximeter sensor can become
useless. This is apparent in the graph where the gradient of the lines increase
as saturation falls from 97% to 70%. It is normal to find that if a sensor has
any error associated with it this error gets greater at lower levels of SATs.
This is known as the magnification of errors. It is for this reason – the
magnification of errors - that sensor accuracy can not be tested on the finger
of a volunteer at normal SATs values because this type of test will not reveal
the magnitude of errors at lower oxygen saturation values.