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The Pulse Oximeter Signal

Pulse oximeter signals are usually collected at points of the body where there is a capillary bed with a strong pulsatile signal - typically finger tips, toes, and forehead. Some argue that these sites have a blood supply larger than is necessary to oxygenate the local tissue because in our evolutionary past they have played important roles as heat dissipation sites.

Red and Infrared generated by LEDs are shone alternately into the tissue. The peripheral pressure wave or the pulsing of the blood causes both red and infra red to be absorbed in a pulsatile fashion. The strength of the transmitted or reflected light is measured by a detector (photo diode).

The signal of interest is the ratio of the of the absorption pulse for the red and the infra red. The signals are normalised to remove effects of variations in LED intensity and photo diode sensitivity. The ratio of the normalised signals is known as the R value. Due to the changing absorption of light at different oxygen saturations of the haemoglobin this ratio changes with the oxygen saturation. It is very important to remember that the absorption of light also changes with variations in the wavelengths of the red and infra red even if the SATs values remain constant. Pulse oximeter systems are calibrated to work with very specific wavelengths. The sensors used in the system need to match this to achieve the desired system accuracy. In practice not all sensors are the same. Inaccurate sensors can be identified and should not be used.