1.1 (figure 3.2); since the arterial blood is


this chapter, the first phase of the project is explained. First, a brief
description of the working principle of the SpO2 sensor and its
resulting PPG signal, second the features extracted and their significant
meaning and finally the classification process.


The SpO2 sensor

The hemoglobin
is the protein in the blood responsible for transporting oxygen to the body
organs, when it is carrying 4 oxygen molecules (completely saturated) it is
called oxyhemoglobin (HbO2) otherwise it is called reduced
hemoglobin or non-oxygenated hemoglobin (RHb). The oxygen saturation measured
by the SpO2 sensor shows the amount of oxygen that is being carried
by the hemoglobin. The SpO2 sensor is a non-invasive method that
measures in addition to the oxygen saturation, the heart rate, and a
photoplethysmogram waveform by being placed on the finger of the individual.


1.2.1       Working Principle

The SpO2 sensor
contains 2 light sources (LEDs); they emit red and infrared signals with 660nm
and 940nm respectively since the absorption trait of the hemoglobin differs
with respect to its synthetic binding and the wavelength of the light sent. The
non-oxygenated hemoglobin absorbs red light (660nm) whereas the oxygenated
hemoglobin absorbs infrared light (940nm). These signals pass through the blood
vessels in the finger where only the unabsorbed ones reach the photodetector on
the opposing side of the sensor; these signals are then sent back to the
monitor for processing where the oxygen saturation is calculated and displayed.

Figure ?3.1: SpO2 working principle


1.2.2       Photoplethysmographic Signal

When the
infrared and red light passes through the finger, they are not only absorbed by
the blood vessels, but they are absorbed, scattered and reflected by other
tissues, bones, skin, and arterial and venous blood. This leads to a photoplethysmogram
waveform having both AC and DC components as shown in the figure below (figure 3.2);
since the arterial blood is pulsatile and acquire AC components it can be
extracted from the non-pulsatile blood and other tissues that acquire DC
components. The resulting signal (shown in figure 3.3) has a unique form that
differs between individuals which made it a good cause to be used as a biometric
technique after extracting its features.