How pulse oximeter works

A pulse oximeter is a device that is used to quickly and easily monitor a person’s oxygen saturation. It means that can measure the level of oxygen within the blood especially in arterial blood without using any invasive means. It measures the oxygen saturation of pulsating blood and it is known as SpO2. Heart rate is automatically calculated alongside the percentage of oxygen. 

We all know that blood carries oxygen which is necessary for our life. Oxygen in blood is specifically carried by haemoglobin molecules, on with oxygen and the other one without oxygen. The haemoglobin without oxygen is called DEOXYGENATED HAEMOGLOBIN and the haemoglobin with oxygen is called Oxygenated HAEMOGLOBIN. Oxygen saturation simply refers to the percentage of the available haemoglobin that carries oxygen. 

Now let’s us see how a pulse oximeter works ? Pulse oximeter uses light to work out the oxygen saturation and light is emitted from the light source which goes across the pulse oximeter probe and reaches the light detector. If a finger is placed in between the light source and the light detector, the light will now have to pass through the finger to reach the detector. Part of the light will be absorbed by the finger and the part not absorbed reaches the light detector. The pulse oximeter uses the property that oxyhaemoglobin and deoxyhaemoglobin absorb light of different wavelengths in a specific way. Both these absorb different amount of light at different wavelengths. Pulse oximeter emits two types of light : 

Red light which has a wavelength of approximately 650 nanometer ;                                                               Infrared light which has a wavelength of 950 nanometer. 

The pulse oximeter works out the oxygen saturation by comparing how much red light and infrared light is absorbed by the blood. Depending on the amount of oxyhaemoglobin and deoxyhaemoglobin present the ratio of the amount of red light absorbed compared the amount of infrared light absorbed changes. Using this ratio the pulse oximeter can then work out the oxygen saturation. The internet circuits and the processing components of the pulse oximeter do some calculations as well. There is a reference curve saved in the memory of the device to calibrate the incoming readings and to normalize them. This increases the precision of the results that it provides. 

SpO2% – 95 to 100 = Normal                                         SpO2% – 91 to 94 = Mild Hypoxemia                             SpO2% – 86 to 90 = Moderate Hypoxemia                   SpO2% – <85 = Severe Hypoxemia. 

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