The oximeter is a device that used to measure the oxygen content in our blood. A pulse oximeter is the most commonly used because it is non-invasive, simple, cheap, easy to use, lightweight, small, compact, very quick responses. Pulse oximeter indicates the oxygen content of blood by measuring oxygen saturation. The blood carries oxygen from the lungs to the entire human body through blood vessels. The hemoglobins present in the blood serve as an oxygen carrier. The oxygen molecules get attached to hemoglobin and travel through blood vessels. The hemoglobin with oxygen is called oxygenated hemoglobin (O2Hb). The hemoglobin without oxygen is called deoxygenated hemoglobin (Hb). The percentage of oxygenated hemoglobin in respect of all available hemoglobin is referred to as oxygen saturation of the blood.
A pulse oximeter passes light through the blood and depending on the absorption of light rays in the blood; the device determines the oxygen saturation level of the blood. Hence, to detect the light absorption, a pulse oximeter must have a light source as well as a light detector.
When, any translucent part of our body such as a fingertip, toe, the earlobe is placed in between the light source and light detector of the device, one part the light gets absorbed and rest of the light passes through the translucent body part (finger is shown the figure) and reaches to the detector.
In the case of human body parts, light absorption characteristics and capacities are different for oxyhemoglobin and deoxyhemoglobin at the same wavelength of light. It is found that red light ray of wavelength 650 nm gets absorbed more in deoxyhemoglobin than in oxyhemoglobin. At the same time, an infrared ray of wavelength 950 nm gets more absorbed in oxyhemoglobin than in deoxyhemoglobin.
In the figure shown below, a finger is placed in between the probes of the oximeter. The upper probe has two light sources side by side, one emits red light, and another emits infra-red light. The lights emitted from the sources above the finger, pass through the finger hence they also pass through the blood circulating in the blood vessels of the finger. During the passing of lights, one portion of both lights gets absorbed in the finger.
By comparing the amount of red light as well as the amount of infrared light absorbed by the finger, a pulse oximeter calculates the concentration of oxyhemoglobin and concentration of deoxyhemoglobin in the blood. The oximeter determines the oxygen saturation level of the blood from the ratio