What is the working principle of the pulse oximeter?

The main measurement indicators of the oximeter are pulse rate, blood oxygen saturation, and perfusion index (PI). Oxygen saturation (SpO2 abbreviated as SpO2) is an important basic data in clinical medicine. Blood oxygen saturation refers to the ** ratio of the combined O2 volume in ** blood volume to ** the combined O2 volume.

The main measurement indicators of the oximeter are pulse rate, blood oxygen saturation, and perfusion index (PI).

The oxygen consumed by the human body mainly comes from hemoglobin (there are four types of hemoglobin in normal blood: oxyhemoglobin (HbO2), reduced hemoglobin (Hb), carboxyhemoglobin (CoHb), methemoglobin (MetHb). Among them, it is combined with oxygen. What is reversibly bound is reduced hemoglobin, and what is not bound to oxygen is carboxyhemoglobin and methemoglobin.) The oxygen carried. It is usually said that the oxygen content in the blood refers to the amount of oxygenated hemoglobin in the blood, and the physical quantity of blood oxygen saturation is used to describe the changes in the oxygen content in the blood.

Blood oxygen saturation refers to the ** ratio of the combined O2 volume in ** blood volume to ** the combined O2 volume.

Normal human arterial blood oxygen saturation is 98*, and venous blood is 75*.

It is an important indicator that reflects the oxygen status in the body. It is generally believed that the normal value of blood oxygen saturation should not be less than 94*, and it is regarded as insufficient oxygen supply below 94*.

So how does it work?

The first oximeter was issued by Millikan in the 1940s. It monitors the ratio of oxygen-carrying hemoglobin to non-oxygen-carrying hemoglobin in the arteries. A typical oximeter has two light-emitting diodes. These two light-emitting diodes face the patient's part to be tested-usually the fingertip or earlobe. One of the two tubes emits a light beam with a wavelength of 660 nanometers, and the other emits 905, 910 or 940 nanometers. The absorption rate of oxygen-containing hemoglobin for these two wavelengths is very different from that of oxygen-free hemoglobin. Using this property, the ratio of the two hemoglobins can be calculated. The testing process usually does not require blood to be drawn from the patient. The usual oximeter can also display the patient's pulse. According to the Beer-Lambert law, the functional relationship between the ratio R/IR and the arterial blood oxygen saturation (SaO2) should be linear, but because biological tissue is a complex optical system with strong scattering, weak absorption, and anisotropy [2~ 4) It does not conform to the classic Beer-Lambert law, which leads to the difficulty in establishing the mathematical model of the relationship between the relative changes in the absorbance of red light and infrared light (R/IR value) and the relationship between the arterial blood oxygen saturation (SaO2) . The corresponding relationship between R/IR and SaO2 can only be determined through experiments, that is, the calibration curve. Most pulse oximeter manufacturers use experimental methods to obtain empirical calibration curves to complete the pre-determined standards of the products.