Perfusion index

Usually PI as a parameter index can reflect the limb perfusion of the test subject, indicating the detection accuracy of the machine. That is, it can still be detected under conditions of low perfusion and weak perfusion.

Moreover, through PI's display, it can also indicate the subject's physical condition, that is, when hypoperfusion occurs, it indicates whether the person being tested has its own causes of heart problems, shocks, etc., but also can reflect Whether there are external factors, such as cold weather, poor peripheral circulation, etc., can be judged by the above conditions!

PI refers to the blood perfusion index, PI value reflects the pulsatile blood flow, which reflects the blood perfusion ability. The greater the pulsating blood flow, the more pulsating components and the greater the PI value. Therefore, the measurement site (skin, nail, bone, etc.) and the patient's own perfusion (arterial blood flow) will affect the PI value. Since sympathetic nerves affect heart rate and arterial blood pressure (affecting pulse arterial blood flow), the human neuromodulation system or mental state also indirectly influences the PI value. Therefore, the PI value will be different under different anesthetic states.

working principle

An initial oximeter was developed by Millican in the 1940s. It monitors the ratio of hemoglobin that carries oxygen to hemoglobin that does not carry oxygen. A typical oximeter has two LEDs. The two LEDs are facing the patient's part to be measured - usually fingertips or earlobes. One diode emits a light beam with a wavelength of 660 nanometers and the other emits 905, 910 or 940 nanometers. Absorption of oxygenated hemoglobin to these two wavelengths is very different from the absence of oxygen. Using this property, the ratio of two hemoglobins can be calculated. The testing process usually does not require blood draw from the patient. The usual oximeter can also show the patient's pulse. According to Beer-Lambert's law, the ratio of R/IR to arterial oxygen saturation (SaO2) should be linear, but biological tissue is a kind of complex optical system with strong scattering, weak absorption, and anisotropy [2～ 4), which does not fully comply with the classical Beer-Lambert law, resulting in the establishment of a mathematical model for the relationship between the expression of relative changes in red light and infrared light absorbance (R/IR values) and arterial oxygen saturation (SaO2) difficult. The relationship between R/IR and SaO2, the calibration curve, can only be determined experimentally. Most pulse oximeter manufacturers use experimental methods to obtain empirical calibration curves to complete pre-manufactured products.