Pulse oximetry uses photoelectric technology for blood oxygen saturation measurement according to Lambert-Beer Law (Lambert-Beer Law). When a beam of light strikes a solution of a substance, there is the following relationship between the transmitted light intensity I and the emitted light intensity I0: I= I0ekCd
The logarithm of the ratio of I and I0 is called the optical density D, and therefore the above formula can also be expressed as: D=In(I/I0)=kCd
Here, C is the concentration of the solution (e.g., blood), d is the path of the light through the blood, and k is the optical absorption coefficient of the blood. k is the light absorption coefficient of the blood. If the path d is held constant, the concentration of blood is proportional to the optical density D.
The absorption coefficients of HbO2- and Hb in blood are different for different wavelengths of light, in the red light (RED) region with a wavelength of 6OO-700 nm, the absorption coefficient of Hb is much larger than that of HbO2; however, in the infrared (IR) region with a wavelength of 80O-1000 nm, the absorption coefficient of Hb is smaller than that of HbO2. is smaller than that of HbO2; near 8O5nm is the iso-absorption point.
The probe used in pulse oximetry is attached to the finger when used. On the upper wall are fixed two light-emitting diodes (LEDs) placed side by side, emitting red light at a wavelength of 660nm and infrared light at 940nm. The lower wall has a photoelectric detector, will be transmitted through the arterial blood vessels of the finger red light and infrared light into an electrical signal, it detects the weaker the photoelectric signal, that is, the light signal penetrates the probe site, the more absorbed by the tissues, bones and blood, etc. There is a photoelectric detector. The absorption coefficients of skin, muscle, fat, venous blood, pigment, and bone are constant for both types of light, so they only affect the magnitude of the DC component of the photoelectric signal. However, the concentration of HbO2 and Hb in the blood changes periodically with the pulsation of the blood, so their absorption of light also changes pulsationally, which causes the signal intensity of the output of the photodetector to change pulsationally with the ratio of HbO2 and Hb concentration in the blood. If expressed in terms of light absorption, red light and infrared light, the change rule of the signal is roughly the same, but the amplitude of the pulsation component may be different, trying to let the above two wavelengths of red light and infrared light through the detection site in turn, and the pulsation component in the two signals are separated out after amplification and filtering, respectively, by the analog/digital converter into a digital quantity, can be calculated according to the following formula for the hemorrhagic oxygen saturation:
SaO2
SaO2= KlR2 + K2R + K3
K1, K2, K3 are empirical constants, and R is the ratio of the amplitude change of the two photoelectric signals over a very small time interval, i.e.
R = ΔRED/ΔIR
The pulsation pattern of the photoelectric signals is in accordance with that of the heart, so the repetitive cycle of the signals can also be detected. of the repetition period also determines the pulse rate. It is customary to refer to the oxygen saturation measured by pulse oximetry as SpO2 to differentiate it from the results measured by other types of oximeters.
Structure of a pulse oximeter
The electronics of a typical pulse oximeter are shown in the figure.
The photodetector in the probe is a photocell that produces a current proportional to the intensity of red and infrared light transmitted to it, but it cannot distinguish between the two types of light. For this reason, a timing circuit is used to control the light-emitting sequence of the two LEDs, i.e.: (1) the red LED is lit; (2) the red IED is extinguished and the infrared LED is lit; (3) both IEDs are extinguished;
This light-emitting sequence repeats itself at a frequency of either 480 times/second (for areas with 60Hz AC power) or 4O0 times/second (5OH7 AC), and this design enhances suppression of ambient light. The instrument is operational when the power is turned on and the red LED in the probe is seen to be flashing. During the cycle when both IEDs are off, ambient light and interference signals are detected, and subtracting them from the red and infrared light signals improves the signal-to-noise ratio. Photocurrent signal is converted into a voltage signal, and after amplification, filtering, signal baseline level conversion and go to the DC component and other signal conditioning process, added to a voltage / current conversion circuit with automatic gain adjustment function, and then by the integral circuit of the signal current integral, its output is an analog / digital converter into a digital signal. In order to ensure accuracy, usually with 12-bit resolution of the analog / digital converter.
The microprocessor performs complex processing of the digital quantities, such as digital filtering, calculating the amplitude of the two photoelectric signals, and deriving SaO2 according to a formula.To further minimize the effect of patient movement and improve the stability of the readings, a series of measured SaO2 values [mostly the current 6s results] are usually weighted moving average. The pulse rate can also be calculated from the pulsatile signal. Finally, the oxygen saturation and pulse rate values are sent to the appropriate monitor for display. The user can control the instrument via the keyboard, set alarm limits and perform other operations.