Blood oxygen saturation is the percentage of Hbo2 to HB, the blood oxygen concentration in the blood. It is a critical physiological parameter of the respiratory cycle. Arterial oxygen saturation (Sa02) monitoring can estimate the oxygenation and hemoglobin carrying capacity of the lungs.
The non-invasive pulse oximetry measurement function is one of the most basic monitoring parameters of the monitor and the vital parameter that reflects the monitor performance. At present, the measurement methods of blood oxygen used on most monitors are based on the absorption of pulsating blood to the pulse of 660nm, 940nm lights. For the various types of monitors used at present, the measurement principle and calculation are different. They have three types: Mindray, Masimo, and Nellcor blood oxygen module.
Mindray blood oxygen module
It uses pulsation oximetry, a continuous, non-invasive method for measuring hemoglobin oxygenation saturation. It measures how much light emitted from one side of the spo2 sensor light source passes through the patient's tissues (such as fingers and ears) and reaches the receiver on the other side. So many factors influence the amount of light passing through the tissue, most of which are constant. However, arterial blood flow changes regularly over time because it is pulsating. We can get the oxygen saturation of the arterial blood and the "plethysmograph" waveform and pulse rate value by measuring the light absorbed by the sensor during the patient's pulse. The measurable wavelength of the sensor is usually 660nm for the red LED, 940nm for infrared LED, and the maximum output power of LED is 4mW. The accuracy of the measurement result depends on the absorption of oxygenated hemoglobin and reduced hemoglobin to certain wavelengths of light. When other substances absorb the same wavelength lights, it will cause false or low SpO2 values in the measurement.
Masimo blood oxygen module
The basic algorithm theory of the Masimo spo2 sensor is the following three aspects:
1. Oxyhemoglobin and deoxyhemoglobin have different absorption of red and infrared light (molecular spectroscopy);
2. The arterial blood volume in the tissue and the light energy absorbed by the blood change during the pulsation period (pulse volume chart);
3. Arteries and veins are variable, and the absorption deviation of venous blood is the main noise entrance in the pulsation process.
The basic principle of the Masimo blood oxygen module is similar to that of the traditional blood oxygen module. The traditional blood oxygen measurement method assumes that all pulsation components in the light absorption signal are caused by the filling of arterial blood. And it calculates by the ratio of the reactive component (AC) of the light absorption of the red and infrared wavelengths to the direct component (DC).
R=[AC (660)/DC (660)]/[AC (940)/DC(940)]
This R-value can be used to find the corresponding blood oxygen saturation in the R-SpO2 table, which is determined based on the blood gas analysis results of the blood gas analyzer on the healthy adult volunteers in the study of tissue hypoxia-induced. The method used by Masimo Set assumes that the arteries and veins are variable, and the wave absorption generated by the veins is the main noise component during pulsation. And further, decompose the red light Rcd (660) and infrared light Ir (940) into the arterial component and the noise component, and calculate the arterial component flea ratio after removing the noise component, namely:
Red (660)=Sr+Nr
Ir (940)=Si+Ni
R=Sr/Si
The reference noise signal is: N'= Red(660-Ir(940)*R
If there is no noise component, that is, N'=0, then: Red(660) = Ir(940)*R,
This relationship is consistent with the traditional blood oxygen measurement method. This noise reference signal method depends on the R-value. The software automatically performs a rolling scan for the possible R-value corresponding to the blood oxygen value between 35% and 100%. The R-value produces a noise N' and uses each possible reference noise signal as a reference to input to process Red and Ir signals through an adaptive correlated noise canceller, outputting a number that corresponds to the possible blood oxygen value in the range of 35% to 100%. The software cycles through the entire processing sequence for the raw data of the last 4 seconds every 2 seconds. Masimo Set blood oxygen saturation value is the sliding average of the arterial blood oxygen saturation refreshed every 2 seconds.
Nellcor blood oxygen module
Principles of Nellcor spo2 measurement
- Oxygenated and non-oxygenated hemoglobin absorb different amounts of red light and infrared light. (spectrophotometric measurement).
- The blood of the arteries in the muscle will change with the pulse (plethysmography).
The monitor measures blood oxygen saturation by injecting red light and infrared light into the arteriole vascular bed and then measuring the change in light absorption during the pulse cycle. Because oxygenated hemoglobin and non-oxygenated absorb different amounts of light, the amount of red light and infrared light absorbed by blood is related to the oxygen saturation of hemoglobin. The monitor uses the pulsatility of arterial blood flow to determine the oxygen saturation of arterial hemoglobin. During systole, a new batch of blood flows into the human blood light bed, the blood volume and light absorption increase. During diastole, blood volume and light absorption are at their lowest. The difference between the maximum and minimum light absorption is the basis for the monitor's blood oxygen saturation measurement, which focuses on the amount of light absorbed by the pulsating arterial blood flow. And it eliminates the influence of non-pulsating light-absorbing substances, such as muscle tissue, bones, and veins.
The amount of light absorbed by hemoglobin varies because of light wavelength, and the average wavelength of the LED is variable. Therefore, the monitor must know the average wavelength of the sensor's red LED to measure the blood oxygen saturation accurately. A resistor of the sensor stores the average wavelength of the red LED in the sensor. The monitor reads the resistance and selects the wavelength coefficients suitable for the sensor's red LED. These coefficients are used to determine the blood oxygen saturation. The monitor will read the resistor when the machine is turning-on, and connecting to a new patient. Also, it has a regular resistor reading during working. In addition, the influence of different muscle tissue thicknesses on the measurement will be compensated by automatically adjusting the LED luminous intensity.
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