Oxygen is the foundation of human life. The contraction and relaxation of the heart causes the blood of the human body to pulsate through the lungs. A certain amount of reduced hemoglobin (HbR) combines with the oxygen taken up in the lungs to form oxygen and hemoglobin (HbO2). About 2% of the oxygen is dissolved in the plasma. These blood is transported through the arteries to the capillaries and then released in the capillaries to maintain the metabolism of the tissue cells. (SO2) is the percentage of oxygen-bound oxyhemoglobin (HbO2) in the blood that accounts for the total capacity of the combined hemoglobin (Hb), that is, the concentration of blood oxygen in the blood, which is an important physiological parameter of the respiratory cycle. Functional oxygen saturation is the ratio of HbO2 concentration to HbO2 Hb concentration, which is different from the percentage of oxyhemoglobin. Therefore, monitoring arterial oxygen saturation (SaO2) can estimate lung oxygenation and hemoglobin oxygen carrying capacity.
2.1 Blood oxygen saturation detection classification
The measurement of blood oxygen concentration is usually divided into two types: electrochemical method and optical method.
The traditional electrochemical method of measuring blood oxygen saturation is to first collect blood from the human body (the most commonly used is to take arterial blood), and then use the blood gas analyzer for electrochemical analysis, and measure the partial pressure of arterial oxygen (PaO2) within a few minutes. Arterial oxygen saturation (SaO2) was calculated. Because this method requires arterial puncture or intubation, it causes pain to the patient and cannot be continuously monitored, so when it is in a dangerous situation, it is not easy for the patient to receive timely treatment. The advantage of the electrochemical method is that the measurement result is accurate and reliable, the disadvantage is that it is cumbersome and cannot be continuously monitored, and it is a damaged oximetry method.
The optical method is a new optical measurement method that overcomes the shortcomings of the electrochemical method. It is a continuous non-invasive blood oxygen measurement method, which can be used in an emergency ward, an operating room, a recovery room, and a sleep study. At present, pulse Oximetry is the most widely used. The principle is to detect the change of blood absorption amount of blood, and measure the percentage of oxyhemoglobin (Hb02) in total hemoglobin (Hb), thereby directly obtaining SO2. The advantage of this method is that it can continuously measure the human body without damage, and the instrument is simple and convenient to use, so it has been paid more and more attention. The disadvantage is that the measurement accuracy is lower than that of the electrochemical method, and the error caused by the lower blood oxygen value is large. Ear oximeters, multi-wavelength oximeters and the newly introduced pulse oximeter have appeared. The measurement error of the latest pulse oximeter can be controlled within 1% to meet the requirements of clinical use. Although they are not satisfactory in some respects, the clinical benefits they generate are widely recognized.
2.2 Principle of non-invasive blood oxygen saturation detection
Functional oxygen saturation is often used clinically to reflect changes in oxygen levels in the blood. The non-invasive blood oxygen saturation measurement is based on the principle that the absorption of light by arterial blood varies with arterial pulsation. Basic research shows that oxyhemoglobin and non-oxyhemoglobin have different absorption rates for incident light of different wavelengths. When monochromatic light illuminates the human body vertically, the absorption of light by arterial blood will change with the pulsation of arterial vessels in the translucent region, while the absorption of light by other tissues such as skin, muscle, bone and venous blood is constant. When the finger is irradiated with the constant light λ1, λ2 of two specific wavelengths, if the incident light wavelength λ1 is appropriately selected (Hb02, Hb has an equal absorption characteristic here, that is, about 805 nm), the Lambert-Bear law is applied and according to the oxygen saturation. The definition can be derived from the approximate formula of arterial oxygen saturation: Sa02 = a bQ
Where: Q is the ratio of absorbance change of two wavelengths (Hb02, Hb), a, b is constant, and is related to the sensor structure and measurement conditions of the instrument.
It is important to note that biological tissue is a complex optical medium with anisotropic, strong scattering and weak absorption. Therefore, it cannot be described by a strict formula in actual measurement, so it is generally determined by measuring the ratio of the change of the absorbance of the two beams. The calibration curve finally obtains oxygen saturation. When the wavelength of the two beams is selected, the wavelength of the incident light is generally selected to be 660 nm and 940 nm.
2.3 Photoelectric sensor for non-invasive blood oxygen saturation detection [2,3]
The blood oxygen sensor is an important component for detecting blood oxygen saturation, and its damage will directly lead to inaccurate detection or the whole machine will not work. The blood oxygen sensor can be mainly divided into a finger type, an earlobe type, a wrap type and an adhesive type according to the shape, and can be classified into an adult type, a child type, and an infant type according to the use. Regardless of the shape and type, the principle of the blood oxygen sensor is the same, and they are composed of a light-emitting device and a receiving device. The light-emitting device is composed of red light having a wavelength of 660 nm (650 nm) and an infrared light emitting tube having a wavelength of 940 nm (910 nm). Photosensitive receiving devices mostly use PIN-type photodiodes with large receiving area, high sensitivity, low dark current and low noise, which convert the received incident light signals into electrical signals.
Most of the newly developed pulse oximeters are finger-held sensor probes. The probe is placed on the fingertip when in use. Two LEDs placed side by side are fixed on the upper wall of the finger sleeve, and the emission wavelengths are 660 nm red light and 940 nm infrared light, respectively. The lower wall is a photosensitive receiving device that converts the red and infrared light transmitted through the finger into an electrical signal. When the oximeter is running, the time-sharing driving circuit allows the two LEDs to emit light at a certain time interval and at a lower duty ratio, respectively, according to the ratio of the luminous intensity of the photodiode to the transmitted light received by the photocell. Calculate the whole blood absorption rate a660 and a940, and then combine the experimentally calibrated coefficients A and B into the above formula to calculate the blood oxygen saturation value.
2.4 oximeter system block diagram
The pulse oximeter generally consists of a blood oxygen saturation detection module, an industrial computer or a PC, and a blood oxygen detection probe (generally a finger sleeve type). Some are directly developed into one or portable. If you use a system that has been developed with a blood oxygen saturation detection module, because the level voltage between the module and the industrial computer or PC is different, they must be connected by a level conversion module. Ability to communicate correctly.
3 pulse oximeter operation
The correct operation of the oximeter is related to the accuracy of the test results. Transmissive pulse oximeters mostly use fingers, earlobes, toes, etc. as the detection site, because these parts are the most easily transmitted parts of light. For pulse oximeters using finger-type sensor probes, it is best to clean the finger parts before the test. Otherwise, if there is too much dirt, it will hinder the transmission of light, which will have a certain impact on the measurement results. . During the measurement, the middle finger is clamped in the finger sleeve, and the nail should be placed on the upper wall of the light-emitting tube. After the clip is attached, it should also be noted whether the four sides of the finger sleeve are tightly sealed to avoid interference from ambient light. After the fingertip is clamped and turned on, the blood oxygen saturation can be read after waiting for the measurement data to stabilize. The current oximeter can generally read the pulse rate value and the pulse waveform.