Oxygen is the basis for maintaining human life. The systole and diastole of the heart make the blood flow through the lungs. A certain amount of reduced hemoglobin (HbR) combines with the oxygen in the lungs to form oxyhemoglobin. About 2% of the oxygen is in the plasma. The blood moves through the arteries to the capillaries. And then oxygen is released to maintain the metabolism of tissue. Blood oxygen saturation is the percentage of the volume of oxyhemoglobin in the blood to the total hemoglobin. It is a vital physiological parameter of the respiratory cycle. The functional oxygen saturation is the ratio of the HbO2 concentration to the HbO2 Hb concentration, which is different from the percentage of oxyhemoglobin percentage. Therefore, the arterial blood oxygen saturation can estimate the oxygenation of the lungs and the oxygen-carrying capacity of hemoglobin.
1. Classification of blood oxygen saturation detection
There are two types of blood oxygen concentration measurement: electrochemical method and optical method.
The traditional electrochemical method of blood oxygen saturation measurement requires human blood sampling (usually the arterial blood). And the electrochemical analysis is performed with a blood gas analyzer. We can get the arterial oxygen partial pressure in a few minutes. With it, we calculate the arterial oxygen saturation (SaO2). This method requires arterial puncture or intubation, which causes pain to the patient and does not support continuous measurement. Therefore, it is not easy for the patient to receive timely treatment when in a dangerous situation. The advantage of the electrochemical method is that the measurement result is accurate and reliable, and the disadvantage is that it is more troublesome and painful for the patient. The optical spo2 measurement is a new type of measurement method that overcomes the shortcomings of electrochemical measurement. It is a continuous non-invasive blood oxygen measurement in emergency wards, operating rooms, recovery rooms, and sleep research. Pulse Oximetry is currently the most widely used. Its principle is to detect changes in blood absorption of light and measure the percentage of oxygenated hemoglobin (Hb02) in total hemoglobin (Hb). The advantage of this method is that it can achieve continuous and harmless measurement of the human body and is convenient, so it has received more and more attention. The disadvantage is that the measurement accuracy is lower than that of the electrochemical method.
2. Principle of non-invasive blood oxygen saturation detection
Clinically, functional oxygen saturation is used to reflect changes in blood oxygen content, based on the absorption of light by arterial blood changing with arterial pulsation. Basic research shows that oxyhemoglobin and non-oxyhemoglobin have different absorption rates for incident light of different wavelengths. When monochromatic light irradiates the human body vertically, the absorption of light by arterial blood will change with the pulse of arteries in the light-transmitting area. While the absorption of light by other tissues such as skin, muscle, bone, and venous blood is constant. Then we can calculate the light absorbed by blood and calculate the spo2 saturation.
3. Photoelectric sensor for non-invasive spo2 detection
The Spo2 sensor is a fundamental component for blood oxygen saturation measurement. And its damage will directly lead to unreliable detection or paralysis of the whole machine. According to the appearance, the sensor can be divided into finger sleeve type, earlobe type, wrapping type, and adhesion type and adults, children, and infants due to different ages. Regardless of the shape, the principal components of the spo2 sensor are the same. And they are all composed of a light-emitting device and a receiving device. The light-emitting device is composed of red light with a wavelength of 660nm (650nm) and an infrared light-emitting tube with 940nm (910nm) lights. Most photosensitive receiving devices use PIN-type photosensitive diodes with a 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 use finger-sleeve sensor probes. When in use, the sensor clips on the fingertip. Two light-emitting diodes placed side by side are fixed on the upper wall of the finger sleeve, and the light-emitting wavelengths are 660nm red light and 940nm infrared light, respectively. The other side is a photosensitive receiving device, which converts the red light and infrared light transmitted through the finger into electrical signals. When the oximeter is running, the time-sharing drive circuit makes the two light-emitting diodes emit light at a certain time interval and with a lower duty cycle. According to the ratio and intensity of the transmitted light received by the photoelectric tube, we can calculate spo2 percentage.
4. Diagram of oximeter system
A pulse oximeter is generally composed of a spo2 device module, an industrial computer or PC, and a blood oxygen detection probe. Some of them are portable and all-in-one designs. If we built with an existing module, we need to add a level conversion module to communicate correctly.
5. Operation and Use of pulse oximeter
The correct use of the oximeter refers to the accuracy of measurement. Transmissive pulse oximeters mostly use fingers, earlobes, toes as detection tissue because they can easily pass through by lights. For pulse oximeters using a finger spo2 sensor, it is best to clean the fingers, especially the nails, before testing. Otherwise, if it is too dirty, it will hinder the transmission of light, which will affect the measurement results to a certain extent. When measuring, clamp the middle finger in the finger sleeve, and put the nail facing the luminous tube on the upper wall. We should also pay attention to whether the finger sleeve is tightly closed on all sides of the finger to avoid interference from ambient light.
All have done, we can start the spo2 measurement. Generally, It can also read the pulse rate and pulse waveform.
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