[Blood Oximeter] Disassembly Part 3 Background Knowledge
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The main measurement indicators of the oximeter are pulse rate, blood oxygen saturation, and perfusion index (PI). Blood oxygen saturation (abbreviated as SpO2) is one of the important basic data in clinical medicine. Blood oxygen saturation refers to the percentage of the combined O2 capacity in the total blood volume to the total available O2 capacity.
Measuring indicators
The main measurement indicators of the oximeter are pulse rate, blood oxygen saturation, and perfusion index (PI).
Pulse rate
Pulse refers to the pulsation of the superficial arteries. A normal person's pulse is consistent with his or her heartbeat.
Heart rate is the frequency of heart beats. Generally, the heart rate is between 60 and 90 times per minute. However, when people are exercising or nervous, their heart rate may increase.
Pulse rate is the number of pulse beats per minute.
Under normal circumstances, the pulse rate and heart rate are consistent. When atrial fibrillation or frequent premature beats occur, the pulse rate is less than the heart rate.
Blood oxygen saturation
Blood oxygen saturation (abbreviated as SO2) is one of the important basic data in clinical medicine.
The oxygen consumed by the human body mainly comes from the oxygen carried by hemoglobin (there are four types of hemoglobin in normal blood: oxyhemoglobin (HbO2), reduced hemoglobin (Hb), carboxyhemoglobin (CoHb), and methemoglobin (MetHb). Among them, reduced hemoglobin reversibly binds to oxygen, while carboxyhemoglobin and methemoglobin do not bind to oxygen.) The oxygen content in the blood is usually referred to as the amount of oxyhemoglobin in the blood, and the physical quantity of blood oxygen saturation is used to describe the changes in the oxygen content in the blood.
Blood oxygen saturation refers to the percentage of bound O2 capacity in the total blood volume to the total available O2 capacity.
The normal oxygen saturation of human arterial blood is 98%, and that of venous blood is 75%.
It is an important indicator reflecting the oxygen status in the body. It is generally believed that the normal value of blood oxygen saturation should not be lower than 94%. Any value below 94% is considered to be insufficient oxygen supply.
Perfusion Index (PI)
Usually, PI is a parameter index that can reflect the limb perfusion status of the person being tested and indicate the detection accuracy of the machine, that is, it can still detect under low perfusion and weak perfusion conditions.
Moreover, the display of PI can also indicate the limb condition of the person being tested. That is, when low perfusion occurs, it indicates whether the person being tested has his or her own reasons such as heart problems, shock, etc. It can also reflect whether there are external factors such as cold weather, poor peripheral circulation, etc., so that judgment can be made based on the above situations!
PI refers to the blood perfusion index (PI). The PI value reflects the pulsating blood flow, that is, the blood perfusion capacity. The greater the pulsating blood flow, the more pulsating components there are, and the greater the PI value. Therefore, the measurement site (skin, nails, bones, etc.) and the patient's own blood perfusion (arterial blood flow) will affect the PI value. Since the sympathetic nerves affect the heart rate and arterial blood pressure (affecting the pulsating arterial blood flow), the human body's neural regulation system or mental state will also indirectly affect the PI value. Therefore, the PI value will be different under different anesthesia states.
How it works
The first oximeter was developed by Millikan in the 1940s. It measures the ratio of hemoglobin that carries oxygen to hemoglobin that does not carry oxygen in the arteries. A typical oximeter has two light-emitting diodes . These diodes are pointed at the patient's test site - usually a fingertip or earlobe. One diode emits a beam of light at a wavelength of 660 nanometers , and the other emits light at 905, 910, or 940 nanometers. Oxygenated hemoglobin absorbs these two wavelengths at a much different rate than unoxygenated hemoglobin. Using this property, the ratio of the two types of hemoglobin can be calculated. The test usually does not require drawing blood from the patient. A typical oximeter can also display the patient's pulse. According to the Beer-Lambert law, the functional relationship between the ratio R/IR and arterial oxygen saturation (SaO2) should be a linear relationship. However, since biological tissue is a complex optical system with strong scattering, weak absorption, and anisotropy〔2～4〕, it does not fully conform to the classic Beer-Lambert law, which makes it difficult to establish a mathematical model that expresses the relationship between the relative change measurement value of red and infrared light absorbance (R/IR value) and arterial oxygen saturation (SaO2). The corresponding relationship between R/IR and SaO2, that is, the calibration curve, can only be determined by experimental methods. Most pulse oximeter manufacturers use experimental methods to obtain empirical calibration curves to complete pre-calibration of products before leaving the factory.
Saturation expression method
Functional saturation:
SO2 = oxygenated hemoglobin / (oxygenated hemoglobin + reduced hemoglobin)
Fractional saturation
SO2 = oxyhemoglobin/(oxyhemoglobin + reduced hemoglobin + carboxyhemoglobin + methemoglobin)
Functional saturation is often used clinically to reflect changes in blood oxygen content.
Reasons for decreased saturation
Diseases of the respiratory and circulatory systems, autoregulatory dysfunction caused by anesthesia, trauma caused by major surgery, and injuries caused by other treatments and examinations
Symptoms: dizziness, weakness, vomiting, and severe cases can be life-threatening.
Evaluation of test results
From a medical analysis, the normal index is that the oxygen content in the blood is greater than or equal to 95; the normal index is that the pulse is between 60-100 beats per minute. If the values you tested do not meet the above two indicators, please test 2-3 times at different time points and keep testing for 2-3 days. If the values still do not meet the standards, it is recommended that you go to the hospital for detailed medical treatment.
Finger Clip Oximeter
By driving a red LED (660nm) and an infrared LED (910nm) in sequence, the blue line represents the receiving tube's sensing curve for reduced hemoglobin when hemoglobin does not carry oxygen molecules. From the curve, it can be seen that reduced hemoglobin has a strong absorption of 660nm red light, while the absorption of 910nm infrared light is relatively weak. The red line represents the receiving tube's sensing curve for oxygenated hemoglobin when hemoglobin carries oxygen molecules in red blood cells. From the figure, it can be seen that the absorption of 660nm red light is relatively weak, while the absorption of 910nm infrared light is relatively strong. When measuring blood oxygen, the difference in the absorption of reduced hemoglobin and oxygenated hemoglobin at different wavelengths is detected, and the measured data difference is the most basic data for measuring blood oxygen saturation. In blood oxygen testing, 660nm and 910nm are the two most common wavelengths. In fact, to achieve higher accuracy, in addition to these two wavelengths, more wavelengths are needed, even up to 8 wavelengths. The main reason is that in addition to reduced hemoglobin and oxygenated hemoglobin, the human body also has other hemoglobins. The one we often see is carboxyhemoglobin. More wavelengths are beneficial to better accuracy.
Target Group
People with vascular diseases (coronary heart disease, hypertension, hyperlipidemia, cerebral thrombosis, etc.)
Lipid deposition in the blood vessel cavity, poor blood flow and difficulty in oxygen supply. Cardiovascular and cerebrovascular patients have viscous blood, and coronary artery sclerosis and narrow blood vessel cavity, resulting in poor blood supply and oxygen supply. The body is " lack of oxygen " every day. Long-term mild hypoxia will gradually decline the functions of organs such as the heart and brain that consume a lot of oxygen. Severe hypoxia will cause " myocardial infarction " and " cerebral infarction ". If oxygen is not supplied in time for emergency treatment, it will cause sudden death. Therefore, cardiovascular and cerebrovascular patients use pulse oximeters to detect blood oxygen content for a long time, which can effectively prevent danger. If hypoxia occurs, oxygen can be supplemented as soon as possible to greatly reduce the chance of disease onset. People with respiratory diseases (asthma, tracheitis, chronic bronchitis, cor pulmonale, chronic obstructive pulmonary disease, etc.)
Breathing difficulties lead to limited oxygen intake. Blood oxygen testing of patients with respiratory system is indeed very important. On the one hand, breathing difficulties lead to insufficient oxygen intake. On the other hand, the persistence of asthma will also cause small branch organs to be blocked, making gas exchange difficult, leading to hypoxia, causing varying degrees of damage to the heart, lungs, brain and even kidneys. Therefore, using a pulse oximeter to test blood oxygen content can greatly reduce the incidence of respiratory diseases. Elderly people over 60 years old
Physiological aging of the heart and lungs, insufficient oxygen intake and inadequate oxygen supply
The human body relies on blood to transport oxygen. When there is less blood, there is naturally less oxygen . When there is less oxygen, the physical condition naturally deteriorates. Therefore, the elderly should use pulse oximetry to check the blood oxygen content every day. Once the blood oxygen level is lower than the warning level, oxygen should be supplemented as soon as possible. People who work more than 12 hours a day
The brain's oxygen consumption increases, and the oxygen supply cannot meet the consumption
The brain consumes 20% of the total body oxygen intake. If the brain is overworked, the brain's oxygen consumption will inevitably increase. However, the human body can only take in a limited amount of oxygen. If the body consumes more, it will take in less. In addition to causing dizziness, fatigue, poor memory, slow response and other problems, it will also cause serious damage to the brain and myocardium. It may even lead to death from overwork. Therefore, people who study or work 12 hours a day must use a pulse oximeter to test their blood oxygen content every day, monitor their blood oxygen health at all times, and ensure the health of their heart and brain.
Blood oxygen monitoring in extreme sports and high mountain hypoxia environments
Real-time blood oxygen monitoring of athletes helps to understand the blood circulation of athletes after heavy exercise, so as to guide the formulation of the amount of exercise for athletes. Passengers and journalists who take the Qinghai-Tibet Railway to Tibet need to undergo blood oxygen testing. Through the monitoring of blood oxygen, problems with blood oxygen carrying or supply can be discovered in advance, avoiding the harm of cyanosis caused by high altitude reaction to the body. Long-term alcohol abusers
The human body needs three units of oxygen to completely decompose one unit of alcohol and turn it into water and carbon dioxide. Therefore, hypoxia is one of the important manifestations of drunkenness. However, long-term alcoholics have developed a certain tolerance to alcohol and hypoxia, and are basically unable to detect when they are slightly drunk. Therefore, carry a blood oxygen meter with you to understand the physical condition and degree of drunkenness of alcoholics in time to avoid alcohol poisoning.
Main ingredients
A microprocessor, memory (EPROM and RAM), two digital-to-analog converters to control the LEDs, devices to filter and amplify the signal received by the photodiode, and an analog-to-digital converter to digitize the received signal for the microprocessor. The LED and photodiode are placed in a small probe that contacts the patient's fingertip or earlobe. The pulse oximeter also generally includes a small LCD display.
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