Shulou(Shulou.com)11/24 Report--

Usually, if we want to analyze some information about blood, we need to get blood samples in a variety of unpleasant ways. However, if we want to know the blood oxygen, we just need to put our finger into the device, and it will immediately tell us the heart rate and oxygen saturation, and this is a completely non-invasive way. So how does the oximeter do it?

If you look closely at the place where the oximeter puts its finger, you will find a flashing LED light, and opposite the LED light is a photodiode. Our fingers are placed between the LED lamp and the photodiode, and the oximeter glows to the finger, which is then received by the photodiode on the other side and converted into an electrical signal.

If you've ever accidentally covered your phone's flash with your finger, you'll find that the skin is actually translucent and it turns red on the other side. Because your blood absorbs some light and passes through another part of the light. The scientists experimented with this, and they learned that the absorption spectrum of hemoglobin (the protein that carries oxygen in the blood) is very different in two states, the oxygenated state and the non-oxygenated state.

In general terms, this means that the color projected by it will change. Although these changes cannot be distinguished by the naked eye, the instrument can. And if we use two different wavelengths of light to measure, we will find that the difference becomes more obvious. So, in fact, the oximeter has two LED lights, one glowing red and the other emitting infrared light that we can't see. And the two of them do not glow steadily, but flicker in turn, and then by analyzing the light signal received by the photodiode on the other side, we can accurately judge the bleeding oxygen saturation.

As shown in the image above, this is the absorption spectrum of oxyhemoglobin and deoxyhemoglobin. The Abscissa represents the wavelength of light, and the ordinate represents the molar absorptivity, that is, the ability to absorb light. The red line represents oxyhemoglobin, while the blue line represents deoxyhemoglobin. We can see that in the leftmost red light region, the ability of oxyhemoglobin to absorb this wavelength is weaker than deoxyhemoglobin. But as the wavelength increases, the situation reverses at the rightmost infrared, and oxyhemoglobin actually absorbs a little more light than deoxyhemoglobin.

However, our fingers are not only blood, but also skin, bones, nails and other things, so how can light tell us exactly the oxygen saturation of blood? Our blood doesn't just stay on our fingers, it pulses according to the beat of the heart. Therefore, through a small amount of signal analysis, the microprocessor in the pulse oximeter can isolate the pulse components of the signals it receives and ignore all non-blood signals. This will tell us the heart rate and determine the percentage of oxyhemoglobin.

But we should know that none of these devices are perfect. In some cases, they may give the wrong readings, such as carbon monoxide poisoning. For oxygen meters, the hemoglobin that carries carbon monoxide is the same as the hemoglobin that carries oxygen.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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