Eli5: how does the finger clip with the laser at the doctors measure my oxygen level?
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From the FDA: “A pulse oximeter is a device that is usually placed on a fingertip. It uses light beams to estimate the oxygen saturation of the blood and the pulse rate. Oxygen saturation gives information about the amount of oxygen carried in the blood. The pulse oximeter can estimate the amount of oxygen in the blood without having to draw a blood sample”.
[LINK](https://www.fda.gov/medical-devices/safety-communications/pulse-oximeter-accuracy-and-limitations-fda-safety-communication)
Oxygen turns your blood bright red, without oxygen your blood is very dark. The light passes through your finger and a sensor on the opposite side measures the color of the light to determine how much oxygen it contains.
Blood cells with oxygen in them are redder than blood cells without oxygen. The light reflects more from those cells, and when it detects a pulse of blood, it sees “how red” the pulse is.
I’m a paramedic.
The little thing is a Pulse Oximeter. It displays your SpO2. SpO2 is the Saturation of partial Oxygen. A red blood cell has a protein called hemoglobin. There are 4 hemoglobin sites on a normal RBC. The oximeter reads the percentage of hemoglobin that is saturated. It uses Infrared light spectrum to assess this not a laser. It also reads the pulse wave in the fingertip.
While in most people, this reading correlates to oxygen levels, but in some cases, the reading is wrong. Any reduction in the blood flow to the finger will give a false reading (cold, having your blood pressure taken on that arm, low blood volume, sickle cell crisis, etc). Any white or visible light in the sensor will give a false reading. Now, Carbon Monoxide will pull oxygen off the hemoglobin and bind itself to the hemoglobin. An oximeter will read 100%, even though your oxygen is depleted. Cyanide prevents oxygen from disengaging from the RBC and blocks O2 from entering the cells. You will read 100%, but will have signs and symptoms of hypoxia (without oxygen).
One side of the clip has a light and the other has a sensor. When you clip it on your finger, it shines both red visible light and infrared light through you finger and it hits the detector on the other side after passing through your finger. Hemoglobin is the protein in red blood cells that carries oxygen. Saturated hemoglobin (full of oxygen) and unsaturated hemoglobin (not full of oxygen) absorb different amounts of infrared and visible light, so by comparing the ratio that the sensor detects to the ratio that was emitted, it can tell how much of the hemoglobin in your blood is saturated (carrying oxygen) and how much isn’t.
It’s a type of spectrometer, which is a device used to make measurement with light. In this instance you pick specific wavelength(s) of light which interacts with your blood differently depending on whether it’s oxygenated or not. You shine that through your finger, and then use a detector to check how much actually made it through (some of it was absorbed by your blood). You can use that value to calculate what the oxygen content of the blood is because you’ve already researched the relationship between blood oxygenation and transmission/absorbance of this particular wavelength of light.
The reason for differing interactions of light and blood in different oxygenation states is an entirely different but very interesting topic which I will not go into here. But the underlying principles make this type of measurement possible, and spectroscopy is a huge deal in chemistry.
An easy to understand, everyday example is how you can tell how much dye is in a cup of water by how dark it is, with you acting as the spectrometer. You know a darker or more intense color means more dye, and that red color means red dye, blue color means blue dye etc. So by observing the light that exits the cup with your eyes, you can make a rough judgement about how much dye is in there, and a statement about what color it is, importantly without having some magical method of counting dye molecules one by one.
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