How do pulse oximeters guess the oxygen level in your blood?
In: Other
They measure the intensity of certain colors of light that go through your tissue.
The protein hemoglobin in your red blood cells carries oxygen to all parts of your body. If you shine a light through the hemoglobin, its shadow (transmitted light) will be different depending on if it carries oxygen (oxygenated) or if it does not (deoxygenated). To be more specific, deoxygenated blood absorbs more red light than oxygenated blood. For near-infrared light (NIR), the reverse happens, oxygenated blood absorbs more NIR light than deoxygenated.
By measuring the differences in these transmitted colors shining through your tissue during your normal blood flow pulses with a red- and NIR-colored LED, it’s possible to determine the percentage of oxygenated blood vs. deoxygenated blood.
They shine two lights through your finger and into a light brightness sensor. The color of the lights is different, one light will be diminished by oxygen rich blood cells and the other not. By comparing the brightness difference of the lights, the blood oxygen level can be calculated.
It doesn’t guess it, it’s directly measured via light absorption. Oxygenated hemoglobin (HbO2) absorbs different wavelengths of light in different intensities than the non-oxygenated form (Hb). By using 2 LEDs with different wavelengths, you can work out how high the percentage of the oxygenated form is.
Technology Connections, a YouTube channel which looks at electronics recently had a video [explaining how these work](https://youtu.be/4pZZ5AEEmek).
As an additional note, the common pulse ox cannot well distinguish between hemoglobin that is carrying oxygen and that which is carrying carbon monoxide (gas given off from incomplete combustion). So, such a device would not change its reading as you experience carbon monoxide poisoning. I think there is a version of this technology which can tell the difference, but the need is relatively unusual and so I don’t believe it is commonly implemented in the “every day” ones you might expect to encounter. I also don’t know much of anything about them.
As mentioned in all the other posts, you shine two lights of different wavelengths, measure the difference in absorption of the lights, and then apply some simple maths to find the ratio of the absorption.
I just want to add that this is then compared to a graph of absorption vs oxygen saturation to spit out the number.
This graph was initially obtained by healthy individuals breathing a low O2 mixture of gas, having a pulse oximeter connected, and then samples are taken to determine the saturation at various absorption ratios. This was done down to a SpO2 of 70% and further extrapolated down to 50%
Latest Answers