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Pulse oximetry
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The pulse oximeter measures the oxygen saturation of the blood, that is, what proportion of the hemoglobin is carrying oxygen. And it does it without a needle: it sends light at specific wavelengths through tissue, mainly the nail bed, that strip of skin under the fingernail. It is quick and noninvasive.
The trick is in the color. Deoxygenated hemoglobin, the kind that has already released its oxygen, and oxygenated hemoglobin, the kind still carrying it, do not absorb light the same way: each one absorbs different wavelengths. The device measures how much light was lost crossing the finger, and an internal algorithm converts that loss into a saturation value. It is like holding your finger in front of a lamp and working out what is inside from the color of what makes it through.
And like any device, it has a blind spot. What the oximeter is looking for are the very small arterial pulsations of the arteriolar and capillary beds, the finest vessels in the finger. If tissue perfusion is low, that is, if little blood is reaching that tissue, the device may not be able to get a reliable signal.
In brief
Pulse oximetry measures the oxygen saturation of the blood quickly and noninvasively by passing light through tissue, mainly the nail bed.
Deoxygenated and oxygenated hemoglobin absorb light at different wavelengths, and an algorithm inside the oximeter converts that absorption into a saturation value.
It picks up very small arterial pulsations in the arteriolar and capillary beds, so it may not give a reliable signal when perfusion is low.
Sources (1)
- National Center for Biotechnology InformationPulse Oximetry. StatPearls
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