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Good question – How do those little Pulse Oximeter thingies they clip to your finger at the hospital work?

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[[This question came in from Karl as he was sitting in the Hospital]]

If you have been to the hospital recently, they have probably clipped a little digital device to your finger that made your finger glow red. The device is called a Pulse Oximeter and it is able to display the oxygen level of your blood. It is nice because it is non-invasive.

Pulse Oximeters come in two forms: 1) little stand-alone units that clip on and give a reading, and 2) big units with alarms, trigger levels and so on. Here is a little clip-on unit:

And a larger one:

Why do they do this at the hospital? Probably the reason they use the little clip-on units is because it looks cool and it makes you feel like they are doing something important. But ignoring this obvious reason, it would tell a doctor that something is going wrong with the oxygenation of your blood. A reading of 95 to 100 is considered normal. 90 and above is acceptable. If it drops below that, something may be wrong. Like you’ve stopped breathing (easy to detect without a Pulse Oximeter), or you are having a lot of trouble breathing (ditto), or your heart isn’t doing its job, or they sucked all of the oxygen out of the room (pilots use pulse oximeters and people summiting Mt. Everest would have low oxygen levels). But if you go to the emergency room with a broken arm and they put a little clip-on Pulse Oximeter on you, chances are it’s just for show, and it’s billable.

The place where a pulse oximeter is particularly useful is during Anesthesia. The drugs are changing pulse rates and respiration rates, and the anesthesiologist has direct control over oxygen levels the patient receives.

So how do these devices work? They work by shining two different kinds of light through your finger and comparing the differences in the way the light is absorbed. Red light and infrared light are absorbed differently depending on oxygen levels. As a side benefit, the device can also check your pulse, since the increase in the amount of blood with each heartbeat also affects the light. Infrared light passes through your finger essentially not caring about the oxygen level, while red light is absorbed depending on oxygen level.

This is an excellent video that shows you how the two different sources of light give readings in a Pulse Oximeter:

More info:
- How Blood Works
- How Anesthesia Works

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