The Paramedic’s Guide to Blood Oxygen and the Apple Watch Series 6

Originally published at: The Paramedic’s Guide to Blood Oxygen and the Apple Watch Series 6 - TidBITS

As a paramedic, Rich Mogull has spent decades using pulse oximeters under extreme conditions and environments. While he found the Apple Watch Series 6’s blood oxygen sensor to be quite accurate with just a little focus on positioning, he explains why it’s hard to find the value and use cases for home blood-oxygen monitoring.


That’s a really interesting comparison Rich. I had read elsewhere that the Apple Watch measurements weren’t that reliable for blood oxygen. But it seems to do pretty well. My partner suffers from sleep apnea but the lack of any alarm function on the Apple Watch when the reading drops (which the cheap finger devices offer) prevents it being much use except in review.

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Thank you for that, that was a very instructive read and I found it really valuable.

Thanks for a good article based upon hands on experience. I to found the Watch 6 to match simultaneous Masimo readings with an occasional reading being 1 percentage point lower.
The automatic readings showed greater variation but I’ve not compared them as trying to wear the fingertip device over the same time frames is bit of a pain. So I’m impressed and intrigued but then I didn’t buy the watch for just O2 saturation—it’s another benefit.
I do find that using Apple’s Health App isn’t the best experience. Try viewing a specific measurement over the last month, my flicking finger sure gets a workout. I’d love to see Apple do a better job in data presentation, with ability to show variability ranges over the selected time period but perhaps also show the latest period over historical data over all your data—current month daily mean over all readings. But that’s just the data nerd in me…

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Oh i forgot the question I had… elevation ranges, what does Apple consider to be a medium or high elevation reading? I always thought 3000 feet was medium while 8000 was high l yet I get a high readings at 7056 on the Donnor Pass.

I have extensive experience of one particular fingertip blood pulse oximeter and a variety of hospital and ambulance ones, from the patient perspective.

As an electronic engineer, I was interested to calibrate my cheap HKD100 (about USD13) fingertip blood pulse oximeter against the hospital one that looked like it was built for battle. The readings were never more than 2% different - and those reading from different fingers, which can give different readings - and usually identical, but that was in ideal conditions such as constant ward temperature of 19C/66℉ and me comfortable and at rest.

Lowest recorded SpO2 in hospital was about 63% briefly - probably under-reading of my actual oxygen saturation in my body core, but I have poor circulation in extremities when cold.

In one ambulance trip at 3am on a cold winter night, I saw my SpO2 measured by the ambulance oximeter plummet from low 90’s - 80’s - 70’s - 60’s etc and finally settled at 25%, where it stayed for at least 10 seconds, though it seemed like longer. I prayed hard and concentrated on controlled breathing with all my strength.

Obviously I survived, but one doctor pointed out that such a reading could be expected from “a dead fish” and was probably meaningless, saying more about my blood circulation than my oxygen saturation in core organs.

The point of this being that finger pulse oximeters of any type - fingertip or “military grade” looking instruments - can easily fail to give meaningful absolute readings - even when correctly attached to the “best” finger (index or middle finger of handwriting hand), but are still useful for relative readings to indicate a trend.

In such conditions as I described - a person with poor circulation when cold especially - I wonder if the Apple Watch may actually give a more meaningful reading, as the wrist would get better blood supply than the fingertips?


Your wrist certainly has better circulation, but taking a measurement from reflected light is more difficult than from light transmitted through a body part.

This is one of those questions that I think will only be answered by actual usage, not theoretical assumptions.

Replying to my own post. A little test of the watch reveals that Apple uses 1,500 meters (4,921 feet) as the point where it records as being a high altitude measurement. I live at around 4,515 feet so on the cusp… I’m. thinking it would be good for Apple to record/reveal the recording altitude in the Health App as it does for barometric pressure.


After purchasing and using the Apple Watch Series 6, I called Apple and they confirmed there is an alarm for low blood oxygen while sleeping. However, they would not tell me the low reading numbr that would trigger the alarm.