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Answered Underwater Pulse Oximeter?

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John Hank

New Member
Jul 8, 2020
Hi Guys,

New on here and I had a question that’s been bugging me. I just got into spear fishing and I learned a little bit about Shallow Water Blackout. I figured the freediving section was most appropriate for this question. So my question is, why aren’t pulse oximeters standard equipment for divers to prevent blackout? I know there are dive watches, but they don’t seem to have this feature. Pulse Ox’s generally go on the ear or finger, but there has got to be a way to adapt it for Continuous monitoring while diving. My thinking is when you surface, you can watch your O2 levels, and wait for them to return to normal before diving again, or monitor them on the dive. Why hasn’t this been invented yet? There has to be a reason.

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Even if it acccurately reported O2 it would not be wise to depend on it to prevent blackout. There are other factors involved.
Hi John,

SpO2 monitoring would be interesting to evaluate changes in technique or strategy, for instance speed, weight, fins, start of freefall, recovery breath, etc. I believe this measurement has not been well commercialized due to the following factors:
- Measurement at the finger would not be accurate due to peripheral vasoconstriction
- Measurement at the wrist, the most convenient place for the user as it would be directly integrated in the dive computer, is not advised due to low pulse signal and again due to vasoconstriction
- Measurement at the earlobe may potentially work, if the earlobe would not be too cold. Not sure to which extent the mammalian dive reflex would redistribute the blood away from the earlobe though.
- If the earlobe doesn’t provide good results, you could also monitor SpO2 at the forehead. Again, not sure how good the signal would be at depth.

SpO2 monitoring to prevent blackouts? Not a good idea imo:
- Validity of SpO2 measures decreases at SpO2 < 80%. Above 80% it should be within +/- 2-3% digits correct. I don’t know how much accuracy is lost at lower SpO2, but I doubt it would be sufficiently valid. That is, at least in the older SpO2 meters, but I don’t think that a lot of progression has been made in that regard the last few years.
- There is a certain discrepancy (delay) between the actual SpO2 value and the output you see on your monitor. I presume that this is due to the use of a moving average algorithm the device uses to provide an estimation? Maybe a short circulatory delay as well to take into account?
- At what SpO2 threshold would you expect to black out? In other words, what would you use as a safe threshold? It would be reasonable to expect that this threshold could also be affected by for instance your breath up. Don’t forget that amongst others arterial CO2 defines the affinity of O2 with Hb. Low CO2 increases the affinity of O2 with Hb, potentially slightly inhibiting O2 release into the plasma and the O2-consuming tissues. Anecdotally, SpO2 values around 50% or slightly lower are reported as critical threshold (correct me if wrong, I am not sure)? Reports of most hypoxic individuals have shown SaO2 values (=from arterial blood, not mixed venous blood that typical pulseoximetry is measuring) as low as 34-38% without blacking out (see figure I made, in Dutch, but you’ll get the point). Please don’t expect that you will be capable of reaching such low numbers, that will not end well!


O2 monitoring … SpO2
Please watch the comparison between Oximeters at :— 06:28
— 10:45
— 14:23
— 18:34
— 22:18
— 26:59
— 32:07
— 36:20
— 41:52
— 48:21
As you see ‘you can choose’ the O2 monitor you prefer;)

It’s from this video

And another very interesting video.
For registration go to 6:11

The HR in orange don’t decrease very much at the beginning ( no MDR in air)
He breathes again after 5 min
The value of the Sp02 is 66% , but is still decreasing down to 48% after 30s !!!

Make your own conclusions about the usefulness of O2 monitoring

see you
Thanks for the quick and great replies guys, especially dcvf and SDS. The second video of the guy laying down was very interesting in that once he began breathing again, his O2 saturation continued to go down. Wonder if thats how our bodies function, no longer constricting blood as much, or if thats a function of the device. It makes sense what SDS was saying about the relative values of the SPO2 and how probably unuseful it would actually be to the diver. Once you mentioned vasoconstriction, I was reminded of when i was a first responder at a remote ranch in the mountains. People in trouble on the freezing cold lake, I would bring in mildly hypothermic, (but conscious) and their finger pulse 02 reading would be in the 50s. Thanks again for the response guys. Maybe when the technology catches up it'll at least be a useful training aid for diving.
I’ve tried practicing breatholds while attached to a pulse oximeter. The big problem would be the delay in the reading there can be quite a significant lag between what’s happening and what the monitor is displaying (1 minute or so). That’s testing it with the top of the range kit I use at work. So if you were monitoring it yourself you might black out before the equipment warns you, although it would still give an interesting trend you’d never be able to predict how long the lag was at any given time.

The vasoconstriction / cold patient stuff is a bit of a limitation and may contribute to the lag but you can usually get it to work as long as the patient has a pulse. I Regularly use them on patients that are on very powerfull cooling devices or massive amounts of vasoconstrictors. Rarely have a problem.

I wonder if the effect on light spectrum at depth could confuse it, but it has its own light source so probably shouldn’t matter.

Theres another interesting bit of kit called ForeSight which measures tissue oxygen saturation in the brain from a sensor stuck on the forehead. That might be a more usefull way of doing it / predict a blackout better.
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Very interesting thoughts chemicalali. I’ll take a look at Foresight just out of curiosity.
[Chemicalali] "Theres another interesting bit of kit called ForeSight which measures tissue oxygen saturation in the brain from a sensor stuck on the forehead. That might be a more usefull way of doing it / predict a blackout better."

Now you are talking about near-infrared spectroscopy (NIRS). Very interesting tool to assess changes in for instance muscle/cerebral blood volume or oxygenation. It differs from oximeters in that it measures oxyhemoglobin and deoxyhemoglobin within a 'voxel' underneath the optodes, hence assessing O2Hb and HHb in underlying arterial, capillary, and venous blood, as well as the (de-)oxygenation of myoglobin. Still a bit expensive though to buy it yourself for non-research purposes.

Take a look at this Dutch company (Artinis) who developped a submersible NIRS device - already in use in freedivers. Hope to see some experimental data published soon.
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I see the pitfalls in reliable monitoring of SpO2.
As a precaution to shallow water blackout,
Has there been any studies into surface interval
Hyperoxygenation, utilizing a tank of 100% O2.
Going beyond restoring, but hyperoxygenating
Tissue may allow vital headroom to avoid black out. It would be interesting to conduct some clinical studies.
Interesting idea.

Finger or ear lobe isn't going to work. The best place is probably the nostril wall (nare). It could be built into the mask and display on the lens. Nose tends to stay mostly dry, snotters aside, and would be held very securely, and I have had good results with that site on critically ill kids.

It's not going to be a super reliable system though. You will get a lot of null readings, under readings and occasionally over reads. I don't think it should be relied upon to ensure safety. Bad data would be quite distracting during a dive, but it would be interesting to see SpO2 once back on the surface and still while re-oxygenating.

Niche product which would require a lot of R&D. Develop the system, patent it, sell it to a big mask manufacturer. Cressi springs to mind!
Hi John,
I'm not sure if oximeter device will solve your issue, what I would recommend instead is to keep track of your time below the water and make identical if not longer the relaxation time above it. For that purpose can be used any Free diving computer like Suunto D3 (quiet outdated).

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