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I am talking about breath hold induced hypoxic state. Basically at what point during such an exercise would you expect the risk of fainting to be significant or can it happen at any saturation level? Can I assume, that at 50% the danger of fainting is very high, like 90% and at 70% saturation it is extremely low? Is it possible to train to be able to withstand lower saturation levels and not faint? For example would an experienced breath holder be able to not faint at 40% saturation? And finally, if you regularly do breath holding exercises, would you expect your red blood cell count to rise, similar to altitude exposure?

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    Are you asking about fainting purely from hypoxia, as opposed to vaso-vagal syncope induced by breath holding etc?
    – DoctorWhom
    Commented Jul 25, 2019 at 4:44
  • Yes, you can train this. Saying from the head, I think that some divers can go without oxygen for about 10 minutes or more, which suggests that you can faint at very different tissue oxygen saturation levels.
    – Jan
    Commented Jul 25, 2019 at 14:38
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    Although top freedivers can go an amazingly long time under water, the sport is quite dangerous and people have died doing it. In my EMS experience, O2 sats under about 70% begins to produce mental confusion, and under about 55% unconsciousness is likely. I doubt that anyone can remain conscious with O2 sats of 40%.
    – Carey Gregory
    Commented Jul 25, 2019 at 23:57
  • Isn't CO2 buildup in the body more of a problem than O2 saturation? Commented Jul 26, 2019 at 2:36
  • @CountIblis They're both a "problem" but lack of O2 will disable and kill brain cells long before excess CO2 does.
    – Carey Gregory
    Commented Jul 26, 2019 at 14:32

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I think it's hard to say because there seems to be substantial variability. The lowest figure I've heard was 30%, but it's from an anonymous report quoted in The Guardian:

An anaesthetist at a London hospital, who spoke anonymously, recalled one patient who attended A&E saying she felt cold. “When we put the stats probe on her, her saturation was 30% on air,” he said. “We obviously thought that was wrong, as usually patients are likely to have hypoxic cardiac arrests.” But when a blood sample was taken, her blood was very dark and had oxygen levels equivalent to those seen in people acclimatised to high altitudes. The patient was placed on a ventilator and survived for about a week before dying. “I have had a few patients like this,” the doctor said. “Sadly, their outcomes tend to be bad in my experience.”

The same article says:

Typically patients would lose consciousness below an oxygen saturation of 75%.

But I'm having a hard time finding a paper or textbook to back up this specific number.

Somewhat easier to find is data on oxygen saturation for patients whose life support has been withdrawn. (But these were unconscious already.)

It was observed that 0/82 patients had saturations of less than 40% for more than 3 min prior to cardiac arrest and 74/82 for more than 2 min.

Somewhat related: in patients predisposed to unexplained syncope, absolute frontal cerebral tissue oxygen saturation (SctO2) during the head-up tilt test was a good indicator: "patients faint when SctO2 falls below 60%", but this doesn't correlate at all with peripheral saturation (SpO2) in such patients, which remained constant.

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