If you've ever done the wim hof breathing method, it is a very intense experience.

Basically hyperventilation + long breath holds. Probably similar to what free divers do without the mammalian dive reflex due to the cold water. Or like a dangerous game kids used to do when I was in school where you hyperventilate and then have someone press on your chest until you pass out.

But anyway, I'm not sure if the science would back it up, but Wim Hof describes it as over oxygenating the blood and then stopping and letting CO2 ramp up or something. Whether it is significantly dropping the CO2 or increasing oxygen during the hyperventilation phase, isn't it kind of the same thing? Adjusting the ratio.

Anecdotally, when I was doing it regularly I seemed to not get sick at all.