[n8henrie]

> I'm quite sure no freediving instructor would ever teach you this particular method, because it is a surefire way to die underwater on your first attempt

Definitely no instructor involved, just a dumb 20 year-old living in Puerto Rico. It admittedly was dangerous, but I am living evidence that it was far from a "surefire" way to die. It was one of a hundred ways in which I put my life at risk during my 20s. ¯\_(ツ)_/¯

> As for the specifics that may have prevented you from doing what you wanted: If you breathe too shallow or too slow, you won't clear enough CO2.

I'm confident I was doing it sufficiently well to accomplish a longer period of breath holding than I otherwise would have been able to sustain, as evidenced by having done so (in addition to the usual symptoms of lightheadedness, confusion, loss of vision, near-syncope -- yes I agree quite uncomfortable). I know people on HN love to idolize Wim Hof, but in this context minute ventilation is not that difficult of a concept; I'm usually able to estimate the response in a paralyzed patient's PCO2 fairly well when making changes to their tidal volume and rate.

I didn't search for too long, but here's at least one relevant document, in which otherwise untrained subjects were able to achieve a substantial reduction in CO2 (17.4 vs 29.0) with a mere 15 seconds of hyperventilation, leading to an extra 23 seconds of breath holding prior to involuntary breathing moments. The peripheral O2 sat nadir in the hyperventilation group appears to have been identical to the non-hyperventilation group after the first trial (Fig 8b, looks like ~94%) and was only statistically significantly lower on trials 2-5: <https://pmc.ncbi.nlm.nih.gov/articles/PMC10363065/>

[sigmoid10]

90 seconds of breath hold is still way too little to see oxygen drops in a finger pulse oxi. Also explains why you didn't die free-diving. Even with no training you can go much longer than that before risking a blackout (at least under normal circumstances). In fact training free-diving is all about CO2 tolerance and relaxation, so you won't even be able to store more O2. When I was reaching the 80s, I was doing 3+ minute breath holds. The first ~2 minutes my oxi stayed at full O2 sat with basically no change. In principle you should be able to induce a blackout yourself using this method without feeling an urgent need to breathe - if you do the prep stage hard enough. Just make sure you only do this when lying down in a safe position.