Is it really possible to BO without any sign in DYN? (not talking about depth)
By no sign I mean:
1. No significant discomfort / urge to breathe
2. No contractions
3. No change in mental clarity
4. No other feelings out of the ordinary
Would be curious to know the answer (if there is one) assuming both HV and no HV. Would be great if we can approach the above empirically as opposed to theoretically if possible but any thoughts are welcome.
I think certain behaviors might make it easier to black out than others in DYN. For me, one of my major signs of healthy DR vs real, honest low O2 is my vision. So, line diving with your eyes closed or lying on the bottom of anything with eyes closed is not something I would personally ever do or anything I find necessary to relax, and although I know people who claim to like doing DYN relaxed and eyes sometimes closed, this too seems difficult to pull off but might be one way to be dangerous.
When I get healthy vasoconstriction I notice a kind of narrowing (not major tunneling) to my vision and tight feeling around my eyes. On land or in a swimming pool, this will only happen when I am actually low on O2 (maybe around 82% O2 sat) or very high in CO2, but in the ocean with the benefit of cold water or depths past 20M it can happen much quicker, before I am actually hypoxic (like when I am still 97% O2 sat). Then, no matter what, at a certain point my vision actually starts to go--blackness creeps in from the edges (this is around 60% O2 sat, and the onset is sudden). At 60% I am REALLY hypoxic and vasoconstriction is so strong that my legs feel like numb blocks of lead even if they are still moving effectively, and I usually don't train to that level, and wouldn't want to swim at that level outside of a comp and wouldn't want to spear near that level under any circumstances. I have never samba'd, and the lowest I have gone is down to about 55% during a breathhold.
In any sort of dry tests I have done, I may feel contractions at various points, but by the time my blood O2 sat reaches about 80%, any I get are 'very very real'. They are constant, regular, and impossible not to notice. I can't 'feel' bradycardia, but vasoconstriction and shift to anaerobic metabolism I can definitely feel, moreso if I am swimming (ie DYN or coming up from the bottom). Dry, bradycardia really kicks in for me in the low eighties. At this point, I also feel like my mental clarity is suffering. In cold water and--I have to assume since I haven't measured my HR underwater--at depth, it can kick in much, much sooner. Bradycardia and vasoconstriction are definitely intertwined. The sensations I am getting when my blood O2 reaches low eighties are never recreational.
I feel like lung capacity would have some bearing on warning signs in general; somebody with a big lung to body mass would probably have better safety margins in the sensations department.
Like Trux mentioned, medications, drugs, hang-overs, fatigue, etc, would probably play a role in 'signs and symptoms' but besides fatigue and using amino acid sports supplements I can't speak to those variables.
