I think that was posted here last summer:
U.S. Masters Swimming Discussion Forums - View Single Post - Breath-holding exercises killed one, another critical
Nevertheless, hyperventilating prior to holding breath gives the CO2 sensing/reflex mechanism a lag time that can allow someone to pass out while under.
It's sad but I don't think potential SEAL candidates need to practice breath holding exercise to make it. Just being good swimmers would be plenty.
This sounds like a problem in advanced swimmers & yes it would be hard to tell an adult not to do this in your pool area!!
I was actually talking about this in my patho/physiology class a couple weeks ago. It's a very interesting concept. I'll try to put in it lay terms.
Basically the sensation of choking (needing to breath) is caused by the amount of CO2 present in your blood. If you hyperventilate before holding your breath, you disproportionately lower the CO2 levels from what they would be if you just breathed normally.
Making up numbers- if your resting CO2 level is 10, and your choking feeling is 15, then your hyperventilation base CO2 level is 5.
Then when you hold your breath and CO2 is released in your body into the blood, the CO2 level instead of going from 10 to 15, it goes from 5 to 15 (which takes a longer period of time). This makes you feel like you an hold your breath longer.
Hyperventilating does not really increase the amount of oxygen you take in though. So you only have the same amount of oxygen to use. In that you started off with less CO2 by hyperventilating, you trick your body into thinking you have more oxygen because of the waste product deficit.
This can happen in anyone really though. It can happen to advanced swimmers, not advanced swimmers trying to hold their breath, and especially children who have breath holding competitions.
