Causes of shallow water black out

[Spearo]

The following is a real blackout video while hunting. Depth aprox 10 mt, dive times 1:30-1:40, at the end of the video you will see the diver giving a last breath out.
The video is not embedable but you can watch it directly on vimeo
http://vimeo.com/7575835

[rachidzock]

Well that's indeed scary if it was only 10 meters and he blacked out!!...
I always rest at the surface for double the dive time before any other dive, this is enough, isn't it?

[Spearo]

I dont think that we can make such a definition. I mean double the dive time. Personally I prefer feeling completely rested but sometimes its really difficult to do thinking if there are luts of fish around

[rachidzock]

Well you're absolutely right, sometimes double the time isn't enough.. But for the moment I'm taking it as the Minimum resting time even for when I'm feeling relaxed and there are lots of fishes

btw, I've read online about a freediving trainer who measured his own limits (after how long a blackout or samba occurs/contraction start/ levels of O2 and CO2 reaching the normal after how much time resting..etc)
and it seems he reached a pattern that helps him know his limits and the symptoms that indicate approaching to those limits in a way preventing blackout or samba.. I wonder if you or other freedivers/hunters tried to check their limits or to study their body's behavior thoroughly, cause personally I never pushed it to the max so, would you recommend trying this? and what is the best way you recommend to try this?

[mohamed]

well Rachid every person has different limits than the other the human body is build to give alarms before any bad things happen the most important thing in freediving is not to use hyper ventilation in order not to give your brain fake feeling and to listen carefully to your body. for example calves cramp mean the body is low on O2 you need to relax more on the surface...

[Spearo]

There is a nice website dedicated to promote freedive safety education. http://www.divewise.org take look at it.
The owner of the website is a mother Julie Richardson,
On April 19, 2008 Walter and Julie Richardson received a call that their two oldest sons had been involved in a dive accident and were in trouble. Both boys had blacked out on the ascent following a 90 ft freedive. David, 16, blacked out 30 ft below the surface. Robert, 20, dove back down to rescue his brother without time to get another breath, and blacked out after surfacing with David. The motionless pair, floating face down in the water, caught the attention of their three young buddies, who launched a rescue. After successfully reviving the brothers, they contacted the Coast Guard, and the pair was airlifted to hospital where they later made a full recovery.

Nearly losing two sons at once to freediver blackout compelled Julie to become a voice for change in freedive safety. She poured over articles about shallow water blackout and read countless stories about men and women who had succumbed to this silent and insidious killer. She talked to the top experts in the field, many of whom shared her concern and pledged their support in her effort to promote freedive safety education. Julie didn't want her sons to stop freediving, she just wanted them to adopt the best safety protocols to make them safer divers. And she wanted this for not only her sons, but also for all freedivers everywhere.