questions

[ghosch]

So, essentially it's a hypoxic event. Since the PO2 in your lungs has been reduced during the dive it was fine at depth (due to pressure) but as you reduce your depth the PO2 falls and there is a risk of hypoxia?

[freedivernd]

That sudden fall of PO2 nearer the surface can definitely add to the onset of SWB. As can a rapid, unrestricted, exhale at the surface after a deep dive. (granted, I have experienced none of this.. only what I have read from trusted authorities..sorry, I'm not one to be writing up references, but suffice it to say it comes from reliable sources and you would find the same info)
I have experienced the "full of air" feeling that increased PO2 can have on a freediver.. sitting at the bottom of the line.. telling yourself you have got to go back up. :-\
Fred

[Inspirationdiver]

I am under the impression that this IS the cause of SWB. What else is the diver blacking out from?

[Freedive WI]

When you decend the Po2 in your body goes up, just like on a scuba dive. The air in your lungs get squeezed down, and mostly enters your blood stream.

As you spend time on the bottom, say your at 50', you consume O2. At a certian point the CO2 level gets high enough to stimulate you to breath, and head for the surface.

Sometimes you get distracted, like if you just speared a BIG fish, and stay down a bit too long. When you head up to the surface the water pressure lessens, your lungs re-expand and can actually suck O2 out of your blood stream. They call this the "vacume effect". This can then cause the rapid decrease in PO2 and the Samba, or black-out, then occurs.

Maintaining that stimulus to breath is very important. That's why freedivers NEVER hyperventilate. Slow, deep, stomach (yoga) breaths are the best way to prep for a freedive. Also, making sure that your surface intervals are twice as long as your bottom times will help. Having a buddy on hand is the only real way to be safe.

I never push things underwater, even with a buddy, as the times vary greatly compared to dry static times. Lying on the couch my P.B. dry static is just under 5 minutes (4:51), but in the water I keep my times down to a minute, or a minute and a half, on deeper dives, and no more than 2 minutes on shallow dives, like when I am sprearfishing.

I have seen people Samba before, but never totally black out. A Samba is like a black out, in that you have a loss of motor-control, but you never totally black-out, as in S.W.B. Your face turns purple and your lips turn blue which is then followed by some twitching adn slumping back into the water, usually face-first.

There is something called a "deep water black-out" that can happen on deep, constant ballast, dives. That means if you are SWIMMING, no sleds or ropes, down to 250'+ and make a sudden turn you can have a problem getting the blood shifted back into your legs, as it is been pooling in your body core during the decent. There are some freedivers who have started to experiment with a "hypothermic diving system" to try and combat some of this. That means they wear a normal freedivng jacket, with attched hood, but nothing on their legs. THey then do dives in 39 degree water and have the cold water slow the blood flow of their legs down. This is still in the experiemtal stages and I have no intention of trying it at the moment!

On soem of the deep dives narcosis is really begining to be a problem. We all know how bad it is to go to 300' on just air, now imagine if you were down there working your butt off, like by trying to swim back to the surface! On some of the deep sled attempts there have been some delays, as people are narced out of their minds on a 530'+ sled dive where you have air in your system!

Hope this answers some questions.

BTW: there has been some talk of mounting a PO2 meter inside of a face mask to warn you when to ascend. Terry Maas has mentioned this, and I think that NAvy might even have played around with the idea. It would work on the basis of using a pulse-oximiter (sp?), like thosed used by anestesiologists during surgery.

Jon

[freedivernd]

Inspirationdiver,
SWB occurs when a certain state of hypoxia (anoxia ?) is reached. Some contributing factors being change in PO2, low blood pressure, fatigue, low hydration, improper breath up. If change in PO2 was the sole cause of SWB than a dive of a given duration/depth that causes a black out once would cause a black out everytime.