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Shallow water blackout and the fainting game

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Apr 19, 2015
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Shallow water blackout scares the poop out of me, it´s this whole „without warning“ thing that does it, you know, one moment you are happy as Larry, high fiving Octopusses, annoying scuba divers then next second sinking ass first to Davy Jones locker.

Now, can anybody remember doing the fainting game at school? The one where you take a deep breath or two and then somebody squeezes your chest and five minutes later you wake up on the floor of the classroom with a colossal boner to much embarrassment and giggles?

What are the thoughts that the same mechanisms behind the fainting game are the ones behind shallow water blackout.

My theory is that after taking a deep breath your chest is squeezed by pressure during the dive then just under the surface on the way back the pressure is relaxed and you black out.

Perhaps, in my humble opinion, if you don´t pack your lungs to the max and leave a bit of „slack“ you could be in less danger of blacking out.
 
The one where you take a deep breath or two and then somebody squeezes your chest and five minutes later you wake up on the floor of the classroom with a colossal boner to much embarrassment and giggles?

I see some troubles coming up with the hydrodynamics during DYN or CWT [emoji38]
 
On a more serious note, the blackout is likely due to the squeezing of the carotid artery, which can also occur when heavily packing.
Problem is, the risk to reduce hypoxic blackout can't be minimize by avoiding that. And this one is the nasty, unpredictable one...
 
Those abbreviations describe two different disciplines in freediving. DYN is short for dynamic, which is essentially diving distances in the pool (or open water), using fins. CWT is short for constant weight, which the freediving discipline of diving depth with fins, and not using any kind of sled or weights which can be dropped, or pulling yourself up and down the diving line.
 
SWB is caused by the partial pressure of oxygen (ppO2) dropping below the level needed to keep you conscious as you surface.

When you dive your lungs are compressed and ppO2 rises. Over the course of your dive you consume O2 but the ppO2 is sufficient to keep you conscious as your lungs are compressed. Then, as you swim back to the surface, your lungs expand and ppO2 drops (as you have been consuming O2).

If the ppO2 drops too low then you black out. This is why it is called SWB as it happens near the surface. I have just heard some refer to it as an ascent blackout too.


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Precisely, the same thing happens with the fainting game, you hyperventilate, then hold your breath for a moment or two then somebody compresses your chest and releases it suddenly, down you go.
 
Nah, I'm pretty sure that's due to a reduced blood flow to the brain. Hyperventilation and the resulting low CO2 levels cause already a constriction of the carotid arteries. This gets worse when the chest is compressed after a full inhale. Thus, the blood flow gets too low, the oxygen levels in the brain drop fast and you pass out although there's still enough oxygen in the rest of the body . Fainting due to "systemic" hypoxia takes longer, and the shallow water blackout illustrates pretty well how partial pressure of oxygen in the lungs and blood are connected and how they can cause a sudden blackout.
 
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