Shallow water blackout

[Karloss]

As a diver and snorkeler I have been informed about a shallow water blackout. However I seem to have forgotten the physiology behind it. Could someone be kind enough to refresh my memory on how and why it happens. I recently had a school friend drown in Mexico while snorkeling and I feel this may have been the culprit.

 

[Jim Lapenta]

Essentially what happens is this. You know that CO2 triggers the breathing response. Reduce the amt of CO2 and you lessen the urge to breath. When a freediver hyperventilates before a dive they saturate the tissues with O2. Then at depth due to the partial pressures increasing they have enough O2 to perhaps stay down longer, go deeper, or both before CO2 buildup says it is time to go up and breath. What happens though is that they may have used up so much of the O2 that as partial pressures decrease there is no longer enough % of O2 in the blood stream to maintain consciousness. As they get closer to the surface this becomes more pronounced and they simply blackout and drown unless someone gets to them. The reason it is called shallow water blackout is because the drop in PPO2 levels is so great in that first atmosphere and especially the last half of that. Many incidents I've read of had the blackout happening between 10 and 20 feet.

 

[dumpsterDiver]

Actually I think most BO occur at the surface and often after the diver has taken a breath.

 

[Akimbo]

Lots of thoughtfully explained info:
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