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Thread: Preventing Shallow Water Blackout

  1. #1
    Guppy
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    Preventing Shallow Water Blackout

    I have read the Wiki article on shallow water blackout (Shallow water blackout - Wikipedia, the free encyclopedia) and understand it occurs when O2 level drops below a "point of no return".

    Would it be correct to say that, if a diver does not push too hard to stay below water, namely at the first feel of needing to breathe the diver comes up, then the chance of blackout is reduced?

    I ask this because even though we are taught to dive in pairs, because of the low visibility, the diver staying on top really cannot see the diver below so he's kind of on his own anyway...

  2. #2
    Shallow water blackouts are more than ignoring the urge to inhale.

    If I hold my breathe underwater I will get an urge to inhale at some point. If I attempt to ignore this urge by body will betray me. I WILL inhale and drown. This is not a shallow water blackout.

    Here is how the urge to inhale works. Your brain detects rising CO2 levels and this makes you inhale. It just happens to be that when CO2 levels rise in your body, oxygen levels drop. Your brain does NOT detect the dropping oxygen level.

    If you hyperventilate you can lower the CO2 level in your body. It does NOT raise the oxygen level. Let's look at the following tables:

    Code:
    CO2	O2
    1	10
    2	9
    3	8
    4	7
    5	6
    6	5
    7	4
    8	3
    9	2
    10	1
    11	dead
    Let's say that as CO2 units rise, O2 levels drop at a corresponding rate. When you are at 3 units of CO2 there is no urge to inhale. At 6 units of CO2 your brain starts suggesting you inhale. At 9 units of CO2 your brain starts screaming at you to inhale. At 10 units of CO2 you inhale whether you want to or not. Now, let's say we do some hyperventilating before we hold our breathe:

    Code:
    CO2	O2
    -3	10
    -2	9
    -1	8
    0	7
    1	6
    2	5
    3	4
    4	3
    5	2
    6	1
    7	dead
    Now, at 6 units of CO2 your brain starts suggesting you inhale. But you can ignore that; it is just a suggestion. A few seconds later, you blackout. You blackout before the CO2 levels reach a point that forces your brain to inhale.
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  3. #3
    Grouper
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    Thanks for the info 888, always wandered what caused that
    Follow the bubbles...

  4. #4
    Shark
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    there is actually a little more to it...

    it has to do with the partial pressures of O2 dropping as we ascend, which essentially causes the O2 in your blood to be drawn back into the lungs, instead of vice versa (which is what normally happens)

    shallow water blackout typically happens upon ascent...

    lets say that we need 16% of O2 in the air we breathe (its an actual number, but varies a little from individual to individual)
    no matter how much we hyperventilate, we still only take in 21% O2 in a breath on the surface... now, where this gets us in to trouble is not at depth, where the O2 in our lungs is still enough to support us because of partial pressures... a real quick intro to partial pressures - essentially as we descend, the effects of whatever we breathe are increased... For instance, at 33 feet / 10 meters, although we are breathing 21% O2, it actually has a physiological effect of 42% O2 (.42 partial pressure), at 66 feet it is 63%, 99 feet is 84%, and 132 feet is 105%. that is the super simplified version... now, as we are free diving, the amount of O2 in our lungs is constantly dropping.

    because we are at depth, lets say 66 feet, even once the O2 level has dropped even to 8% in our lungs, O2 is still being absorbed into your bloodstream because the partial pressure (which is .24 / 24% at 66') is higher than 21% that your bloodstream typically saturates to... At this depth, even with that low of residual O2 in your lungs, you are actually perfectly fine, in fact, can stay a bit longer (assuming you don't have to ascend). Where the problem arises, is that on your way up, once you pass 33', you pass that magical 16% that is needed to survive, and the O2 actually starts coming out of your bloodstream and back into your lungs because of the way that gases behave under pressure... take long enough to finish your ascent, and on the way up, enough O2 has come out of your blood that your brain shuts down due to lack of O2, and you blackout...

    now, why don't we hyperventilate excessively - because that lowers the CO2 levels so much that our brain doesn't have those markers to breathe - its actually possible to have shallow water blackout without hyperventilating at all...
    -cody / on vacation from vacation...
    PADI MSDT Instructor, US Coast Guard Captain - Master Near Coastal

  5. #5
    Thanks for that mitsuguy. That makes perfect sense.
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  6. #6
    Grouper
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    Even theough I took Nitrox, I like how you explained pp02, thanks!
    Follow the bubbles...

  7. #7
    Shark
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    Quote Originally Posted by mzbdiver View Post
    Even theough I took Nitrox, I like how you explained pp02, thanks!
    no worries... I've found thats the way most people understand it... the math is a necessity too, obviously, but I give examples like that when I teach Nitrox...
    -cody / on vacation from vacation...
    PADI MSDT Instructor, US Coast Guard Captain - Master Near Coastal

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