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Purely theoretical question about hypoxic gasses

Discussion in 'Marine Science and Physiology' started by divezonescuba, Aug 23, 2018.

  1. divezonescuba

    divezonescuba ScubaBoard Supporter ScubaBoard Supporter

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    Hello:

    Just a theoretical question, with a ballpark answer desired, biostatistics aside.

    X16 is suppose to be the minimum required to support life. About how many breaths would it take with different hypoxic mixes on open circuit on the surface before I might reasonably expect to pass out?

    This is open circuit, so you would be breathing gas with consistent fo2 rather than breathing gas with increasingly less fo2.

    The reason why I am asking this is that some open circuit trimix divers seem to think that you cannot test breathe your regulator with even mildly hypoxic mixes on the surface.
     
  2. Beau640

    Beau640 Solo Diver

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    You can absolutely test breathe a hypoxic mix. You just can't be walking around breathing it and shouldn't breathe it for more than a couple breaths. Have to be careful but literally taking two breaths off a regulator while sitting comfortably to test it will not make you pass out.
     
  3. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    Depending upon the mix, it would be a lot like breathing the air at very high altitude. With my Shearwater computer, I had to change it from the default setting to keep it from warning me that I was breathing an hypoxic mix when I set it for air at our altitude.
     
  4. Superlyte27

    Superlyte27 Cave Instructor

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    On a 10/70 mix, more than a few breaths and I start feeling woozy. On 15/50 it takes a while. I think I could probably maintain it indefinitely, but at what decreased mental capacity. If you want to pay for the gas in my bottles, I’ll do some testing. :)
     
  5. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    Remember that you are not breathing a poison. You are just not getting as much oxygen as you would like for a while. How long that "while" lasts depends upon the mix, and what Pete describes in post #4 sounds about right.
     
  6. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

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    It depends on the individual and the circumstances as to when unconsciousness would occur. I agree with the above posters in that a couple of test breaths on a mildly hypoxic mix will not be harmful.

    The 16% figure you mention comes from the idea that, during normal respiration, the oxygen percentage in exhaled air is roughly that. If you hold your breath for any length of time, the oxygen percentage in the gas that you exhale afterwards will be lower because more oxygen is taken up in the pulmonary circulation.

    Unconsciousness occurs at an inspired oxygen percentage of roughly 8-10%, which is probably why Superlyte27 feels woozy after just a few breaths on his 10% mix on the surface. An interesting thing happens when you get to mixes with oxygen partial pressures that are lower than that in the venous blood in the pulmonary capillaries (30-40 mmHg, equivalent to about 4-5% O2 in a breathing gas) though: the pressure gradient reverses and oxygen begins to leave the body rapidly. Unconsciousness occurs very quickly under these circumstances. The saturation diving community has to be careful with this; maybe some saturation rules crept into your friends' safety consciousness. @Akimbo probably has some good stories about this.

    Best regards,
    DDM
     
  7. Akimbo

    Akimbo Lift to Freedom Volunteer Staff Member ScubaBoard Supporter

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    I always wondered what the origin of the 16% limit was. Thanks.

    I don't know about how "good" the stories are but some are very illustrative. The first I learned of the virtually instant blackout that occurs when exposed to anoxic and near-anoxic environments was on the US Navy's Mark II Deep Dive System that was built to support SeaLab III. Washington engineers, in their infinite wisdom and general lack of diving expertise, routed 440 VAC inside the PTC (diving bell). Obviously this is very dangerous on many levels so they figured their butts were covered by protecting all electrical systems with a very low pressure helium purge... which leaked many times faster than expected. In practice, people would forget to turn the pure Helium supply off after dives and testing, especially when it was in the shipyards for overhauls.

    The bell would fill with pure helium, displacing the air down the bottom hatch. The stories were that more than one "yardbird" (shipyard worker) climbed into the bell and fell back out to the deck plates before they could get more than halfway into the hatch. I came onboard months after it left the yards but these stories were part of my classroom sat training. The diving medical officers didn't have an explanation why they blacked out so fast. You would think there would be enough Oxygen in the bloodstream and tissues to keep you conscious for 30 seconds to a minute.

    This phenomenon was clearly demonstrated to me during pre-dive checks that included testing every BIBS masks -- the oral-nasal masks hanging in every lock for emergency and treatment gasses. The plumbing systems in this particular chamber still contained deep mix; 1% in this case for a 0.3 PPO2 at 850'. I was going through the checklist, pressed the first BIBS mask against my face, and that is the last I remember until I woke up flat on the deckplate with a knot on the back of my head. Fortunately, the training included "never strap the mask on your face until you analyze the gas". I would be dead if the mask didn't fall of on my way down. I was too embarrassed to report it since I knew better.

    I couldn't remember any warning or even exhaling before going black. The closest explanation I have heard is the progression of symptoms relating to shallow water (hypoxic) blackout in a PFI freediving class. Basically, less critical functions are progressively shut-down as the Oxygen level diminishes. It makes sense but you have to wonder what environmental conditions would have selected these traits in human evolution. Apparently the reflex to breathe from CO2 buildup continues to function long after blackout occurs.

    I have written to various state's Attorney Generals over the years suggesting pure Nitrogen as an available, humane, and reliable means of execution. My first choice is to be shot by a jealous husband at 90, but anoxia is my distant second choice... not that I actually get to choose.
     
  8. northernone

    northernone Great White Rest in Peace

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    Enjoyed the in depth replies up thread.

    For what it's worth here is a first hand data sample. With 22 minutes on 12% I'm fine. Just feels like altitude. Pushups, situps and rest periods breathing normally. Got the video somewhere.

    I did feel mentally bad a few hours afterwards. However, I wouldn't hesitate to breath 12% to the surface in an emergency based on what I experienced.

    Cameron
     
    Akimbo likes this.
  9. Superlyte27

    Superlyte27 Cave Instructor

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    Neither would I considering the likely alternative is certain death. :)
    But I wouldn’t go planning on it either.
     

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