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Thread: Effect of in-water oxygen pre-breathing at different depth on decompression-induced b

  1. #31
    RBW Member Paua is an unknown quantity at this point Paua's Avatar
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    Cool Re: Effect of in-water oxygen pre-breathing at different depth on decompression-induc

    Hi,

    Actually cabin presure in flight was, in the early days, only about 0.3bar for the space capsules, but was pure O2.

    At launch however this was boosted up to about 1.2 bar to ensure no contamination from outside air (slight positive pressure inside) and provide a check for leaks.

    This creates a very high fire risk and was the cause of the Apollo 1 fire that killed Gus Grissom, Ed White, and Roger Chaffee on the launch pad.

    Later Apollo craft were modified to allow a 1.2 bar air atmosphere at launch that was gradually replaced by pure O2 after launch, pressure also being gradually reduced to 0.3 bar while in orbit.

    Tests done in Skylab in the 1970's showed the significantly lower fire risk in a atmosphere with diluent gas so on later space flights I believe air at 1 bar was used, but slightly higher pressure during launch for reasons mentioned above.

    Space suits however have pure O2 at 0.3 bar thus the requirement for astronauts to "decompress" into them.

    Cheers,

    Paua

  2. #32
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    Re: Effect of in-water oxygen pre-breathing at different depth on decompression-induc

    Quote Originally Posted by Simon Mitchell  View Original Post
    Hello,

    This is an interesting line of research.

    Also, the magnitude of any "benefit" is not clear from this study. The correlation between numbers of venous gas emboli and risk of DCS is not as precise as it might intuitively seem. Thus, the strategy cannot be considered as provent to reduce risk of DCS. It may also increase the risk of oxygen toxicity on long dives with high oxygen exposure. Interesting space to watch though.

    Simon M
    Simon, although its clear that there is no direct correlation between "bubble score" and DCS but I have always considered that "few or no bubbles is good" and "many bubbles" can be bad" .....
    (please correct me if this wrong)

    Most of the studies published (like hydration before the dive, prebreathing O2, exercise before dive ) always mention their concussion in terms of "bubbles measured"....

  3. #33
    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: Effect of in-water oxygen pre-breathing at different depth on decompression-induc

    Quote Originally Posted by gtzavelas  View Original Post
    Simon, although its clear that there is no direct correlation between "bubble score" and DCS but I have always considered that "few or no bubbles is good" and "many bubbles" can be bad" .....
    (please correct me if this wrong)

    Most of the studies published (like hydration before the dive, prebreathing O2, exercise before dive ) always mention their concussion in terms of "bubbles measured"....
    Hello,

    You are exactly right. Venous bubble counts are frequently used as an outcome measure in decompression experiments because they are convenient, relatively easy to obtain, and an ethically acceptable end point. The caveat is that the end point is soft, and that a study that shows a reduction in venous bubble numbers resulting from some strategy cannot be interpreted as demonstrating a decrease in DCS as a result of that strategy. A reduction in bubbles (probably) can only be good as you imply, but it still does not prove a reduction in clinical DCS.

    Studies using actual DCS as an end point need to be much larger and are ethically more troublesome... which is why they are as rare as rocking horse sh1t. Indeed, the only ones in recent times were the seminal NEDU studies on temperature effects and bubble vs gas content models.

    Hope this makes sense.

    Simon M

  4. #34
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    Re: Effect of in-water oxygen pre-breathing at different depth on decompression-induc

    [quote=jaksel;310783]Really? You don't dive in cold waters very often, do you?

    Depends on your definition of cold. 10C in a 3mm suit is cold, 4C in a dry suit is also cold, for me

  5. #35
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    Re: Effect of in-water oxygen pre-breathing at different depth on decompression-induc

    Quote Originally Posted by Simon Mitchell  View Original Post
    Hello,

    You are exactly right. Venous bubble counts are frequently used as an outcome measure in decompression experiments because they are convenient, relatively easy to obtain, and an ethically acceptable end point. The caveat is that the end point is soft, and that a study that shows a reduction in venous bubble numbers resulting from some strategy cannot be interpreted as demonstrating a decrease in DCS as a result of that strategy. A reduction in bubbles (probably) can only be good as you imply, but it still does not prove a reduction in clinical DCS.

    Studies using actual DCS as an end point need to be much larger and are ethically more troublesome... which is why they are as rare as rocking horse sh1t. Indeed, the only ones in recent times were the seminal NEDU studies on temperature effects and bubble vs gas content models.

    Hope this makes sense.

    Simon M
    Hi Simon,

    In the deep stop conference report, I read that they all mentioned there was little correlation between bubble score and injury. I'm not surprised. To me it seems like a difficult way to be measuring for injury or success. The logic of the problem suggests they are going about this backwards. Some reasons I say this;

    All deco apparently makes bubbling, and aggressive deco apparently makes more bubbling over a shorter time. The bubbles that are heard, is gas that has exited the tissues already and about to enter the lung. I would think that this gas no longer posses a risk to the healthy (non PFO) person. The bubbling heard would seem to be just "deco in progress", and not a sign of injury.

    Contrary to expectations - I would think an injury would make less bubbling, because a portion of the gas is instead trapped in the extremities, and is no longer being transported to the heart (where they are listening). With an injury in progress, the bubble score would go down??

    Some of the studies try to suggest the aggressive profile makes more off gassing - but that's not possible. All test profiles have an identical volume of gas to remove during decompression. The only questions are: how much time is taken to do complete decompression (including surface phase), is it done uniformly across the ascent, and the ratio of dissolved to micro bubble form it takes?
    I would expect that the conservative profile will spread out the same volume of bubbling over a longer time frame (giving an artificial lower bubble score). The shallow stop profile will have an initial high supersaturation and off gas rate in the water, and posibly a high initial bubbling, which has not been considered or measured (lower bubble score towards end).

    I would think that for a bubble score to be useful, one would need to calibrate it to a known or expected bubbling for every profile, time and ascent type. Then to compare that baseline value to a measured bubbling for every dive, and then look for a deficiency in off gas activity.

    RossH

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