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NEDU Study

Discussion in 'Technical Diving Specialties' started by klausi, Feb 21, 2017.

  1. klausi

    klausi Barracuda

    # of Dives: 2,500 - 4,999
    Location: Dumaguete, Philippines
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    I wrote a short review about the thoroughly discussed NEDU study on deep stops. In brief, I think that the type of diving tested during this study (heavy exertion at depth, divers being cold during deco) is not very representative of the tech diving me and my dive buddies do in the Philippines (calm diving at depth, always pretty warm). I wouldn't change my deco schedules based on this study, even though it is interesting and well done.

    I also rant a bit about models in diving and science in general.

    Yes, I know the discussion is 3 years old :) I still hope it's interesting to some of you what I wrote.
     
  2. PfcAJ

    PfcAJ Orca

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
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    You should read what Simon Mitchell has to say about the topic and its applicability to technical diving.

    There's a lot of "discussion" on rebreather world and the deco stop forums.
     
    shoredivr and northernone like this.
  3. Kevrumbo

    Kevrumbo Solo Diver

    # of Dives: 1,000 - 2,499
    Location: South Santa Monica Bay/Los Angeles California, USA
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    In summary, the implications of the NEDU Deep Stops Study shows that deep stop strategy may now in fact supersaturate the Slow Tissues later in the deco profile along with an increased risk of DCS upon surfacing. In other words, the Slow Tissues are still on-gassing while you are decompressing the Fast Tissues with Deep Stops at 75% and 50% max depth per the Ratio Deco method for example. The real practical and takeaway point of the NEDU Study is not that the deco profiles tested experimentally in the study's paradigm are non-representative of "real world" deco profiles as performed by sport technical divers, but that the same disadvantageous pattern of Slow Tissue Supersaturation is inherent to all bubble model deco algorithms which prescribe deep stop profiles.

    To compensate, and still choosing to use a Dual Phase/Bubble Model Deco Profile with DeepStops like VPM, RGBM, or Ratio Deco method, you might have to extend the shallower stop times on Oxygen to effectively offgas & clear the Slow Tissues' surfacing supersaturation along with any potential pathological DCS causing micro or proto-bubble formation.

    At present, exactly how much to extend this O2 stop time is arbitrary. For practical consideration, take into account the FN2 of the bottom mix and how many consecutive days of decompression diving you plan to do along with the slow tissue loading of residual inert gas over that time (the tissue loading bar graph of the Petrel/Perdix Computer shows this nicely on surfacing post-dive): So for example using Nitrox50 and 100% Oxygen for deco gases, doing three consecutive days of two open circuit dives per day with a 3hr SIT on the deep wrecks in tropical 28 deg C warm water Truk Lagoon with bottom times & depth ranges of 40 to 60 minutes at 36 to 60 meters breathing the maximum FN2 of Deep Air, I would additionally extend my O2 profile by another 50% to 100% of the original oxygen stop times as calculated by the Ratio Deco method. On the fourth day, I would now a take day-off from diving to further off-gas slow tissues, reset CNS/O2 exposure and get relief for my lungs from any pulmonary inflammation effects due to prolonged high ppO2 breathing.

    Lastly here's my suggested gas content/non-bubble model technique, for those wanting to de-emphasize DeepStops while still utilizing the conservatism of arbitrary extended O2 stops: By using & applying Buhlmann GF's for instance, if I were doing two OC deco dives per day for three or four consecutive days-in-a-row on Air or Trimix on a month long Indo-Pacific/Micronesia tech wreck expedition, then I would program my Petrel Computer from Day 1 thru 4 something like 40/80 for Day 1; 40/70 for Day 2; 40/65 for Day 3 and finally 40/60 for Day 4. On Day 5 take a break from diving, and then continue the same GF's for three or four consecutive dive days with one day-off schedule/pattern.
     
    Last edited: Feb 22, 2017
    leebutler and northernone like this.
  4. klausi

    klausi Barracuda

    # of Dives: 2,500 - 4,999
    Location: Dumaguete, Philippines
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    See, I argue in my blog post linked above that this point is separate from their experimental study. In the theoretical part they compare profiles to a model. And we don't know how much that model corresponds to the reality of bubble formation in tissue. It's not a conclusion from their experiments at all.
     
  5. Kevrumbo

    Kevrumbo Solo Diver

    # of Dives: 1,000 - 2,499
    Location: South Santa Monica Bay/Los Angeles California, USA
    5,389
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    My point again in terms of applicability and practicality to take from the NEDU Study is: you have to determine how much deco stress your Slow Tissues are willing to take if you want to do multiple decompression dives per day consecutively for a week or more, as in Expedition Technical Diving, for whatever particular deco model algorithm you decide to use. If you're feeling very tired by the third consecutive day with a few latent indistinct musculoskeletal "transitory aches & pains" in your limbs/joints, then you have sub-clinical DCS -you're not effectively doing enough shallow stop O2 decompression time to washout out residual inert gas in the venous blood & slow tissues. (Take a day off to further reduce deco stress, and try diving again the day after).

    The real essence of the NEDU Study and the vital parameters to consider are the bottom time and depth of fast & slow tissues' on-gassing exposure, and effective final decompression relief of surfacing Slow Tissue supersaturation to prevent pathological DCS bubble formation. The Fast Tissues are a lot more robust & tolerant than we once thought (because of their greater blood vessel vasculature & perfusion?), and therefore prescribed deep stops to protect them from inert gas supersaturation are not as critical as preventing further Slow Tissues on-gassing and later supersaturation upon surfacing.

    Some more Simon Mitchell snippets (sifting through the Deep Stops Increases DCS thread again):

    ...I am saying (based largely on the results of the NEDU study) that transient high / peak supersaturation in fast tissues does not seem to matter as much as we thought it might, and therefore that protecting fast tissues from supersaturation early in the ascent by using deep stops does not seem as effective as assumed by bubble models. This is especially so when it comes at the cost of increased supersaturation (both in terms of peak levels and duration) in slower tissues later in the ascent. The NEDU study is telling us that this is where the problems seem to come from.

    @Kevrumbo asks:
    . . .Explain from a physiological basis why allowing supersaturation of Fast Tissues in this instance is less risky to those tissues and results in a lower overall incidence of DCS in the shallow stops trials of the NEDU Study. . . ?

    Hello Kev,

    The bubble models and the deep stop approach were originally promoted on the basis that they were more successful at controlling bubble formation. The attempts to evaluate this notion in decompression dives in humans that I am aware of have shown that gas content models (or decompression procedures that have backed off deep stops to some extent) actually produce less bubbles when measured after surfacing. Neal Pollock presented some fascinating work they have been doing at the inner space event at a NOAA / AAUS rebreather diving forum I attended last week. Hopefully this will find its way into the literature at some point soon. In any event, the more we investigate it, the more the "control bubbles by deep stopping" concept appears to need reconsideration. What this is suggesting is that the bubbles are coming from the slower tissues that absorb more inert gas during the deep stops. It also implies that the faster tissues that deep stops attempt to protect from supersaturation are less prone to bubble formation when they become supersaturated. You are seeking a physiological explanation for this, and while I can't be definitive, I would suggest that it makes sense that a tissue washing inert gas out quickly might be less prone to bubble formation and growth than a tissue with slower inert gas kinetics where the supersaturation persists for longer (there's that time integral again).

    Simon M

    Deep Stops Increases DCS
     
    Last edited: Feb 22, 2017
    klausi likes this.
  6. stiebs

    stiebs Manta Ray

    # of Dives: 200 - 499
    Location: Melbourne, Australia
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    This exact argument has been done to death on both rebreatherworld and thedecostop forums. A short search on either of those forums will bring up over 50 pages of discussion including input from some of the world's most respected decompression scientists
     
    shoredivr and rjack321 like this.
  7. The Chairman

    The Chairman Chairman of the Board Staff Member

    # of Dives: I just don't log dives
    Location: Cave Country!
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    Well, it's time to have that discussion here. Not everyone feels welcome over there.
     
  8. tbone1004

    tbone1004 Tech Instructor

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
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    by feels welcome, you mean the list of very well known and respected divers that have been unceremoniously banned for no just cause other than having a disagreement with the owner?
    RBW and TDS have basically been demoted to classifieds...

    definitely time to have the discussion here
     
    The Chairman likes this.
  9. The Chairman

    The Chairman Chairman of the Board Staff Member

    # of Dives: I just don't log dives
    Location: Cave Country!
    48,988
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    Yeah, that too. :D Welcome to the United Nations of ScubaBoard. Disagreeing with the owner here is encouraged and adds to the fun.
     
    northernone likes this.
  10. stiebs

    stiebs Manta Ray

    # of Dives: 200 - 499
    Location: Melbourne, Australia
    588
    33
    28
    I already feel sorry for Simon.......
     
    BenjaminF and rjack321 like this.

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