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Updating Diffused Gas Models?

Discussion in 'Ask Dr. Decompression' started by gcbryan, Apr 15, 2011.

  1. gcbryan

    gcbryan One Bad Hombre

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    What have we learned since the diffused gas models were first presented to us?

    We know that bubble models add bubble dynamics to the discussion. Exercise and hydration has been discussed along with things that might cause on gassing and off gassing to not be at the same rate (cold dive and hot shower or active dive and a nap afterward).

    What else? Looking at some of Dr. Deco's older posts I see that we need to consider that some of the longer half-lives may not contribute to DCS even though they may or may not exist (half-lives that long that is). Some parts of our body may off gas slowly but not be a contributor to DCS and so would not be relevant for our purposes (I may not be drawing the correct conclusions of course).

    Perfusion limited models. Although limited does that substantially effect the way we think the body actually works?

    What have I missed, gotten wrong, and what can anyone else add to this interesting subject?
     
  2. Dr Deco

    Dr Deco Medical Moderator Staff Member

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    Hello gcbryan:

    Halftimes and “Tissues”

    As I have mentioned in the past, and most readers know by now, tissues and compartments are not the same. Tissues can be identified, dissected, and removed from the body intact, while compartments cannot. They are mathematical constructs for making an algorithm to generate dive tables. One must remember that Haldane originally had an anatomical model in mind when he generated his dive tables [1908]. The idea continued for decades, but scientists began to discard the concept in favor of “compartments.” These compartments are good for designing tables but are not good for analyzing physiological effects.

    In order to generate tables for saturation decompression, it is necessary to utilize halftimes of about 600 minutes. These are not compatible directly with physiology as they would not supply real tissues with oxygen or remove metabolic waste products. I suspect that they represent a gas phase [bubble] that is present in [connective] tissue. Since a bubble is in equilibrium with the surrounding tissue fluid, it is eliminated very slowly. Basically, the driving force is the small Laplace pressure from the surface tension on the surface of the bubble. This elimination is slow, but the expansion of this bubble [as the pressure is lowered] would produce pain in connective tissues in knees, etc.

    An Analogy: Perfusion and Diffusion

    Most tissues of the body are loaded and unloaded with inert gas by means of perfusion processes. Surprisingly, only the very fastest of tissues have diffusion as limiting process. That is because the blood flow brings in so many nitrogen molecules that they are in abundance and diffusion limits its movement into the tissue. As an analogy, it is as if several large rail cars brought boxes to be unloaded to a warehouse. The workers on the platform are overwhelmed and the goods start to pile up on the platform. The workers represent the movement of boxes [molecules] and movement into the warehouse is limited – but not by the train [blood flow].

    In our analogy, at the other end of the spectrum, we have one train per day, and it has just one boxcar of boxes. The workers stand around most of the time and have no difficulty putting the boxes in the warehouse. The train [blood flow] is the limiting process. Thus, diffusion is not generally limiting. Diffusion from one tissue to an adjacent tissue is a possibility and would be covered, in part, by the different halftimes.

    Micronuclei

    If we have micronuclei in our tissues and they are enlarged by muscle movement, they could develop into larger bubbles and then grow during the decompression phase of the dive. Not only would these nuclei grow but they also change the gas exchange kinetics. Dissolved nitrogen diffuses into them, becomes gaseous nitrogen, and it is not eliminated because a bubble is in equilibrium with the tissue fluid. Their nitrogen partial pressures are almost identical [the bubble has a bit more because of Laplace pressure from surface tension].

    Dr Deco :doctor:
     
    lamont and Duke Dive Medicine like this.
  3. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

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    Wow. Well said.
     
  4. lamont

    lamont Photographer

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    Has anyone done the work to add an inner ear compartment to buhlmann models to reduce the risk of inner ear hits on trimix?
     
  5. Dr Deco

    Dr Deco Medical Moderator Staff Member

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    As far as I know, the Buhlmann model ascribes definite tissues to each of the halftimes.

    I personally do not believe this. He has heard my side, and he stuck to his side.
     
  6. gcbryan

    gcbryan One Bad Hombre

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    That's what I finally realized. It took a while :)

    I'm learning to look at decompression algorithms for what they do and for an explanation of the physiology I'll read the medical literature and not mix the two :)
     
  7. Mr Carcharodon

    Mr Carcharodon Solo Diver

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    "If we have micronuclei in our tissues and they are enlarged by muscle movement"

    How does muscle movement enlarge a bubble? I have always thought it just aligns polar molecules allowing a small bubble to form, but that growth was from diffusion. Am I missing part of the puzzle?
     
  8. Charlie99

    Charlie99 Solo Diver

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    Google "tribonucleation"
    Typical google results: Production of gas bubbles in fluids by tribonucleation

    While this might not be exactly enlarging existing bubbles, it does cause the formation of additional bubbles, which I assume could later combine, and as Dr Deco said, once the bubbles form, during the decompression phase (when dissolved gas pressure exceeds the pressure inside the bubble, which is just slightly ambient presssure) nitrogen will diffuse into them.
     
  9. Dr Deco

    Dr Deco Medical Moderator Staff Member

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    Hello Charlie99:

    Tribonucleation?

    Actually this mechanism does enlarge existing bubbles, but ones that are micro in size. That is probably much less than a micron. They would be generated by heat energy present in living tissue. Micro voids are present in fluids and these can be enlarged by pressure reductions in a small volume. Tribonucleation means “nucleation by rubbing.”

    Surfaces in the body, such as the walls of the capillaries, can touch with muscle contractions and then rapidly separate when the muscle is relaxed. This mechanism in capillaries was first postulated more than sixty years ago.

    The technical name for the process is “viscous adhesion” and refers to surfaces remaining in contact when in touch with a fluid [liquid or gas] in the space between them. Moist fingers can pick up a smooth surface by this mechanism. When calculating the pressure reduction with a fluid between, the process is referred to as “Stephan adhesion.”

    Dr Deco :doctor:
     
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