• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

Perfusion vs Diffusion and Slower Compartment Questions?

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

  1. gcbryan

    gcbryan One Bad Hombre

    # of Dives: 1,000 - 2,499
    Location: Seattle
    16,084
    10,073
    113
    Based on another thread and thinking about this subject again I have a couple of questions for anyone who can shed some light!

    Haldane/Buhlmann dissolved gas theory is considered to be perfusion limited in that one tissue may be saturated and another nearby tissue may not be but unless that nearby tissue is able to help out via perfusion no gas exchange will occur (according to the theory anyway).

    1) Are any theories in mainstream use other than perfusion limited models? I think RGBM and VPM are still perfusion limited aren't they?

    2) I think I've read explanations of why Buhlmann added the last several slower compartments but I can't recall what they were now. I'm especially talking about half-lives of 305, 390,498, and 635 minutes. I think I read something along the lines of they were added just to make the calculations come out but that nothing in the human body really has half-lives that slow. Can anyone elaborate or point me in the right direction here.

    Thanks!
     
  2. huwporter

    huwporter DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Sydney, Australia.
    211
    178
    43
    Not quite - in Haldane/Buhlmann, all compartments are considered independently exposed directly to ambient pressure, so tension in any one compartment doesn't affect any other.

    DCIEM uses serial compartments (so compartment 2 is only exposed to compartments 1 and 3, not ambient etc). RNPL (and possibly BSAC 88) uses diffusion. I believe VPM uses a Haldanean model to track compartment tensions, and bubble theory to calculate ascent profiles. (RGBM, not sure about).

    There is some relevant discussion here:
    http://www.scubaboard.com/forums/as...786-last-body-part-reach-full-saturation.html

    Cheers,
    Huw
     
  3. gcbryan

    gcbryan One Bad Hombre

    # of Dives: 1,000 - 2,499
    Location: Seattle
    16,084
    10,073
    113
    Thanks Huw! Good link.

    Regarding the first part I guess what I was really asking wasn't answerable within the context of a diffused model. I've briefly read about the other models you mentioned.

    I guess what I was asking is whether a perfusion limited model is that far off base even by being perfusion limited. To the extent that anyone knows, how much off gassing takes place outside of the constraints of a perfusion limited model?

    In other words is this really that unrealistic or does it take account for most of the off gassing anyway (in spite of the limitations)?
     
    Last edited: Apr 15, 2011

Share This Page