Decompression controversies

[rongoodman]

Interesting talk, thanks for posting it. I just returned from a week of recreational diving on the Aggressor III in Belize and one of the things I watched over the week was the tissue compartment loading graph on my new Perdix. It was very reassuring to see how quickly the fast compartments fell back to normal and how slowly the slower compartments built up over the week with Nitrox and GF 40/85.

 

[dberry]

The "heat maps" are definitely a great way to illustrate the differences in the decompression models, but I want to clarify something for my own understanding. Please correct me here...

The calculated gas loadings (level of saturation) of the 16 tissue compartments shown in the heat maps are those calculated by the different models. E.g. ZHL16 model predicts compartment #14 would have x% supersaturation at a certain point in the deco profile, and the VPM-B model predicts compartment #14 would have y% at the same point. That is, unlike the NEDU (Navy) study which used incidence of DCS and the presence of veinous gas bubbles, the gas loadings in the heat maps are those predicted by the models themselves (and not based on anything resembling data.)

The problem with the spiffy colors in the heat maps is a temptation (for me, at least) to subconsciously view the maps as being similar to an MRI image - i.e. an actual measurement on an actual subject in real time.

On the one hand, the real time data would be really nice to have, but there would be problems with ethics and finding more volunteers OTOH, the fact the maps were generated using the VPM's own predictions of supersaturation of the slow tissues with the VPM profile is pretty significant. Hoisted by their own petard, as it were?

Finally, can someone enlighten me on where Buhlmann came up with his 16 tissue compartments? Based on actual measurements on different types of tissues (muscle, joints, nerve sheathes, etc.), or just varying kinetic parameters to empirically match the rates of DCS observed in divers. In other words, is there a connection between "slow compartment #14" and (e.g.) my liver or spinal cord or some other specific bit of the anatomy? (My understanding at this point is the answer is "not in the least", but my ignorance is vast.)

-Don

 

[Rhone Man]

The problem with the spiffy colors in the heat maps is a temptation (for me, at least) to subconsciously view the maps as being similar to an MRI image - i.e. an actual measurement on an actual subject in real time.

Like the old saying goes, never confuse precision with accuracy.

There are still an awful lot of unknowns that are papered over with best guesses in terms of tissue modelling.

 

[Diver0001]

Finally, can someone enlighten me on where Buhlmann came up with his 16 tissue compartments?

-Don

I doubt that it was one decision. Erik Bakker's article suggests that the ZHL-12 compartments were based on actual trials and the ZHL-16 compartments were mathematical extrapolations.

All of this was done over a period of 30 years so it's unlikely that it was just one decision.

R..

 

[JohnN]

Very informative presentation. Thank You !!

 

[victorzamora]

The "heat maps" are definitely a great way to illustrate the differences in the decompression models, but I want to clarify something for my own understanding. Please correct me here...

Don, you're definitely right that the "heat maps" are mathematical models and not actual data. The "compartments" being tracked don't necessarily correlate directly to actual tissues. The takeaway from the heat map is that a particular set of patterns correlates more highly to DCS than another....and VPM produces heat maps more like the bendier one. Coincidence or correlation, I believe, is still up for debate. I find it logical to believe there's a correlation there.

 

[Pao]

Regarding the heat maps:
It all came from that NEDU study. It was a very good study with the investigators controlling (as much as allowable) the possible variables that might affect the outcome such as thermal stress and workload. The two most significant things about this study (aside from the actual result) are 1) it actually used a true endpoint (DCS - God bless those brave volunteers!) and not a surrogate endpoint (VGE) and 2) it is the only valid empirical/real world data as of the moment. This reminds me of the beginnings of the Evidence-Based Medicine movement a while back where empirical data was challenging "common-sense" pathophysiologic models. Well it turns out that there are very good "common-sense" explanations for these effects and now the paradigm has shifted to everything needing to be study/evidence based. In this case both the empirical data and mathematical models agree - like having your cake and getting to eat it too.

An interesting point in the paper was the algorithms used: VVAL18 Thalmann for the Haldanian model and BVM for the bubble model. How do these compare to the ZHL-16 and VPM models?

 

[dberry]

I doubt that it was one decision. Erik Bakker's article suggests that the ZHL-12 compartments were based on actual trials and the ZHL-16 compartments were mathematical extrapolations.

All of this was done over a period of 30 years so it's unlikely that it was just one decision.

R..

Good point. I wasn't thinking specifically of ZHL-16 vs. ZHL-12, but rather how he modeled / measured any of it. I just finished reading one of the original Buhlmann papers from 1967 that focused on the slowest half-time compartments; I know his studies continued for decades and I'm sure they became more sophisticated, but... Yikes! The "half-times" were based on how many subjects got bent after a given decompression time. Sort of a safe / not safe determination, then a rather rough fit to a function with multiple exponential decays. I'll have to look up some of the more recent papers he published - I find the necessary methodologies both fascinating and a bit terrifying.

Definitely very hard research to conduct and even harder to make definite conclusions from, and by all accounts the models have greatly decreased the incidence of DCI/DCS. Although the studies I read were largely based on saturation diving conditions that would be of most interest to the people funding the research. I suppose the results correlate well enough to NDL and non-saturation "rec/tek" decompression (?)

Coincidence or correlation, I believe, is still up for debate. I find it logical to believe there's a correlation there.

Seems very reasonable to me, although my opinion is worth exactly $0. Will VPM proponents be weighing in shortly? The old SB "popcorn" emoji might come in handy.

 

[dberry]

I certainly agree with Pao: God bless the volunteers (for NEDU and the studies by Buhlmann, and other researchers.) In that 1967 Buhlmann paper some of the volunteers got bent 5-6 times. OK, most were immediately recompressed, and none "showed symptoms after 72 hours." Still... that's really taking one for the team.

 

[leadduck]

Good point. I wasn't thinking specifically of ZHL-16 vs. ZHL-12, but rather how he modeled / measured any of it.

I think the number of compartments and their halftimes is just a discrete approximation to a continuum; similar to sampling rate in digitized music. The experiments are done to determine the range of halftimes that need to be considered, and their allowed critical supersaturation (M-values, resp. (a,b)-values).
For the history of ZHL-12,16A/B/C,8ADT see Bühlmann decompression algorithm - Wikipedia, the free encyclopedia

The most recent one has only 8 compartments. A larger number of compartments just makes the calculation of the ceiling a little more precise, but does not change the overall shape of the ascent. Since the compartments are calculated independently, tracking 8 compartments takes about half as many arithmetic operations as 16. That means longer battery life time for the dive computer.