Decompression controversies

[kensuf]

Just because you had a negative test for a PFO does not necessarily mean you don't have one.

I was tested for a PFO and tested negative. I was tested again 15 years later after getting bent a number of times over a 3 year period, and tested positive. I was then tested a third time and tested negative. The surgeon confirmed the PFO when he closed it with an amplatzer device, and I haven't had any problems since.

Both negative tests were a trans-esophagael echocardiogram (TEE). The positive was a trans-thoracic echocardiogram (TTE). The TEE was supposed to be the gold standard, but I know of at least one other person that used to get bent a lot and now has a bionic heart that also tested negative on a TEE. The best my surgeon figured was that the prophyl I was on for the TEE skewed my ability to do a deep valsvalva while they had the probe jammed down my throat.

 

[Tassi Devil Diver]

One of the things that always concerned me about bubble models like VPM was the lack of human testing, when compared with the extensive testing behind the Buhlmann model. This was the primary reason why I never adopted the use of such models and I have always avoided computers and planning software that uses these bubble models.

I found it bemusing that training agencies and dive computer manufacturers were so keen to adopt deep stops despite the lack of evidence of their effectiveness.

It's interesting to note the changing narrative concerning bubble models like VPM. In the early days the primary selling point of VPM was that total decompression time could be reduced by utilising their deep stops. Now the promoters of VPM have back peddled on this over the years, and slowly changed the narrative behind the models to the point they now deny this was ever the case.

Great to see that particularly through the efforts of Dr Simon Mitchell that actual situation regarding bubble models is now being revealed. The time and effort put in to this by Dr Mitchel is to be applauded.

 

[Freewillow]

Well said, Tassi devil Diver. Deep stops, when diving without Helium, does not make a lot of sense for most french speaking divers.

 

[LiteHedded]

One of the things that always concerned me about bubble models like VPM was the lack of human testing, when compared with the extensive testing behind the Buhlmann model. This was the primary reason why I never adopted the use of such models and I have always avoided computers and planning software that uses these bubble models.

I found it bemusing that training agencies and dive computer manufacturers were so keen to adopt deep stops despite the lack of evidence of their effectiveness.

It's interesting to note the changing narrative concerning bubble models like VPM. In the early days the primary selling point of VPM was that total decompression time could be reduced by utilising their deep stops. Now the promoters of VPM have back peddled on this over the years, and slowly changed the narrative behind the models to the point they now deny this was ever the case.

Great to see that particularly through the efforts of Dr Simon Mitchell that actual situation regarding bubble models is now being revealed. The time and effort put in to this by Dr Mitchel is to be applauded.

and dr. doolette. big thanks to him for bending all those sailors so we have some data

 

[exposure]

This is a great presentation for what I know about decompression. I am far from them. But if starting deep like VPM is not a good option (and I tend to agree with that), what's your thoughts about mimicking VPM by using GF on ZHL-16?

 

[UWSojourner]

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

I just wanted to make something clear on the heatmaps and the predictions of supersatation. All of the models (VPM, GF, NEDU profiles) were measured by the same yardstick. The colors in the map do not represent one thing in one model and another thing in the next model. Tissue compartment on gassing and off gassing was modeled using Buhlmann's 16 compartment model -- which is the tissue compartment model actually used by GF and VPM, though not the Navy.

The real difference between VPM and GF, and in the NEDU's shallow and deep stop models, is not the way in which on gassing and off gassing are modeled. What makes the difference is the theory, the prediction, about the best way to expose yourself to supersaturation. The NEDU's study put a significant question mark on the deep stop strategy of supersaturation exposure.

 

[Patoux01]

what's your thoughts about mimicking VPM by using GF on ZHL-16?

Hmmm. While the question isn't pointed at me obviously, I'd like to ask: where is GF mimicking VPM?
That's a recurring argument, but I still can't figure it out. Rossh brought it up again a month ago, showing how VPM B+3 was very similar to GF 40/70 (or was it the other way round? ) Deep Stops Increases DCS
Just posting that in case you hadn't seen that page on the thread, not sure I'm bringing anything valuable here...

 

[Storker]

what's your thoughts about mimicking VPM by using GF on ZHL-16?

I don't see any mimicking.

Both models are based on hypotheses, one with actual experiments as a fundament, the other... not so much. Now, any model has a number of adjustable parameters (in this case the most obvious ones are the "conservatism factor" in VPM and GFhi/GFlow in the Buhlmann-based models), and it's not a shock that two fundamentally different models of the same phenomenon can give very similar results with a careful choice of parameters. That doesn't mean the two models have the same predictive value outside the given dataset and with carefully chosen parameters¹.

“With four parameters I can fit an elephant, and with five I can make him wiggle his trunk”
(attributed to John von Neumann)


¹ At a certain period in our history, the geocentric and the heliocentric model of the solar system had pretty similar predictive value within the available data and given the right parameters. That didn't mean that they were equally correct.

 

[Agility]

Is there some odd genetic factor that plays a part? I believe so since some people get the niggles more than others.

Dr. Marroni from DAN Europe is doing studies in this field. There are bubblers and non-bubblers. Possibly something to do with how much nitric oxide they produce in their endothelium.
my conclusion for the time being: If you know you are a bubbler you need to dive far more conservatively than the tables (and computers) suggest.

 

[dberry]

Hmmm. While the question isn't pointed at me obviously, I'd like to ask: where is GF mimicking VPM?
That's a recurring argument, but I still can't figure it out. Rossh brought it up again a month ago, showing how VPM B+3 was very similar to GF 40/70 (or was it the other way round? ) Deep Stops Increases DCS
Just posting that in case you hadn't seen that page on the thread, not sure I'm bringing anything valuable here...

I thought the video linked by the OP did a nice job of showing how GF effectively gives profiles that look more like VPM. To save you time, just check out the video between 19:00 - 20:00 (red line = full ZHL16, light blue = VPM), then between 22:00-23:30. The second example uses a pretty extreme GF (20/90), but makes the point very clearly. Whatever your preferred GF, the effect is to give more stops at greater depths compared to "pure" ZHL-16. I suppose the real point being made in the video is "don't ignore the slow compartments" (which the flavor of VPM illustrated and extreme GFs tend to do.)