"re-assessing deco profiles and deep stops", plus other bits..

[matt]

Fascinating thread, good to see the old-timers offering good commentary - less good the bitchy under-current.

I'm not a theorist and don't have any religion to deep-stops; I'm using 15/85 but I'm inclined to try some other numbers going forwards (I started on 20/80 back in last century). I've never (touch-wood) been bent.

Simon - do you "fool" the schedule into thinking you came off the helium early (c.45m seem popular - but no gas switch)? This gives a big reduction on the Vision at least - I know a few who use this successfully for some very deep open-ocean dives.

Matt.

However, I am happy to share aspects of my personal story. I was like Wayne Gerth in the early 2000s (as beautifully articulated by Bruce Partridge in his post). Everyone got caught up in the theoretical attraction of deep stop approaches to decompression. I used VPM for a period of time, and then gradient factors configured to mimic the bubble model approach to decompression (GFs ~ 20-90). I did over 500 hours on rebreathers using these approaches, including quite a number of dives over 100m and suffered no problems other than two episodes of musculoskeletal "niggles". In recent years as the deep stop issue has become controversial and the evidence we have been debating has emerged, I have altered my gradient factor approach and now use 35 - 75 on a Shearwater, or settings on Kev Gurr's VGM that produce similar profiles depending on which rebreather I am wearing. These changes have not been driven by perceptions of how I feel, but I have had no problems over a further 300 - 400 hours. I don't know whether this is right, but it seems to me to be an appropriate response to the current state of the available evidence. My decompressions are quite conservative, but I am often the only physician in very remote locations!

Simon M

 

[Mark Chase]

Fascinating thread, good to see the old-timers offering good commentary - less good the bitchy under-current.

I'm not a theorist and don't have any religion to deep-stops; I'm using 15/85 but I'm inclined to try some other numbers going forwards (I started on 20/80 back in last century). I've never (touch-wood) been bent.

Simon - do you "fool" the schedule into thinking you came off the helium early (c.45m seem popular - but no gas switch)? This gives a big reduction on the Vision at least - I know a few who use this successfully for some very deep open-ocean dives.

Matt.

WHich again begs the question are we overcompensating for helium?

If not then doing this just makes the deco super agressive and your getting away with it.

Persoanly i do it on dives where the plan ends up significantly over 3 hours but i diluient switch by flushing in off board bailout.

ATB

Mark

 

[matt]

CDM-18 is just a means of comparing lots of models in one go with a single input (my understanding)

I'm not sure about that Gian. Combined Decompression Model is an 18 tissue neo-haldanian model that checks for both deco issues as well as ICD.

I don't think it's a multi-model checker - but I could be wrong.

Matt.

- good to see you back Gian!

 

[matt]

This CDM 18 is not so good. For example, that same dive in Buhlmann ZHL16B is a 54 minute dive (34 min deco), and it also fails the CDM 18 tests in the same manner. Most models do fail CDM18.

The problem is the CDM 18 design - because its a composite of several models, a plan from any one single model will not pass, including the very models CDM 18 is composed of.
.

I thought it was a tuner not a checker. That's how the author describe it to me at least.

Matt.

 

[matt]

WHich again begs the question are we overcompensating for helium?

If not then doing this just makes the deco super agressive and your getting away with it.

Persoanly i do it on dives where the plan ends up significantly over 3 hours but i diluient switch by flushing in off board bailout.

ATB

Mark

I've done the bit in bold too - but I felt like shit. I mean, 4.5hrs in the water, 32 mins at 115m and I had to sleep for 16 hours when I got out and skip the next day.

I've been holding the He since then and feeling great on similar dives (5hr, 130m) - but I'm lurking in the water up to an hour more than those keeping the He but dropping it from the computer. Maybe I'm a chicken. But I can see that the deco is over (MLV test) well before the Vison is done.

I think we're all stopping in the water far longer than needed. But I don't have any fact - just anecdotal evidence.

Matt.

 

[gianaameri]

I thought it was a tuner not a checker. That's how the author describe it to me at least.

Matt.

It is a tool to analyse dives in which you/others got bent to determine the likely point of the dive when the damage occurred and the tissues involved, and to analyse your dive plan to understand which point of the dive may put you at risk.

The above using man-tested models (human testing is no longer permitted) which gives to me a greater level of confidence than gel-based bubble theories.

I find it a very useful as a tool, and to me it tells that VPM models are trading deep stops for shallow stops (i.e. think that by adding deep stops you can shorten the shallow stops), and this on some specific dives may not be a good idea.

I do suggest you ask the author of CDM-18 for a better explanation.

He is very approachable and provides good "tech-support" for the CDM-18 to help you through it at the begining (my personal experience).

He is also extremely bright.

 

[anarchista]

I find it a very useful as a tool, and to me it tells that VPM models are trading deep stops for shallow stops (i.e. think that by adding deep stops you can shorten the shallow stops), and this on some specific dives may not be a good idea.

Deep stops add more dissolved gas in free tissue. What about the need for longer decompression increases on small depths.

FOR the SAME LEVEL of CONSERVATISM.

rc greet

 

[gianaameri]

Deep stops add more dissolved gas in free tissue. What about the need for longer decompression increases on small depths.

FOR the SAME LEVEL of CONSERVATISM.

rc greet

Gel has no brain, so it did not think of that (joke) !

 

[anarchista]

i.e. think that by adding deep stops you can shorten the shallow stops

Deep stops add more dissolved gas in free tissue. What about the need for longer decompression increases on small depths.

FOR the SAME LEVEL of CONSERVATISM.

greet rc

 

[matt]

It's no more or less accurate than any other theoretical model - it's just different. I can't get worked up about it.

Matt.

It is a tool to analyse dives in which you/others got bent to determine the likely point of the dive when the damage occurred and the tissues involved, and to analyse your dive plan to understand which point of the dive may put you at risk.

The above using man-tested models (human testing is no longer permitted) which gives to me a greater level of confidence than gel-based bubble theories.

I find it a very useful as a tool, and to me it tells that VPM models are trading deep stops for shallow stops (i.e. think that by adding deep stops you can shorten the shallow stops), and this on some specific dives may not be a good idea.

I do suggest you ask the author of CDM-18 for a better explanation.

He is very approachable and provides good "tech-support" for the CDM-18 to help you through it at the begining (my personal experience).

He is also extremely bright.

 

[gianaameri]

It's no more or less accurate than any other theoretical model - it's just different. I can't get worked up about it.

Matt.

CDM-18 is based on man-tested models which is an argument in its favor.

Gel-based models are in no way superior, but also have a place in the planning.

Neither should get anybody worked-up.

They are just useful tools, but it is an eye opener when VPM in a simple dive is shown to fail (arterial bubbles) when the other man-tested models had flagged a risk (fact).

 

[rossh]

CDM-18 is based on man-tested models which is an argument in its favor.
Gel-based models are in no way superior, but also have a place in the planning.
Neither should get anybody worked-up.
They are just useful tools, but it is an eye opener when VPM in a simple dive is shown to fail (arterial bubbles) when the other man-tested models had flagged a risk (fact).

.
If this comparison was a 1 to 1, as in model A compared to model X, then we would be looking at a valid comparison. Then Model B to model X, and so on. We would consider the differences in model design concepts, and make allowances for these.

However, the CDM18 is a composed of the sum of models A + B + C + D, compared to model X. The problem is that A + B + C + D is now being view as a "super model" that can now rule over the rest. This super model has the assumptions of all it contributors built in, but only the longest parts of each is visible through this A + B + C + D design. Missing from this design is shorter aspects of each model. This creates a longer profile bias in CDM that requires any other single model under review to comply with in order to pass. But no single model under test will comply, including the models CDM is composed of. Please keep that in mind when using CDM18.

As Matt suggests above, every model has its own concepts, and they all seem to work, so no one model is any more correct that the next. Any two models will make different profile shapes, and each might consider the other to be lacking, or over compensated is some area of the profile.
.

 

[anarchista]

Missing from this design is shorter aspects of each model.

Can you give an example physiological shortening decompression in a passage.

I can: Model Hills (short deep hyperbaric exposure), near saturation of the blood vessel was high. Some of the dissolved gas is separated into a blood vessel portion to spread the tissue.
page 33-34 "Opportunities Selection Decompression for Apparatus Diving type CRABE" Ryszard Klos 2011

greet rc

 

[Igor P]

Deep stops add more dissolved gas in free tissue. What about the need for longer decompression increases on small depths.

FOR the SAME LEVEL of CONSERVATISM.

rc greet

The problem is you can not prove you are correct. Comparing different models is not that easy. Every dodel works it's own way.
Dissolved gasses model looks only on saturation of tissues and maximum supersaturation states and on supersaturation limits dictates decompressio. Bubble model tracks bubble growth and based on bubble growth dictates decompression.

This are two complettely different models. Logics of one is not in sinchronisation with the other. So person that understand logics of one can not judge logics of the other if she doesent understand it. Whole conflict of opignions in this thread is born here in this two logics differences.

Resurchers just seek for bubbles and explain them like decompression risk.
In bubble models we diferentiate stable bubbles and unstable bubbles, stable that do not grow and are not harmful and unstable that grow and are harmful So part of those bubbles resurchers see for us do not mean risk and part of them do mean risk.

 

[gianaameri]

.
If this comparison was a 1 to 1, as in model A compared to model X, then we would be looking at a valid comparison. Then Model B to model X, and so on. We would consider the differences in model design concepts, and make allowances for these.

However, the CDM18 is a composed of the sum of models A + B + C + D, compared to model X. The problem is that A + B + C + D is now being view as a "super model" that can now rule over the rest. This super model has the assumptions of all it contributors built in, but only the longest parts of each is visible through this A + B + C + D design. Missing from this design is shorter aspects of each model. This creates a longer profile bias in CDM that requires any other single model under review to comply with in order to pass. But no single model under test will comply, including the models CDM is composed of. Please keep that in mind when using CDM18.

As Matt suggests above, every model has its own concepts, and they all seem to work, so no one model is any more correct that the next. Any two models will make different profile shapes, and each might consider the other to be lacking, or over compensated is some area of the profile.
.

The 3 VPM profiles in the study have been shown NOT to work insofar they generated arterial bubbles and arterial bubbles have been shown to be associated with neurological damage and cutaneous DCS (if I read correctly the studies I linked).

As to CDM-18, it is NOT the "sum" of severalmodels. It is the "best of" several model where "best" is defined as the safest output of the combined models (ask author for more accurate interpretatuon).

As planning tools VPM and CDM-18 in combination, rather than in isolation, lead to a more reasoned and researched decision and risk assessment of the dive profile.

Logic and math suggest there is great value in Vplanner and CDM-18 used in combination to reduce risk.

I would have never dived the VPM profile in the study no matter how many ethical committees would have said it was fine to do so.

 

[rossh]

The 3 VPM profiles in the study have...

Here is a question for you to consider Gian.

What would you expect to see in a profile that was at the boundary of fast / no injury. That is - the standard model calibration settings. i.e. the gray area of no DCS, but also not the safest. The no margin area?

Do not confuse this question with "safe", conservative or relaxed profiles that are used everyday.

Foot note, we have not seen what it looks like inside other models at the same calibration position yet.

 

[gianaameri]

Here is a question for you to consider Gian.

What would you expect to see in a profile that was at the boundary of fast / no injury. That is - the standard model calibration settings. i.e. the gray area of no DCS, but also not the safest. The no margin area?

Do not confuse this question with "safe", conservative or relaxed profiles that are used everyday.

Foot note, we have not seen what it looks like inside other models at the same calibration position yet.

Your claim to "no injury" is at best wishful thinking.

You have no evidence to support your statement that a profile producing arterial bubbles results in "no injury."

The scientific research I linked and the book by John Lippmann more generally point exactly to the opposite.

Any suggestion that a profile generating arterial bubbles should be acceptable in recreational diving has no basis in good reason and good science to date.

BUT if people want to dive such profile "on faith" and "hope" it is for them to make that choice as part of their dive plan.

However, I suspect it would be irresponsible if hyperbaric specialist and training agencies would advocate such unreasonable behavior based on current knowledge and peer-reviewed scientific research.

Maybe VPM 0 is not as conservative as we once thought, and should come with a big and clear warning and explanation, but it does not mean VPM is bad as a planning tool.

It has its logic, basis, and purpose as an algorithm, but the study fails VPM 0 under the tested conditions.

There is nothing to support the allegation or suggestion that VPM 0 is validated or safe, under the tested conditions (although the divers did not report adverse symptoms after the dive).

 

[anarchista]

The problem is you can not prove you are correct. Comparing different models is not that easy. Every dodel works it's own way.
Dissolved gasses model looks only on saturation of tissues and maximum supersaturation states and on supersaturation limits dictates decompressio. Bubble model tracks bubble growth and based on bubble growth dictates decompression.

This are two complettely different models. Logics of one is not in sinchronisation with the other. So person that understand logics of one can not judge logics of the other if she doesent understand it. Whole conflict of opignions in this thread is born here in this two logics differences.

Resurchers just seek for bubbles and explain them like decompression risk.
In bubble models we diferentiate stable bubbles and unstable bubbles, stable that do not grow and are not harmful and unstable that grow and are harmful So part of those bubbles resurchers see for us do not mean risk and part of them do mean risk.

You have entered a mechanism in the model bubbles, decreases the saturation of free compartments?
Model Buhlmann (prior or modified) that has the long confirmation, in the range slow tissue.

Sorry but no specific discussion, perishing in a dialectical ravings.

greet rc

 

[rossh]

Your claim to "no injury" is at best wishful thinking.

Well the study reports no injury.....

Basic scuba training tells us to avoid fast deco, and to take it easy, for the long term prospects. That is why we all add conservatism to our dives.

Gian, you skipped over the question. What do you expect to see at the boundary of least deco, in respect of microbubbles? i.e. the minimum deco setting, the fast profile setting.

What do you think you might see if we took a close look into most any other model at the minimum deco / fast setting (ZHL 100/100 for example).

 

[gianaameri]

Well the study reports no injury.....

Basic scuba training tells us to avoid fast deco, and to take it easy, for the long term prospects. That is why we all add conservatism to our dives.

Gian, you skipped over the question. What do you expect to see at the boundary of least deco, in respect of microbubbles? i.e. the minimum deco setting, the fast profile setting.

What do you think you might see if we took a close look into most any other model at the minimum deco / fast setting (ZHL 100/100 for example).

Gee, did you want the scientists to perform an autopsy on the divers which had fizzed-up using VPM 0?

John Lippmann has shown us that "no symptom" does not mean "no injury." (fact)

Other scientists have shown us that there is a strong link between ARTERIAL bubbles and spinal injury and cutaneous DCS. (fact)

If you look at the mechanics of wound healing, you can see how the body can heal itself after an injury (possibly asymptomatic), but not necessarily in case of nerve damage (my understanding is that the body can find new pathways rewiring itself, but nerve damage does not "heal" and is irreversible hence "injury" is irreversible, although may be asymptomatic).

So, I do not associate "no symptom" with "no injury."

Also, I do not associate arterial bubbling with "injury free."

I would not perform a dive schedule which has shown to produce ARTERIAL bubbles.

I would not perform a dive schedule which man-tested models (i.e. CDM-18) flag as a "DCS risk."

Also, I have no clue - and so no other person on earth knows - what I "expect to see at the boundary of least deco, in respect of microbubbles."

My feeling, which is not science, is that on long shallow dives when I performed a brief rapid ascent I fizzed big time on the venous side, but that this resolved itself as quickly as the fizz when opening a Coke bottle - hence it may have made me feel scary queasy very briefly, but did not kill me.

ARTERIAL fizzing, science instead is telling us that it is another matter alltogether relative to "microbubbles" on the venous side.

Show evidence that microbubbles (whatever this may mean in actual unit of measure) on the arterial side cause no injury on humans, and I will believe your hypothesis that there is some kind of level (you state size, number, frequency, duration) of arterial microbubbles that we can accept and aim for in our recreational dive planning.

I don't know (and neither do you)!

Till then, I go with the conclusion of the scientific study which shows VPM 0 is no good, under the tested conditions.