Deep Stops Increases DCS

[Storker]

I get 106 min vs 74 min, but I don't think you can say anything sensible about differences in stop distribution when the total run time is very different.

 

[Macan]

What do you get for total run time if you run VPM+0 against un-tweeked Bulhmann?

50m @ 30 min, 21%

VPM-B +0

Depth Stop Run Mix pO2 EAD

Des 50 - 1 21 - -
Lvl 50 28 30 21 1.25 50
Asc 30 - 32 21 - -
Stp 27 1:00 33 21 0.77 27
Stp 24 2:00 35 21 0.71 24
Stp 21 3:00 38 21 0.65 21
Stp 18 4:00 42 21 0.59 18
Stp 15 5:00 47 21 0.52 15
Stp 12 7:00 54 21 0.46 12
Stp 9 10 64 21 0.40 9
Stp 6 15 79 21 0.34 6
Stp 3 28 107 21 0.27 3
Sfc - - 107 21 - -

Dive # 1, VPM-B +0
Elevation = 0 m
CNS = 18%
OTU's = 47
Decozone start = 37 m

ZHL-C+GF 100/100

Depth Stop Run Mix pO2 EAD

Des 50 - 1 21 - -
Lvl 50 28 30 21 1.25 50
Asc 12 - 33 21 - -
Stp 12 3:12 37 21 0.46 12
Stp 9 7:00 44 21 0.40 9
Stp 6 13 57 21 0.34 6
Stp 3 28 85 21 0.27 3
Sfc - - 85 21 - -

Dive # 1, ZHL-C+GF 100/100
Elevation = 0 m
CNS = 16%
OTU's = 43
Decozone start = 37 m
Gas 21 = 4409 ltr.

 

[Diver0001]

32 min difference in run time. I assume that VPM gave the shorter of the two?

R..

 

[leadduck]

Anyone here prepared to do a 170ft dive with 30min BT on GF 100/100?

Probably not as the DCS risk probably will be in the range of 5%.

Please note that Bühlmann himself made his tables more conservative than GF 100/100. If we compare for a dive 30min @ 170ft, we get:

------------------------------------------------------------------------
Depth                 10  20  30  40  50  60  70  80  90  100       TTS
------------------------------------------------------------------------
GF 100/100            30  18   9   5   1                             68
Bühlmann 1986 Table   32  16   8   4   3   3                         71
Navy Manual 2008          72  11   5                                 93
VPM-B                 30  19  13   9   5   5   3   3   1             93
VPM-B +2              38  22  14   9   6   6   3   3   2    1       109
------------------------------------------------------------------------

As you can see, Bühlmann's recommendation of 1986 was already slower than GF100/100.
More important: Comparing VPM-B+2 to Bühlmann ZH86, VPM-B+2 adds 20min below 30fsw, but only 18min at 30fsw and above. So from the aspect of integral supersaturation of medium compartments and DCS risk, original Bühlmann 1986 is probably the better choice.

EDIT: had a typo in the Navy 2008 row, shifted by 10fsw

 

[Dr Simon Mitchell]

Having deco ruined forever, by a one sided interested who got his ideas turned down by his peers in 2008.

I challenge you to provide evidence that I had any ideas related to these debates "turned down by my peers" in 2008 or at any time for that matter. The only person systematically peddling ideas that are in conflict with what would be widely accepted by my "peers" is you. I have pointed many of these out with reference to the literature, but this does not stop you blindly charging on, more or less fabricating any narrative that you need to support whatever argument you are making (for example, your idiosyncratic rewrite of the widely accepted paradigm for DCS pathophysiology in order to deprecate the relevance of the high VGE counts that appear associated with use of your model).

B is acting privately, using influence to bully the public, outside of the peer review system (my emphasis)

Virtually everything I have said in the various debates with you is said in one or more of the following publications which all appear in high quality, non-open access, peer reviewed journals.

MITCHELL SJ, DOOLETTE DJ. Pathophysiology of inner ear decompression sickness: potential role of the persistent foramen ovale. Diving Hyperbaric Med 45, 105-110, 2015

DOOLETTE DJ, MITCHELL SJ. Recreational technical diving part 2. Decompression from deep technical dives. Diving Hyperb Med 43, 96-104, 2013

DOOLETTE DJ, MITCHELL SJ. Hyperbaric conditions. Comprehensive Physiol 1, 163-201, 2011

VANN RD, BUTLER FK, MITCHELL SJ, MOON RE. Decompression illness. Lancet 377, 153-164, 2011

MITCHELL SJ, DOOLETTE DJ. Selective vulnerability of the inner ear to decompression sickness in divers with right to left shunt: the role of tissue gas supersaturation. J Appl Physiol 106, 298-301, 2009

DOOLETTE DJ, MITCHELL SJ. A biophysical basis for inner ear decompression sickness. J Applied Physiol 94, 2145-2150, 2003

DOOLETTE DJ, MITCHELL SJ. The physiological kinetics of nitrogen and the prevention of decompression sickness. Clin Pharmacokinetics 40, 1-14, 2001

Ross, has it occurred to you that for me to come on these threads and strongly articulate opinions that are at odds with those widely held by my scientific colleagues would rapidly result in a perception that I am a nutcase. Why would I do that?

Simon M

 

[leadduck]

Nowhere is it written, or shown, that all dives are some how made equal by matching up the total run times. It is done only as a convenience for testing procedures. Some researchers like to use this to an unfair advantage in comparison too, as its always biased toward the shallow profile.

The reason why studies compare profiles of equal runtime is: they compare different models for their efficiency of decompression, i.e. they look for the best tradeoff between runtime and DCS risk. The way to do that is: take two models and set each conservatism parameters so that they produce a profile of the same runtime, then run many test dives and compare DCS outcome. The method with less DCS risk is the more efficient one.

Why do you think this is biased toward the shallow profile? Do you think that a shallow profile is always more efficient?

There is a little bit too much weight been given to the value of surface stress. As successful deco procedure is controlled in the dive portion, and that is where the eventual dive outcome is determined.
If a higher stress in the surface portion passes without issue, then it never was an issue in the first place.

Most DCS cases happen post-surfacing due to surface stress. That's the main problem with spending too little time at 30fsw and shallower. The dive outcome is of course determined by the in-water deco procedure -- because it influences surface stress.

 

[RainPilot]

Probably not as the DCS risk probably will be in the range of 5%.

Please note that Bühlmann himself made his tables more conservative than GF 100/100. If we compare for a dive 30min @ 170ft, we get:

------------------------------------------------------------------------
Depth                 10  20  30  40  50  60  70  80  90  100       TTS
------------------------------------------------------------------------
GF 100/100            30  18   9   5   1                             68
Bühlmann 1986 Table   32  16   8   4   3   3                         71
Navy Manual 2008      72  11   5                                     93
VPM-B                 30  19  13   9   5   5   3   3   1             93
VPM-B +2              38  22  14   9   6   6   3   3   2    1       109
------------------------------------------------------------------------

As you can see, Bühlmann's recommendation of 1986 was already slower than GF100/100.
More important: Comparing VPM-B+2 to Bühlmann ZH86, VPM-B+2 adds 20min below 30fsw, but only 18min at 30fsw and above. So from the aspect of integral supersaturation of medium compartments and DCS risk, original Bühlmann 1986 is probably the better choice.

Interestingly, to me, the Navy 2008 profile is an identical runtime to VPM-B but with the shallower stops extended dramatically. I suspect that if the NEDU test had been run on those 2 schedules, the results would have been the same but we might spend a 1000 posts less arguing about actual vs "fudged" profiles etc etc.
It also seems that the conservancy settings on VPM seem to less about adding a padding factor (like lowering GF High would achieve) and more about undoing the tissue stress from the deeper stops. So why overdo the deeper stops to start with?

 

[tursiops]

It also seems that the conservancy settings on VPM seem to less about adding a padding factor (like lowering GF High would achieve) and more about undoing the tissue stress from the deeper stops. So why overdo the deeper stops to start with?

Bingo!

 

[shoredivr]

No, I'm genuinely curious. How does one properly measure decompression stress?

EDIT: And when I claim that someone has said something, I'd better be able to back that up with more than just my own claim. Seems fair, does it not?

If previous patterns hold true, your question will not be answered.

 

[rossh]

The reason why studies compare profiles of equal runtime is: they compare different models for their efficiency of decompression, i.e. they look for the best tradeoff between runtime and DCS risk. The way to do that is: take two models and set each conservatism parameters so that they produce a profile of the same runtime, then run many test dives and compare DCS outcome. The method with less DCS risk is the more efficient one.

Why do you think this is biased toward the shallow profile? Do you think that a shallow profile is always more efficient?



Most DCS cases happen post-surfacing due to surface stress. That's the main problem with spending too little time at 30fsw and shallower. The dive outcome is of course determined by the in-water deco procedure -- because it influences surface stress.

I can't agree with the idea that forcing two dissimilar / profiles / models, into making them equal runtime, some how magically evens them up. Different models, different on/off gas patterns, different saturation characteristics. If you want to know what a specific model says and thinks about how to make a deco profile, then don't try to tell it how to do its job. Models have their own distinct formula, and fiddling run times to some arbitrary dimension, does not improve on that.

I get this kind of question all the time in this format: I want to make your model put an extra stop here, or do some other aspect different. The answer is always: stop fiddling with it: the model does not care for your ideas or modifications.

******

Most DCS do appear on the surface, maybe from an over pressure / blockage / circulation problem that originated in the dive. It may take many hours to appear, but the underlying cause is usually a dive environment problem.

Surface stress is at a maximum at surface time, and declines from there, rapidly at first. One normally has to bring a problem to the surface, for it to go wrong. There are exceptions (not now). Perhaps the only real surface generated injury would be just plain old going too fast in deco, but no one much is pushing the envelope these days.

Think about this - what can you do wrong on the surface? Ascend in elevation, hot bath, gross exercise, or ... ?? not much else really. The surface period is mostly about waiting and see if it worked, or crossed fingers to see if you got away with some poor preparation / bad procedures, etc.

Remember that tissues are dynamic. A fast tissue becomes a slow one when its made cold and it circulation drops off. Any tissue with a blockage, becomes a slow one, and a tissue that gets to DCS injury status, takes on an infinite time, that needs a re-compression to free up the trapped gas and restore the circulation.

Something else that's not thought about much - we look at all these tissue graphs and point at values, and move things around.. etc. But we are looking at successful deco off gas patterns. There are no faults to be seen.

What do you think tissue graphs look like when you get a DCS? If your bad shoulder injury kicks in, or you get a cold water suit flood in one arm? Or blow though the stops by accident? If you could actually measure this stuff while the physiological changes were taking place, what would you see? The tissue pressures would be off the chart, or zero, depending on where we measure the condition. The point being all our charts and measures are built on successful deco. Trying to see that some arbitrary values is right or wrong - probably just kidding yourself, particularly if its minor.




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