Deep Stops Increases DCS

[Kevrumbo]

I was a little to hasty with my reply above. After re-reading the study and thinking about it for awhile I still come to the same conclusion that the study was flawed but now don't believe the NEDU was being careful for reasons explained below.

I've been in the automation/control business for almost 40 years and one of the cardinal rules I try to follow is you only change one thing at a time. If you change item 1 and item 2 and there is a problem was it due to item 1, item 2, or some combination of the two items. There are too many variable factors in the study to reach a conclusion especially when one of the factors directly influences DCS namely the TST or total stop time. The researchers in taking the variability out of TST created a variability in the amount of off-gassing by introducing deep stops. They write: "This is the largest man-trial of individual decompression schedules of which we are aware and the only laboratory comparison of deep stops and shallow stops that is not confounded by differences in TST."

You try to reduce the variables as much as possible other than the variable you are trying to study (deep stops in this study). The variables in the study are: pre-dive factors, different algorithms (WAL-18 Thalmann and BVM(3)), and amount of off-gassing between the first stop and the surface. The pre-dive factors included sleep, alcohol, caffeine, exercise, illness, and medications. The pre-dive factors were not controlled but information about them was obtained by self-reporting through brief interviews.

There's too much variability. The results to me are inconclusive; Was DCS due to the deep stop or was it due to shortening the TST in spite of what a deep stop algorithm computed (or would have computed)? Here's what I would have done:

Control the pre-dive factors: no alcohol or caffeine, everyone gets full night sleep (no alarm clocks), everyone eats the same number of meals before the test (no snacks), no one taking medications, no illnesses, same amount of exercise for everyone. Use the same algorithm -- Buhlmann with GF's. For the deep stop divers, use 30/90 or 30/85. For the shallow stop divers, use 90/90 or 85/85. By using a GF(Lo) a deep stop can be introduced while maintaining the same GF(Hi) limit. This will increase the length of the shallow stops but maintain the same end point for final tissue content on surfacing. Every diver needs to dive in both groups.

Yes, that argument & criticism of the experimental paradigm was all covered previously in this thread. But the more compelling interpretation by UWsorjourner (and Simon Mitchell's lectures on the NEDU Study during the 2013 Bikini Expedition), had to do with the Integral of Supersaturation Concept:

A couple of good questions at this point might be:

1) what is integral supersaturation? and

2) why does Ross seem desperate to discredit integral supersaturation [with regards to VPM]?

What is integral supersaturation (ISS)?

Integral supersaturation is a pretty simple concept. Decompression sickness occurs due to the release of dissolved gases after a dive. How many bubbles will form and how big they get is a function of 1) the speed of the release (that's the pressure of the gases being released), and 2) the time those pressures are allowed to exist. Higher pressures can be endured for a short time and lower pressures for a longer time. The thing to remember is that the TIME you're exposed to supersaturation is important.

ISS is a measure of the speed and time of decompression. Simply looking at charts of peak supersaturation gives you an idea of speed, but is not a good measure of the overall supersaturation-time you're exposed to. Integral supersaturation measures both pressure and exposure time and so can be thought of as an index of decompression stress (supersaturation pressures you're exposed to and the time you're exposed to them) when comparing dives with a similar run time. Pretty simple idea. Its not the rosetta stone of decompression, but it is an interesting and useful measure as Dr. Doolette has described.

Consider the chart below. The chart shows ISS for the dives in the NEDU study with VPM-B+7 and GF 53/53 added. The VPM and GF parameters were chosen to get to the same runtime as the NEDU A1 & A2 profiles.

As noted numerous places by Doolette and others, the only credible explanation for the A1 profile performing so much better than the A2 profile is that the ISS of A1 was significantly lower. As Dr. Mitchell addressed here, even though A1 had much higher early supersaturations, those supersaturations were short in duration. What seems to have been much more significant is that A2 had about 50% more supersaturation-time exposure once the diver surfaced.

Now compare VPM and GF on the chart. It's pretty obvious that VPM and A2 (the higher risk profile) are closer; GF and A1 (the lower risk profile) are closer together. And that pattern of significantly more supersaturation exposure upon surfacing doesn't end with the NEDU study. . .

So. . . ISS seems to indicate VPM (and other bubble models) behave more similarly to the A2 profile shown to be more risky in a very closely monitored study of actual dives performed by the US Navy. Its that simple. For some other similarities see here. . .

Deep Stops Increases DCS

 

[EFX]

Furthermore......

From the USN's NEDU shallow vs. deep efficiency study are the following two quotes:

"The U.S. Navy has a continuing need for more efficient decompression procedures. For instance, the newly introduced U.S. Navy air decompression tables have substantially longer air decompression times than the tables they replaced but provide only small reductions in estimated pDCS. Redistribution of these long air decompression times according to a bubble model resulted in substantial reductions in estimated pDCS and was the motivation for the present work." [page 2]

"In this context, two decompression schedules for the same dive depth and bottom time differ in efficiency if one has a shorter required decompression time for the same pDCS or a lower pDCS for the same decompression time." [page 2]

According to the navy, efficiency can mean both lower decompression time or lower pDCS (probability of Decompression Sickness). Given the motivation in paragraph one to lower pDCS the last requirement in the second paragraph becomes unattainable. If bubble models lower pDCS by inserting deep stops then deco time will necessarily increase because a deep stop increases inert gas loading. That is what the study showed. Not only did the pDCS not stay the same it actually increased resulting in more DCS hits.

Dual phase (bubble) decompression models like VPM-B track inert gas tensions in the dissolved and free gas states. In describing the VPM-B Asser Salama in "Deep into Deco" writes, "The method of calculating the ascent ceilings and how they propagate through the ascent was also altered, as the increase in bubble size forces VPM to reduce the allowable gradients for the subsequent parts of the ascent profile, which in turn tends to increase the length of the shallower stops, especially when it comes to deeper dives." [emphasis mine]

The Navy had their cake but couldn't eat it. What does this mean for us as recreational divers (rec and technical)? It certainly does not mean that deep stops are not warranted as the Navy contends but points to a need for more carefully controlled studies.

 

[Kevrumbo]

Furthermore......

From the USN's NEDU shallow vs. deep efficiency study are the following two quotes:

"The U.S. Navy has a continuing need for more efficient decompression procedures. For instance, the newly introduced U.S. Navy air decompression tables have substantially longer air decompression times than the tables they replaced but provide only small reductions in estimated pDCS. Redistribution of these long air decompression times according to a bubble model resulted in substantial reductions in estimated pDCS and was the motivation for the present work." [page 2]

"In this context, two decompression schedules for the same dive depth and bottom time differ in efficiency if one has a shorter required decompression time for the same pDCS or a lower pDCS for the same decompression time." [page 2]

According to the navy, efficiency can mean both lower decompression time or lower pDCS (probability of Decompression Sickness). Given the motivation in paragraph one to lower pDCS the last requirement in the second paragraph becomes unattainable. If bubble models lower pDCS by inserting deep stops then deco time will necessarily increase because a deep stop increases inert gas loading. That is what the study showed. Not only did the pDCS not stay the same it actually increased resulting in more DCS hits.

Dual phase (bubble) decompression models like VPM-B track inert gas tensions in the dissolved and free gas states. In describing the VPM-B Asser Salama in "Deep into Deco" writes, "The method of calculating the ascent ceilings and how they propagate through the ascent was also altered, as the increase in bubble size forces VPM to reduce the allowable gradients for the subsequent parts of the ascent profile, which in turn tends to increase the length of the shallower stops, especially when it comes to deeper dives." [emphasis mine]

The Navy had their cake but couldn't eat it. What does this mean for us as recreational divers (rec and technical)? It certainly does not mean that deep stops are not warranted as the Navy contends but points to a need for more carefully controlled studies.

What it now means for recreational "sport" technical divers is that if you choose to use deep stops with a Dual Phase Bubble Model, and decompression profiles with 50% & 100% O2, then be prepared to extend your profile time on 100% O2. Efficiency is not paramount for us as it is for the Navy in mission tactics & operations (i.e. exfiltrate a SEAL team quickly out of the water with the least probability of DCS) , but rather Effectiveness in doing the requisite time on Oxygen at 6m depth is more important -however long it takes and may vary from diver to diver - to ensure a symptom free exit from the water . . . (the caveat though becomes O2/CNS exposure, which is another prudent reason to take a day-off after consecutive days of decompression diving).

 

[Redshift]

There are threads about this study, can we not go there here? Has nothing to do with the original post.

 

[EFX]

OK. Me bad. Moderator, would you move my posts and responses to the "Deep Stops Increases DCS" thread. Thanks.

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A ScubaBoard Staff Message...

Done.

Storker
SB Moderator

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[Ayisha]

...their conclusion: "The practical conclusion of this study is that controlling bubble formation in fast compartments with deep stops is unwarranted for air decompression dives."

To the quoted sentence they should have added "without extending the time at shallow stops.". rjack321 makes that point in the thread "Are People backing away from VPM?"

bold added

This was my takeaway too after reading the study and hundreds of posts and threads regarding the study and what it means. I haven't refreshed my memory, but it appeared that a short deep stop paired with a longer "safety" stop were a good compromise. YMMV.

 

[Ayisha]

There were a few times in this thread and others and on other boards that "padding shallow stops" when implementing a short and shallower deep stop is prudent in limiting bubble growth in fast tissues and off-gassing effectively. There were some exchanges I recall between Dr Mitchell and KevRumbo regarding this as well. I don't have time to re-read these threads, but here is a post right on this thread that Dr Mitchell says sums it up for him:

UWSojourner has summed it up for me.

...

A useful post is here.

 

[Kevrumbo]

There were a few times in this thread and others and on other boards that "padding shallow stops" when implementing a short and shallower deep stop is prudent in limiting bubble growth in fast tissues and off-gassing effectively. There were some exchanges I recall between Dr Mitchell and KevRumbo regarding this as well. I don't have time to re-read these threads, but here is a post right on this thread that Dr Mitchell says sums it up for him:

Simon Mitchell's recommendation (as he first told me in 2013 as both my attending & treating Physician on the Bikini Atoll Expedition), in light of the NEDU Study is a Buhlmann ZHL-16 GF Lo/Hi of 40/70 as a starting reference base -although he primarily dives CCR with Helium for his Expedition Dives.

Kevrumbo said:
The best most prudent compromise to practically apply from the NEDU Study & discussion, is to do the Deep Stops, and extend out the O2 profile at 6m such that you have a surfacing Gradient Factor of 60% or less (per the readout of a Petrel Computer upon surfacing from your O2 deco stop) --to ensure inert gas elimination from those Slow Tissues. This is especially warranted if you're doing multiple deco dives per day for a week or more -and I would also recommend taking a day-off/break after three consecutive days of multiple deep deco dives per day. . .

Simon replies:
I would agree that this is a workable compromise. Deep stops can be safely incorporated into a dive profile if you want to use them. However, there is no evidence that you gain anything by doing so, and the available evidence suggests that their use is not the most efficient use of deco time. Thus, if you have a fixed amount of decompression time, the decompression will become less safe if you over-emphasize deep stops.

Kevrumbo:
Simon, I'm simply not willing to risk bubble nucleation & formation in my Fast Tissues for the sake of not loading/supersaturating my Slow Tissues later on in the deco profile (per indication of those "heat maps" by UW Sojourner); essentially a "Robbing Peter to pay Paul" dilemma. And yes, I have plenty of time and an 11L Cylinder (AL80) full of O2 to clean-up those Slow Tissues. . .

Simon Mitchell replies:
Hello Kev,

I guess this is the sticking point. It is an article of faith for you that allowing fast tissues to supersaturate early in a profile that places less deep stops in your ascent is harmful, and there is probably nothing I can do to change your mind on that. However, I must point out that you only believe that because someone has told you it is so. It is an attractive theoretical assumption that many people believe(d) in the absence of any confirmatory data. The point is, that there is now data that challenge the idea. As UWSojourner's heat maps have illustrated the NEDU deep stops profile did reduce fast tissue supersaturation compared to the shallow stops profile, but this did not result in better outcomes. If tight control of fast tissue supersaturation early in the ascent is as important as you believe, why did the profile with the best control of fast tissue supersaturation early in the ascent produce the highest DCS rate?

Anyway bud, if you do what you say you are going to do and significantly pad your shallow oxygen decompression it may not matter too much what you do earlier. Just don't have a seizure please!

I hope you have a fabulous trip. I may have mentioned I am going back with Pete Mesley in November.

Simon

 

[rossh]

The sham and BS explanations continue......

I'm sorry for being so blunt, but it seems such directness is needed to wake up some people to the junk science being offered by the experts, and the deliberate implied but invalid arguments by some (committed) scientists.

Some truth follows:

1/ The nedu did NOT test VPM or GF. The Nedu did NOT test the deep stops as used in DIR, RD, or VPM type profiles. The Nedu actually tested two shallow stop types - one with extended shallow stops (60 -30 ft). Because the profiles were extremely long (2.5 x normal) and "low risk" shallow profiles, they had to add back in some stress through cold. Hence the nedu test is actually a test of long shallow stops vs effects of cold. The association to deep stops or condition is implied only. (see below)

But most importantly, the tissue Supersaturation explanation offered as validation of the Nedu test result, applies to conditions of shallow type profiles. Actual real deep stops profiles, do not exhibit the same supersaturation gas patterns. Hence the test is NOT relevant to deep stops, and the 3 claimed "plausible explanations" offered by Mitchell, are all shown to be false. here and here


2/ There is no such thing as + 7 VPM - that is a complete fabrication - made up to to fit this fake argument. However, *if* one wanted to actually fit a real VPM-B profile to the nedu test, it would need to start at -10. Such a thing does not exist, but it clearly demonstrates that the nedu test never got close to deep stops or VPM-B. (see below)


3/ The Integral SuperSaturation (ISS) does NOT represent or measure decompression stress in any useful manner. Its too simplistic to be used for values of overall decompression ascents. ISS is also NOT validated or calibrated or published as a definitive measure in scientific and decompression literature. But is it hyped up to BS levels for by the proponents of this sham. see here


4/ AND the existing Schreiner and Haldane equations that are already built into ZHL and VPM models fully cater for all the negative effects discussed in staying deeper and integral saturation. That is the entire purpose of these equations - gas tracking and delaying the ascent as needed. They works just fine in ZHL, GF, and VPM, multilevel, extended times and ...and including deep stops. Mitchell's arguments of trying to single out deep stops and ISS, within VPM-B only, is a sham, and a lie to himself and everyone else.



Diagrams here:

So, Kevin, Sorry to be nasty, but you have been suckered by Junk science, made up advertising type sham diagrams by trolls, and a scientist who did not do his home work.


****


But hey - why believe me? Consider this instead. The outcome of all this by the "experts" was to suggest and adopt profiles like 40/70, which if you look closely, is almost the same as a VPM-B+3 profile. They are not a perfect match, but are close in many aspects.

So its OK to use GF to make and emulate VPM-B style profiles, but not the actual model? That makes the whole argument against VPM-B as hypocrisy.

******

More info here: all diagrams

Yes, that argument & criticism of the experimental paradigm was all covered previously in this thread. But the more compelling interpretation by UWsorjourner (and Simon Mitchell's lectures on the NEDU Study during the 2013 Bikini Expedition), had to do with the Integral of Supersaturation Concept:

A couple of good questions at this point might be:

1) what is integral supersaturation? and

2) why does Ross seem desperate to discredit integral supersaturation [with regards to VPM]?

What is integral supersaturation (ISS)?

Integral supersaturation is a pretty simple concept. Decompression sickness occurs due to the release of dissolved gases after a dive. How many bubbles will form and how big they get is a function of 1) the speed of the release (that's the pressure of the gases being released), and 2) the time those pressures are allowed to exist. Higher pressures can be endured for a short time and lower pressures for a longer time. The thing to remember is that the TIME you're exposed to supersaturation is important.

ISS is a measure of the speed and time of decompression. Simply looking at charts of peak supersaturation gives you an idea of speed, but is not a good measure of the overall supersaturation-time you're exposed to. Integral supersaturation measures both pressure and exposure time and so can be thought of as an index of decompression stress (supersaturation pressures you're exposed to and the time you're exposed to them) when comparing dives with a similar run time. Pretty simple idea. Its not the rosetta stone of decompression, but it is an interesting and useful measure as Dr. Doolette has described.

Consider the chart below. The chart shows ISS for the dives in the NEDU study with VPM-B+7 and GF 53/53 added. The VPM and GF parameters were chosen to get to the same runtime as the NEDU A1 & A2 profiles.

As noted numerous places by Doolette and others, the only credible explanation for the A1 profile performing so much better than the A2 profile is that the ISS of A1 was significantly lower. As Dr. Mitchell addressed here, even though A1 had much higher early supersaturations, those supersaturations were short in duration. What seems to have been much more significant is that A2 had about 50% more supersaturation-time exposure once the diver surfaced.

Now compare VPM and GF on the chart. It's pretty obvious that VPM and A2 (the higher risk profile) are closer; GF and A1 (the lower risk profile) are closer together. And that pattern of significantly more supersaturation exposure upon surfacing doesn't end with the NEDU study. . .

So. . . ISS seems to indicate VPM (and other bubble models) behave more similarly to the A2 profile shown to be more risky in a very closely monitored study of actual dives performed by the US Navy. Its that simple. For some other similarities see here. . .

 

[PfcAJ]

I don't think I'd say 40/70 and VPM+3 is a close match.

Lets look at a fairly standard issue dive
200ft, 18/45, 50%, 100%, 30min BT via Deco Planner:

VPM starts stops at 130', 40/70 at 90'.
VPM has a total of 11mins of stop time before the 70' gas switch, 40/70 has 7.
VPM has 28mins on oxygen, 40/70 has 34mins.
VPM's total deco time is 62mins, 40/70 is 66mins.

Imo, id rather get on a deco gas sooner and move VPM's deep stop time to somewhere useful (aka the oxygen stop).

So based on that, you're doing *less* total deco, *less* time on deco gases, and *less* time on oxygen.

Nope.