Doolette's Alert Diver Interview

[boulderjohn]

My question is, if our current bubble models with deep stops do not add on adequate shallow time and we are manually adding on more, and our buhlmann models with GF potentially are lacking the advantages of the deep stops, do we need a new model that does both?

I am now gong to write my version of what I understood Simon Mitchell to say about this in the deep stops thread. If others have a disagreement with my understanding, please chime in.

The short answer is that after more than a hundred years of working on this, we still cannot say with certainty what the ideal decompression profile may be, and that ideal profile may be different for each of us. We know that a range of profiles with first stops at different depths have been reasonably successful, and we can infer that the ideal lies somewhere in that range. Scientific studies will slowly give us better and better evidence that will help us find that ideal. As individual divers, we have to look at that slowly mounting evidence and make our own decisions.

 

[boulderjohn]

f our current bubble models with deep stops do not add on adequate shallow time and we are manually adding on more, and our buhlmann models with GF potentially are lacking the advantages of the deep stops,

I wanted to make this point separately from what I wrote above--note that there is no definitive proof for either of the two premises above. As I understood the deep stops thread, there is evidence that some of the deep stops models start with stops that are too deep and do not provide a benefit, with or without the extended shallow stops.Does that mean that the ideal (whatever it is) lies somewhere in between those deep stop models and the neo-Haldanean models? Does it mean that the neo-Haldanean models were right all along? I don't know, but I will continue to read the literature.

 

[boulderjohn]

One particular line of thinking, riddled with fallacies of different kinds, interferes with our ability to find the ideal. That line of thinking is "I have always done ABC and been fine, so therefore ABC is the correct thing to do." Depending upon the specific argument being made, such an argument usually blends both the single cause fallacy and the post hoc fallacy. Decompression includes a number of factors, and it is a fallacy to assume one of those factors in particular causes the outcome. That one factor may have no bearing on the outcome at all, and it may even work against the positive outcome--things might have been even better if that single factor had been eliminated.

As an example, think what you would say to a diver who said, "I always snap my fingers every 5 minutes during decompression, and I have never been bent. Therefore, snapping your fingers every 5 minutes during decompression is an important part of the decompression process."

As a second example, think what you would say to a diver who said, "I always descend 10 feet for one minute before beginning my ascent, and I have never been bent. Therefore, descending 10 feet before an ascent is an important part of the decompression process."

 

[KevinNM]

The problem with improved theories is that basing your practice on a new theory without actual human testing is quite possibly going to result in less safely. VPM and others didn't appear to improve safety in actual testing despite clever theoretical claims as to how they would. And the whole field of medicine is littered with highly appealing theories that when tested showed minimal or negative improvements in outcome.

Without careful testing you just have theory, and careful testing requires a lot of money.

And in some cases it's just impossible to conduct that study: Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials. - PubMed - NCBI

 

[boulderjohn]

The problem with improved theories is that basing your practice on a new theory without actual human testing is quite possibly going to result in less safely.

I am not sure I saw where anyone suggested blindly jumping on a new theory without giving it proper evaluation. As I said a couple of posts ago, "As individual divers, we have to look at that slowly mounting evidence and make our own decisions." those decisions should give appropriate weight to the credibility of the evidence in its favor.

 

[huwporter]

I've no doubt that you can treat gas breaks as oxygen and get away with it (all you are doing is pushing your GFHi more aggressive - I'll run some numbers later) in most cases, particularly if you are fit etc.

So I was interested enough to run some numbers; looking at, for example, 30 minutes @ 90m (300 ft) with standard OC gases, with (unmodified) Buhlmann 20/85 I get 66 mins on oxygen.

Doing 12 mins on, 6 mins off on 12/65 trimix for a total of 66 mins on oxygen without counting gas breaks as O2 time, pushes that to GFHi 87 (due to on-gassing in some compartments during the gas breaks).

Doing 12 mins on, 6 mins off on 12/65 trimix for a total of 66 mins including gas breaks as O2 time pushes that to GFHi 106. Totally doable if you are fit, wouldn't be my choice (Hey, I do this for fun), but I know people that have run significantly higher GFHi-s in the past.

 

[atdotde]

My question is, if our current bubble models with deep stops do not add on adequate shallow time and we are manually adding on more, and our buhlmann models with GF potentially are lacking the advantages of the deep stops, do we need a new model that does both? It would be more conservative and require longer deco obviously but would incorporate both the deep stop and the complimentary extra shallow stop time rather than people having to add it on/make it up themsevles in the shallows. Is the DCS rate for our bubble models and buhlmann GF models currently unacceptable to warrant a new model?

Doing longer deco is almost always reducing DCS risk (unless you are really stupid and wait longer at depths where you are on-gasing significantly). There is no problem making your algorithms more conservative so they let you stay in the water longer. In particular with gradient factors, you can lower them (mainly GFhigh) and you will get longer decompression times (while adjusting GFlow so you get deeper initial stops like in the bubble model of your liking). Tuning gradient factors should already provide you with the freedom you are looking for. The question (also in the deep stop discussion) is where to place your stops for a fixed runtime. But if you are willing to deco longer, then it is very easy to make it saver.

 

[Kevrumbo]

I find it interesting that we have a few threads on SB over a hundred pages long about the NEDU study and deep vs shallow stops and the correct distribution of each. We have people that feel strong one way or the other. We have people that say they do deep but then add on extra shallow time.

My question is, if our current bubble models with deep stops do not add on adequate shallow time and we are manually adding on more, and our buhlmann models with GF potentially are lacking the advantages of the deep stops, do we need a new model that does both? It would be more conservative and require longer deco obviously but would incorporate both the deep stop and the complimentary extra shallow stop time rather than people having to add it on/make it up themsevles in the shallows. Is the DCS rate for our bubble models and buhlmann GF models currently unacceptable to warrant a new model?

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 (for example, exfiltrating 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 to washout the slow tissues -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 and pulmonary/lung inflammation due to prolonged O2 exposure, which is another prudent reason to take a day-off after consecutive days of decompression diving).

The way you could do this using GF Buhlmann zhl-16 with a Petrel/Perdix Dive Computer is to initially enter for example, a deep stop GF's lo/hi of 30/85 and at least complete this deco profile as displayed if for whatever reason you have to get out of the water as soon as possible (due to weather, great white shark etc). But if you have time and enough O2 supply, you can reset the GF hi, realtime during the dive at the 6m O2 stop, to a lower more conservative surfacing value like 60 for instance and complete the extended O2 profile as displayed.

From a dialog between Simon Mitchell and me:

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! . . .

Simon

Deep Stops Increases DCS

To reiterate: Try to find a practical balance or compromise of Bubble/Dual Phase Models like RGBM, VPM, Ratio Deco -minimizing Fast Tissue Supersaturation stress with deep stops- while extending out shallow time on Oxygen to effectively off-gas the resultant inert loading on the Slow Tissues as implied by the NEDU Study.

Here's another method: By using & applying Buhlmann GF's for instance, if I were doing two OC deco dives per day for three or four consecutive days-in-a-row on Deep Air on a month long Indo-Pacific/Micronesia tech wreck expedition, then I would program my Petrel Computer from Day 1 thru 4 something like 40/70 for Day 1; 40/65 for Day 2; 40/60 for Day 3 and finally 40/50 for Day 4. On Day 5 take a break from diving to further off-gas those slow tissues & reset CNS O2 clock, and then continue the same three or four day consecutive deco dive day with one day-off schedule/pattern.

 

[beester]

On pulmonary damage: I was involved in a scientific test on pulmonary damage (Phd research). I see if I can find the details on the final results and testing methodology but as I remember as a guinea pig it was the following:

- 2 hour dive at a ppo² of 1.6 in a static tank.
- No gas breaks
- 15 minutes before dive a pulmonary test (spyrometrie, and exhalation sample to check certain enzymes exhaled, blood test)
- After dive every immediately same tests and every 30 minutes after for 5 sequences.
- 16 test persons, 14 male 2 female, age bracket 30-60 years old.

With my layman eyes (I have scientific experience but in history... not real science ;-) ) I remember the results being inconclusive not showing any increased exhalation of those enzymes nor different blood values that could pin point additional pulmonary short term damage.

On deepstops: The jury is indeed still out. I still use a 20/85 gf (so traditional GUE style ascend) except that I will pad some on the 6m and 3m stops. I've heard about a DAN study done in Italy in a 40m(130ft) basin, testing specifically on deepstops. But I don't have details on methodology so I'm not drawing conclusions. The gist of it was that for some test persons deepstops reduced bubble generation (doppler) when compared to following a traditional buhlmann model. However for other test persons it was the reverse.

In the end it's like what Kev's excellent post points out... however I want to make a personal remark on "article of faith" point made by Dr. Mitchell. I've done many dives in the 40m-60m range, both on air (in my bad bad old days) and on trimix. I've tried to register how I felt after doing those dives and I think I have a good understanding what works for my body. However playing around with deepstops in this depth range doesn't give very different results. In the end it's just playing with the distribution of a couple of minutes deeper or shallower.

However how do you proceed beyond that... I've now done about 10 dives (this year) in the 70-90m depth range with bottom times up to 35 minutes. This is a totally different ascend strategy, the risks and repercussions when making an error are greater. Since I don't have enough dives in this depth range (or deeper) to make reliable estimate on what works or not, I do follow (act of faith) people I trust and know. People that are doing many many dives on a much bigger scale than mine (100m+ with bottom times which can be more than 1 hour). Of course what works for them might not work for me, because in the end DCS is a result of not only physics but also biology, and all our body's are slightly different. But it's the best I can do.

If one of my teammates does ask to do a bit more shallow, we do... in the end the ascend strategy is a compromise.

Just my 2 cents.

 

[boulderjohn]

I've heard about a DAN study done in Italy in a 40m(130ft) basin, testing specifically on deepstops. But I don't have details on methodology so I'm not drawing conclusions.

The study was done at recreational depths only, and the deep stops were at 1/2 the maximum depth. Those stops were then followed by a single safety stop at the standard depth. The study then essentially compared a two stop profile at reacreational depths to a one stop profile at recreational depths.

What we now call deep stops at technical depths tend to be much deeper than 1/2 maximum, and they are followed by more complex ascent profiles.