Deep Stops Increases DCS

[Kevrumbo]

Look at the Bottom Mix Gas used in the NEDU Study (essentially Deep Air):

(Abstract, p.i)). . .Divers wearing swimsuits and tshirts, breathing surface-supplied air via MK 20 UBA, and immersed in 86 °F water were compressed at 57 fsw/min to 170 fsw for a 30 minute bottom time during which they performed 130 watt cycle ergometer work. . . Results indicate that slower tissue gas washout or continued gas uptake offsets the benefits of reduced bubble growth at deep stops.

(Conclusion p.18) The practical conclusion of this study is that controlling bubble formation in fast compartments with deep stops is unwarranted for air decompression dives.

This is the simple point to take away from the study:

Of course you're going to have significant residual inert Nitrogen, potentially on-gas N2 at your deep stop and perhaps even at intermediate deco stops on Eanx50 which may encroach on critical slow tissue M-values --if you were using a working bottom mix with a high fractional N2 content to begin with like Air. Plan accordingly, use a computer to track your inert tissue loading (i.g. Shearwater Petrel) and be prepared to extend your 6m depth 100% Oxygen deco profile along with a stand-by In-Water-Recompression Table as a DCS treatment contingency if necessary should you choose to use Deep Air on mandatory decompression dives, especially on multiple dives per day consecutively over a week or more (i.e. like an "Expedition Trip"). Better yet, take a day-off to further purge those slow tissues of residual N2 after three or four consecutive days of decompression diving. . .

 

[lv2dive]

Kevin,

You are going well beyond the parameters of the study when you start talking about EAN50, oxygen, and what you call expedition trips. I don't think that the study authors would agree with your bolded statement, as representative of a "simple point" to take away from their study.

Look at the Bottom Mix Gas used in the NEDU Study (essentially Deep Air):


This is the simple point to take away from the study:

Of course you're going to have significant residual inert Nitrogen, potentially on-gas N2 at your deep stop and perhaps even at intermediate deco stops on Eanx50 which may encroach on critical slow tissue M-values --if you were using a working bottom mix with a high fractional N2 content to begin with like Air. Plan accordingly, use a computer to track your inert tissue loading (i.g. Shearwater Petrel) and be prepared to extend your 6m depth 100% Oxygen deco profile along with a stand-by In-Water-Recompression Table as a DCS treatment contingency if necessary should you choose to use Deep Air on mandatory decompression dives, especially on multiple dives per day consecutively over a week or more (i.e. like an "Expedition Trip"). Better yet, take a day-off to further purge those slow tissues of residual N2 after three or four consecutive days of decompression diving. . .

 

[BDSC]

This is the simple point I took from the study,


CONCLUSIONS


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


Rubicon Research Repository

 

[Kevrumbo]

Kevin,

You are going well beyond the parameters of the study when you start talking about EAN50, oxygen, and what you call expedition trips. I don't think that the study authors would agree with your bolded statement, as representative of a "simple point" to take away from their study.

This is the simple point I took from the study,

CONCLUSIONS

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

Rubicon Research Repository

BDSC (and Kate again). . .Simon Mitchell MD, Ph.D lectured our charter group during a June/July 2013 two week Bikini Atoll Expedition, about the implications of this very study. His opinion and practical recommendation to me as my attending Physician & Hyperbaric Researcher was to extend the O2 deco stop profile (and correspondingly monitor CNS tox) if I chose to perform deep stops per Ratio Deco, or to use GF 40/70 Buhlmann zhl-16 --all electively on open circuit Air bottom mix. And to take a break or day off if feeling super tired or experiencing the "niggles" after consecutive days of deco diving because of that heavy residual inert N2 load on my slow tissues. . .

 

[BDSC]

And to take a break or day off if feeling super tired or experiencing the "niggles" after consecutive days of deco diving because of that heavy residual inert N2 load on my slow tissues. . .

I have seen this term a few times now. Exactly what do you guys mean by "niggles"?

 

[Kevrumbo]

I have seen this term a few times now. Exactly what do you guys mean by "niggles"?

Sub-clinical non-acute type I DCS in a musculoskeletal joint space or limb --either a very dull ache or a slight sore intermittent impulse. . .

 

[BDSC]

OK. Thanks!

 

[lv2dive]

And that's fine, I believe he told you that. But that is not the same as saying that is the conclusion of that study.

John (and Kate again). . .Simon Mitchell MD, Ph.D lectured our charter group during a June/July 2013 two week Bikini Atoll Expedition, about the implications of this very study. His opinion and practical recommendation to me as my attending Physician & Hyperbaric Researcher was to extend the O2 deco stop profile (and correspondingly monitor CNS tox) if I chose to perform deep stops per Ratio Deco, or to use GF 40/70 Buhlmann zhl-16 --all electively on open circuit Air bottom mix. And to take a break or day off if feeling super tired or experiencing the "niggles" after consecutive days of deco diving because of that heavy residual inert N2 load on my slow tissues. . .

 

[Kevrumbo]

And that's fine, I believe he told you that. But that is not the same as saying that is the conclusion of that study.

Kate, nobody "practically" dives the profiles the subjects did in the experimental paradigm. I'm simply applying and perhaps extrapolating the conclusion to real world decompression dives with deep air bottom mix over several consecutive days, and finding cause for problems I had on the Bikini Expedition with type one symptoms as I went on a contiguous week of two deco dives per day.

 

[EFX]

The study did that. So they were careful but not so careful in their conclusion

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.