Deep Stops Increases DCS

[rjack321]

I don't think the heat maps really care if the dives are OC or CCR, have gas switches or don't. The heat maps are just looking at patterns of supersaturation and continued on gassing.

The heat map below is for OC 200ft 30min dive, 18/45, EAN50 at 70ft, 100 at 20ft. The profiles are generated by VPM-B+3 and GF 75/75. Each of those profiles then is shown with an extra 5 minutes of time spent at 70ft.

In addition the integral supersaturation for those profiles is shown. Integral supersaturation is simply a measure of time spent in a supersaturated state.

I would note the following:
1. The profiles surface at around 90 minutes.
2. The VPM-B+3 profile has deeper stops limiting early supersaturation. Of course this causes the higher supersaturations once the diver surfaces.
3. The VPM-B+3 profile has roughly 30% more supersaturation obligation once the diver surfaces (546.7 / 423.6).
4. The charts pretty much repeat the visual pattern of the NEDU study for A2/A1 profiles.
5. The additional 5 minutes at 70ft really doesn't impact the overall picture.

View attachment 200372

Thanks so much, this pretty much answered all of my "how is this relevant to an OC tech dive with gas switches" questions.

 

[Divetech99]

I just want to summarize some of the information that resulted from the debate last year over on RBW.

During that debate the NEDU profiles (A2-deeper stop, A1 shallower stop) were compared to profiles that were generated by VPM-B and GF. The comparisons were done in a number of different ways in order to see how similar VPM-B and GF might be to the tested NEDU profiles.

As more and more information was digested and compared, my conclusion was that the VPM-B profile would likely behave quite a bit like the A2 profile (i.e. higher DCS) and a GF profile more like the A1 profile. So I do think the study has a lot to say to recreational divers who plan decompression dives. Obviously everyone has to decide for themselves, but here are some links that might help you think through the issues.

Profile similarity
Compares shapes of the A2 and VPM profiles

Heat map similarity
Visually compares patterns of supersaturation in the A2,VPM, A1 and GF profiles

Deep stop skew similarity
Compares and index of how much a profile "skews" toward deeper stops

Risk function similarity
Compares a profile risk function for each of A2,A1, VPM and GF.

In addition, Dr. Doolette has a presentation here that I think is very good.

Happy New Year!

@UWSojourner, thanks for your post and for summarizing the data for us. My questions below to you (and others) is bordered on a more practical level post the NEDU study and several in-depth discussions surrounding it:

1. Post NEDU study, what algorithm/model are you using for Deco dives and Non-Deco dives? Did you use a different algorithm pre-NEDU?

2. Within your current dive algorithm in question 1, how did you alter the conservative factors such as GF or P1/P2 on dive computers? In other words, did you change your GF settings or computer settings, and if so, to what?

3. Since most dives are non-Deco, what is your current non-Deco (within NDL) dive practice? Just to clarify the question, some just use 15 feet for 3 min, others use 30ft@1 min-15ft@1-10ft@3min, etc.

4. Do you use a computer for Deco and NON-Deco dives? If so, what computer do you use? If no computer, what tables do you use?

Tom, AJ and others, please feel free to share your current practice.

 

[Freewillow]

@UWsoJourner,

I am french speaking and I follow a Bühlmann G16 with or without GF and I have a tough time to reconcile it with your nice summary. Could you help by telling me (us) what certain acronisms are ( just the full words behind the letters). Ex RBW, REDU .. Thanks :

 

[Divetech99]

Rbw is rebreather world. It's a site similar to scubaboard where the NEDU study was extensively discussed. Search this thread as someone posted the link. NEDU is the US Navy experimental diving unit that conducted the study. Start with the rbw thread. It will give you a lot of background.

 

[Freewillow]

Thank you for the explanation. I have seen the presentation from Dr. Doolette . Very very good indeed. Thanks for sharing

 

[UWSojourner]

3. Since most dives are non-Deco, what is your current non-Deco (within NDL) dive practice? Just to clarify the question, some just use 15 feet for 3 min, others use 30ft@1 min-15ft@1-10ft@3min, etc.

The NEDU study doesn't really impact how to approach NDL and even light deco. I think the current prescription for NDL is pretty much what its always been ... control your ascent rate and do your safety stops if possible. Whether its 2 at 20 and 3 at 10 or 5 at 20, or 1 at 30 and 4 at 15, etc., etc. probably doesn't matter much.

And know what your computer is telling you. I've seen a number of situations where divers come up complaining that their watch is beeping at them. You look at the computer and its indicating 10-15 minutes of deco, but they didn't understand. It doesn't matter what algorithm you dive if you ignore it.

If you're using the computer to keep you out of deco, or to just do light deco, then VPM, RGBM, GF, etc. seem fine to me. Last time I looked (it was awhile ago) RGBM was pretty conservative close to the NDL, as was VPM. For example, 90ft 25 min air ... VPM-B+3 exits at about 46 min, GF40/70 at about 44min. So if that's the kind of diving you're doing don't ditch your computer based on the NEDU study.

 

[Kevrumbo]

An anecdotal account (and pure speculation of an incident last Nov 2014 to a diver at Blue Lagoon Dive-Ops Truk) --Vestibular/Inner Ear DCS symptoms as a result of not performing Deep Stops, or ascending too fast to the first intermediate deco stop. Again, along with the risk of type II Neuro DCS, you should take this into account when deciding to omit Deep Stops: are you willing to risk a rare but possible Inner Ear or type II DCS hit for the sake of not late supersaturating/loading your Slow Tissues???

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. . .

 

[UWSojourner]

An anecdotal account (and pure speculation of an incident last Nov 2014 to a diver at Blue Lagoon Dive-Ops Truk) --Vestibular/Inner Ear DCS symptoms as a result of not performing Deep Stops, or ascending too fast to the first intermediate deco stop. Again, along with the risk of type II Neuro DCS, you should take this into account when deciding to omit Deep Stops: are you willing to risk a rare but possible Inner Ear or type II DCS hit for the sake of not late supersaturating/loading your Slow Tissues???

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. . .

If you're integrating the information from the NEDU study and are concerned about type II hits, then remember ...

... the deep schedule produced a rate of DCS II cases that was 94% more than the shallow schedule.

... the deep schedule produced a rate of DCS I cases that was 287% more than the shallow schedule.

... the deep schedule produced a hit rate that was 223% more than the shallow schedule.

 

[Kevrumbo]

If you're integrating the information from the NEDU study and are concerned about type II hits, then remember ...

... the deep schedule produced a rate of DCS II cases that was 94% more than the shallow schedule.

... the deep schedule produced a rate of DCS I cases that was 287% more than the shallow schedule.

... the deep schedule produced a hit rate that was 223% more than the shallow schedule.

Then as a practical application, best compromise & precaution, extend your O2 Deco profile to ameliorate the above risk as indicated by the NEDU Study (which btw did not use any hyperoxic Deco Gases at all, only just using inefficient Air Decompression). Otherwise risk a vestibular or Neuro/Fast tissue hit by omitting Deep Stops . . .do you understand?

 

[UWSojourner]

Then as a practical application, best compromise & precaution, extend your O2 Deco profile to ameliorate the above risk as indicated by the NEDU Study (which btw did not use any hyperoxic Deco Gases at all, only just using inefficient Air Decompression). Otherwise risk a vestibular or Neuro/Fast tissue hit by omitting Deep Stops . . .do you understand?

I certainly understand that you're free to respond to the study as you see fit. And dropping your high GF to 60 seems pretty conservative which I think is good if you have the time.

However, I also understand that others are responding differently … most raising their low GF and reducing their High GF as they can. Firm recommendations are difficult, but these posts by Dr Mitchell are worth reading:

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