Deep Stops Increases DCS

[tomfcrist]

Simon. As a leader in your field, answer me this one simple question.

Is the increased occurrence of DCS a direct reflection of doing extremely prolonged deco stops between 40' and 70'?


Additionally, without even running the study, would the outcome be highly predictable solely based on the suggested profiles?

To me, sight unseen it would be obvious what would happen.

What is fundamentally different about a real world deco plan on any bubble model available for a dive like this? The time spent at the deep stops was unrealistic. The trade off became no longer worth staying deep. There is balance in every deco strategy as we all well know, and this study put an extra 100lbs on the left side of the scale.

 

[ajduplessis]

The problem IMO with the study is that the 2 groups used different profiles, but never swapped the groups/profiles to see if the numbers added up again. The results show one polulation sample incurring more DCS than the other. Maybe the one group was more prone to DCS that the other in the first place?? The tests and results for me means nothing.

What I will agree on is that most divers that utilise deep stops, stop too deep, add extra time to their stops or move to slow between stops in general.

The other problem is that the folks that do less severe deco dives are now confused as to what they should be using.

 

[Kevrumbo]

Tom,

You are missing the point. Of course it is possible to complete safe decompressions using any of the commonly available models, and yes, it would be difficult demonstrating significant difference in risk between any two algorithms in real world diving; there are just too many variables - including how the algorithm is applied. The question addressed by the NEDU study was which approach is more efficient. Thus, if you have two profiles that are conducted in identical manner (as described in my previous post) other than the distribution of stop depths, which one results in the least DCS? In the NEDU study the deep stops approach appeared less efficient which probably also means that if you do keep all other factors equal, a deep stops algorithm may be less safe. But this does not mean that a deep stops algorithm cannot be used to perform safe dives. I think this is the message David is trying to convey, and you are misquoting him to some extent.



I can assure you that my views are very confluent with David's.

Tom, I did not come on here to get involved in another debate about deep stops. All of the points you have raised have been addressed in detail on other forums; particularly the RBW deep stops thread that others have linked to. If you really are interested in understanding this subject more completely, then your time would be well invested in reading that thread. In particular, you will find many posts that address the likely relevance of the NEDU study to technical diving.

The perspective that the study is irrelevant because profiles don't look typical of those used in technical diving is simplistic in the extreme. The most plausible reason for relative failure of the NEDU deep stops profile has been investigated, and its potential relevance to the profiles we dive has been explained multiple times. One relevant post can be found here:

Decompression Profile Project - UTD Ratio Deco vs Bulhmann - Page 51

I do agree that the title of this thread is somewhat pejorative, and it would not have been my choice. We do not know what optimal decompression is, but there is a strong signal in the emerging scientific literature that there it may be disadvantageous to over-emphasize deep stops, especially if there is not a corresponding extension of shallow stops to manage slow tissue supersaturation. What, exactly, constitutes "over-emphasizing" is not certain, but it may extend to some profiles generated by current deep stop algorithms. This perspective is certainly a change from the prevalent data-free belief of a decade ago that deep stop approaches were superior.

Simon M

Thank you again Simon, for the practical real world "caveat" to take away from the NEDU study, and your lecture/recitation during our 29June -11July Bikini Atoll Expedition 2013 -and most of all gratitude for the post-IWR follow-up treatment as my attending physician during that trip (I owe you a bag of IV Plasmalyte and Caldolor, but will you take a pint of beer next time we meet?)

And now we come back to what I've been saying since Post #2:

Look at the Bottom Mix Gas used in the NEDU Study link above (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 main practical point IMO/IME, to take away from the study:

Of course you're going to have significant residual inert Nitrogen and potentially on-gas N2 at your deep stop & perhaps even possibly at intermediate deco stops on Eanx50 which may encroach on critical slow tissue M-values as well --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 IWR contingency protocol.

 

[tomfcrist]

The problem IMO with the study is that the 2 groups used different profiles, but never swapped the groups/profiles to see if the numbers added up again. The results show one polulation sample incurring more DCS than the other. Maybe the one group was more prone to DCS that the other in the first place?? The tests and results for me means nothing.

What I will agree on is that most divers that utilise deep stops, stop too deep, add extra time to their stops or move to slow between stops in general.

The other problem is that the folks that do less severe deco dives are now confused as to what they should be using.

The one large group dive both profiles. There were a few test subjects that only dove once, however the majority dove both profiles multiple times.

 

[shoredivr]

Did you just say that Dr Mitchell hasn't read the study?

Rebreatherworld thread has a rossh, this one has a tom.....

 

[LiteHedded]

rossh is financially invested in VPM. i assume Tom is an RGBM guy
not surprising

 

[tomfcrist]

rossh is financially invested in VPM. i assume Tom is an RGBM guy
not surprising

For what its worth, I'm not invested in either. I use RGBM for certain dives and ZHL16 for others.

 

[LiteHedded]

for what it's worth I've done a ton of VPM diving and have never been bent. but there is a growing number of divers that are having issues with it
i'm with GUE these days on deep stops. slow your ascent but making it a 'stop' is probably not the way to go.

 

[PfcAJ]

Isn't rgbm the Naui tech algorithm?

 

[tomfcrist]

Isn't rgbm the Naui tech algorithm?

Yup, the RGBM algorithm is promoted by Naui tech. However, it is not the only algorithm used by Naui tech divers...Many algorithms are discussed, and certainly the pro's, cons and limitations of each are discussed as well. The reality is that unless you can teach tech courses in a location(or on a vessel) where a 3hr SIT is attainable, you are better off jumping to Buhlmann (unless of course you feel the need to spend a couple hundred bucks on Gap software) as the software is readily available and relatively cheap, and the Multi level dive computers that are capable of planning decompression dives generally support that algorithm(as well as VPM and now Liquivision supports RGBM on the Xeo).

RGBM is not the only answer, although neither is Buhlmann.