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'?
Hello Tom,
Thank you for engaging in this discussion constructively.
There is one important point to clear up. The NEDU test profiles were not "manipulated" or arbitrarily stretched etc etc as has been implied several times. They were the outputs of two models (one gas content model and one bubble model) developed by the USN. The gas content model is actually in use in the USN in certain applications, and one aim of the study was to help decide whether the bubble model should be used instead. The decompression times seem long in relation to what you / we are accustomed to but remember these were air dives with no oxygen decompression. Also, the USN models have to some extent been calibrated against navy databases of dives of known outcome and these longish durations are what is required (according to their data) to decompress from typical navy dives with an acceptable risk of DCS. Remember too that a typical navy dive will usually involve working harder and maybe getting colder than many of us are accustomed to. Just performing significant work during the bottom phase of the dive markedly increases the decompression time requirement for safe decompression and that is why the NEDU study replicated typical navy dive conditions by imposing significant work throughout the bottom time. Finally, the NEDU bubble model profile is not markedly different from one prescribed by VPM running on a high conservatism level around +7 (which falls within the original model parameters) or from one that was actually recommended for the same dive by Bruce based on RGBM at the 2008 Deep Stops Workshop in Salt Lake City. The latter was discussed in Wayne Gerth's paper in the Proceedings of that workshop. I don't know whether you have the document, but if you PM me your email address I would be happy to send you the pdf.
Thus, based on the above, I don't think it is possible to say that the outcome would be highly predictable.
However, to truly appreciate the significance of the NEDU study outcome you have to think a bit more deeply than mere consideration of what the profiles "look like" or how they compare to the profiles we typically use. The "truth in the universe" that we are seeking in such a study is whether distributing stop time deeper is a good strategy. We all know that you can do a decompression with lots of deep stops safely, especially if there are also lots of shallow stops too. But the crucial question in respect of the efficacy of deep stops is that if you have a fixed amount of decompression time, is it better to spend more or less of that time deep. This was the case in the NEDU study where there was a fixed amount of decompression time, and the bubble model distributed more of the stop time deep in comparison to the gas content model. Obviously you know the result.
That result occurred despite the fact that the NEDU bubble model profile did what bubble models are supposed to do: it protected the fast tissues from supersaturation early in the ascent, but at the expense of increased supersaturation late in the ascent (which is virtually inevitable if you distribute a fixed amount of decompression time deeper). This has been depicted in various ways, but it can be clearly seen in the heat map diagrams created by Kevin Watts which I linked to in my previous post. This suggests that the fundamental premise of bubble models (that controlling supersaturation and therefore bubble excitation early is protective) may be flawed. And do remember as you criticise this recent data, that this premise was completely untested; there was NEVER ANY clinical outcome data that supported bubble models. We all just jumped on board with the idea (yes, including me) because it was so theoretically attractive. To be explicit, the NEDU study appears to tell us that protecting fast tissues early at the expense of greater supersaturation in faster tissues later may be a flawed concept. The study is suggesting that the clincal problems are arising from tissues that we are allowing to supersaturate to a larger degree as a result of having done deep stops.
Is all this relevant to the profiles we dive?? I think the answer is a qualified yes. The same difference in the pattern of supersaturation in faster and slower tissues that appeared disadvantageous in the NEDU study can be seen in real world bubble model profiles. This is readily apparent if you look at those heat map diagrams I linked to earlier which compare CCR constant PO2 mixed gas profiles generated by VPM-B+4 and GF 40-74. However, it is probably true to say that the use of oxygen decompression, and the various modifications that seem increasingly common these days (like padding the final oxygen stop to some extent) probably diminish any difference in risk between profiles.... possibly substantially so. Thus, the real difference in risk between profiles with different emphasis on deep and shallow stops as dived in the real world may be significantly less than implied by the NEDU study..... BUT if you want the "truth in the universe" then I think the NEDU study is correct.
For the reasons above, and as I have said many times previously, I am NOT saying don't use bubble models, or don't do deep stops. I just think that discourse on this subject is too heavily tainted with emotion and 'religeon' and the emerging signals (of which the NEDU study is only one, and which divers are entitled to understand) are being obfuscated. The optimal decompression paradigm is far from established but I do think that it is fair to suggest that we are in a phase of reconsidering how much emphasis should be placed on deep stops.
Simon M
---------- Post added December 24th, 2014 at 10:39 AM ----------
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?)![]()
Hello Kev,
Sorry... I have only just noted your PM. I was not ignoring you. It was a pleasure to help you out. A look forward to that beer some time.
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):
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.
I have tried to articulate my thoughts on the real world signficance of the NEDU study in my reply to Tom above. You actually reflect some of them in your commentary, eg extending the shallow oxygen stop. I am less convinced that the air vs helium thing is significant. As I said above, the apparently disadvantageous pattern of supersaturation distribution between faster and slower tissues in profiles that emphasise deep stops to a greater or lesser degree (see the heat map diagrams i linked to previously) also occurs in real world mixed gas dives with oxygen decompression.
I had an entire month at Bikini earlier this year.... pete and I did some fantastic exploration in the Saratoga and we are getting better at using remote strobes. I won't go to Bikini in 2015 but am the doctor on his Truk trip in November.
Hope you are well and happy and enjoying your diving.
Simon M