Hi Tom (with a couple of points for Diver0001),
Simon, the evidence has been on your desk for a while now. Even the NEDU study showed obvious advantages of the deep stop theory in that it proved there is a huge protection from supersaturation at the beginning of the ascent.
This does not constitute evidence that deep stops are effective in reducing bubble formation or preventing DCS. I agree, it is obvious that imposing deep stops will protect fast tissues from supersaturation. I have certainly never disputed that. Indeed, I have pointed out many times that the deep stops in the NEDU deep stop profile did (as you say) protect the fast tissues from supersaturation. You seem to be
assuming that this, of itself, makes it obvious that outcomes must be better if deep stops are performed. But the whole point of the NEDU study is that despite this protection of fast tissues early in the ascent, the deep stop ascent was still associated with worse outcomes. This implies that protecting fast tissues early in an ascent may not be beneficial, and that the harm arising from increased supersaturation of slower tissues later in the ascent as a consequence of performing the deep stops outweighs any benefit of performing the deep stops.
You say this is because the deep stops were too long in the NEDU study. OK, lets shorten them, but keep the total length of decompression the same and compare tissue supersaturations with a "shallow stops" decompression. If the decompressions are the same length and the deep stops profile distributes more of that decompression time deeper then it is a physical inevitability that the deep stops profile will result in more slow tissue supersaturation later in the ascent. Maybe with shorter deep stops the differences in outcome between the two dives might be less obvious and more difficult to detect in a study of practical size, but why would you expect the deep stops profile to now be safer?
You seem to be putting a lot of faith in the notion that the
short deep stops will control bubble formation early without causing excessive disadvantage in terms of slow tissue supersaturation later, but one of the reasons I am comfortable interpreting the NEDU study the way I do is that other studies that have tested dives with short "tech dive style" deep stops do not support your assumption. Indeed the opposite seems to be true. Ironically, bubble models do not appear to control bubble formation very well. That is why Ross spent a lot of time on the RBW thread and elsewhere trying to down play the relevance of venous gas emboli counts in decompression research.
You really can't see where there may be a middle ground? I find that imrobable. The shallow stop model suffered some DCS too. Do you suggest it is improper to assume that if the enormous amount of supersaturation at the beginning of ascent had something to do with that? If you could eliminate that initial supersaturation by adding deport stops for a very short time, and the occurance of DCS plummeted, would that be evidence enough for you that deep stops have a purpose if done correctly?
As I implied in my earlier post to you, I can totally see that there is a "middle ground" or "sweet spot" or some other way of putting it. I just don't think it involves stops as deep as bubble models typically impose them.
Can I make a related point about the assumed benefits of protecting the fast tissues early which may not be obvious to you, or to Diver0001 (thank you for your interest and very reasonable posts by the way)? It is a great idea in theory (which is why we all jumped aboard the bubble model bandwagon in the early 2000s). However, it may be that is largely unnecessary, or at least ineffective from the point of view of preventing bubble formation and / or DCS. Even when fast tissues get supersaturated, they don't stay supersaturated for long (because they are fast). The window of opportunity for bubble formation is therefore short and these tissues may be quite resistant to bubble formation (certainly relatively so). In the evolving pathophysiological paradigm for DCS in which circulating venous gas emboli are increasingly viewed as key players (especially when they get into the arterial circulation) it is the slower tissues where these emboli almost certainly arise that are probably the most important. It may therefore be that the "curve" created by deep stops that you and Diver0001 like to see and consider to be a fundamental part of appropriate decompression may not be as necessary or even desirable as might seem intuitive. Indeed, it may even be a disadvantage because while the fast tissues are being protected in the early part of the curve, the slow tissues are still loading. This hypothesis is consistent with the emerging data in which deep stop and shallow stop approaches have been compared (including the NEDU study).
Finally, if you are asking me whether there is benefit in a middle ground in which some short deep stops are conducted early, and compensated later in the ascent by increased shallow stops to offset increased gas uptake by slow tissues I would have to say that I don't have a definitive answer to that. I doubt it (for the reasons articulated in the paragraph beginning "You say..above), but it has never been tested. But let's not lose perspective on the debate, which has been about deep stops as imposed by bubble models, and the promise of bubble models (at least initially) was that they could produce more efficient (and therefore shorter) decompression
because of the deep stops, and that you could even shorten your shallow stops as a result. I really do think that notion is discredited.
Simon M
