Hello Mikeny9,
You cannot dismiss the relevance of the NEDU study to decompression diving by technical divers using high oxygen content gas just because the divers in the study were using air. The reasons for this were articulated at length in the original deep stops threads on rebreather world. Those threads are still available, and I would strongly suggest that if you really are interested in this subject that you read them. Put simply, if the study had incorporated oxygen decompression then the difference between deep stop and shallow stop outcomes would probably have been smaller and perhaps not detectable to a level of statistical significance in a study of pragmatic size, but there is no reason to expect that the "truth in the universe" about the disadvantage over-emphasizing deep stops would be any different. Indeed, the same disadvantageous patterns of supersaturation distribution across fast and slow tissues that form the most plausible explanation for the NEDU study results have been shown to exist in bubble model decompressions from typical technical rebreather dives.
You have asked several times about the current state of science around this subject. In this regard it is possible to make some useful general observations.
First, there is not and never has been ANY evidence supporting the emphasis placed on deep stops in decompression diving by bubble models, ratio deco, or GF approaches chosen to emulate bubble models. Whatever you think about the studies that I discuss below, you need to keep this key fact in mind at all times.
Second, every bit of related science that has emerged in recent times suggests that bubble models, ratio deco, and GF approaches with a low GF-lo place too much emphasis on deep stops. That evidence can be summarized as follows:
1. The NEDU study. Previously extensively discussed. A truly seminal piece of work which is never likely to be repeated. There has been a lot of contrived criticism of this study mainly from people with a vested interest. We could go on at length about this (and indeed we have), but the thing we should all remember is that every other bit of contemporary evidence (see below) which has emerged appears to corroborate the NEDU study findings.
2. The recent
Spisni study comparing RD and GF 30/80. This study is particularly interesting because it contained a design flaw in that the RD decompression was longer than the GF one. If you want to evaluate the efficacy of the deeper stops and the RD S curve in comparison to another approach which involves shallower stops and no S curve, then you at the very least need to make the profiles the same length (because, all other factors being equal, a longer decompression should always be safer). Put simply, the experimental design imposed a significant inbuilt advantage for the RD approach. Despite this, there was a trend toward less instances of high bubble grade in the divers using the GF approach (which did not reach statistical significance) and inflammatory markers were more elevated in the divers using RD. You have asked several times what is the evidence that debunks RD. “Debunks” is probably too strong a word, but this study provides a signal that the RD approach (as it stood at the time) was inferior to another commonly used approach with shallower stops and no S curve. Moreover, there is some evidence (see the Swedish navy study below) that the comparator for RD in this study (GF-lo of 30) also places too much emphasis on deep stops. In other words RD may have looked worse if compared to an approach with even less emphasis on deep stops.
3. The Blatteau study published in Aviation Space and Environmental Medicine in 2005. They showed more venous bubbles after a deep stop decompression compared to a traditional gas content model with shallower stops.
4. A Swedish Navy study that has been presented at several conferences but which is not yet published. This compared decompression with GF lo = 30 to DCAP (which prescribed shallower stops) from decompression dives. Again, higher bubble grades in the deeper stop dives.
5. The Ljubkovic study from Journal of Applied Physiology which showed almost universally high bubble grades after decompression from trimix dives using VPM. This was not a comparative study, but it did illustrate that bubble model decompressions do not reliably control bubble formation as has been claimed in the past. This study spawned Ross’s ridiculous crusade to rewrite the DCS pathophysiology text books with his claim that venous gas emboli are irrelevant.
6. Neal Pollock’s work at inner space and on deep diving research cruises. Also not published yet, but presented at multiple conferences (including SPUMS last week) and medical courses. He has been monitoring divers for bubbles after deep decompression dives and correlating findings with the profiles. He has found apparent success in decreasing bubble grades by de-emphasizing deep stops, and (in particular), by padding shallow oxygen stops (a strategy which Ross actively disputes).
With the exception of the NEDU study all of these involved gases other than air. You are correct to point out that only one of them relates directly to RD, but RD emphasizes (or at least emphasized) deep stops as a central part of its approach. Collectively these studies offer accumulating evidence that deep stops as prescribed by bubble models and RD in decompression dives are too deep. The Spisni study (albeit a single small study) offers evidence that this concern is not adequately compensated for by imposition of the S curve and exploitation of the oxygen window as I have heard claimed. And I reiterate my first general point above: all of this needs to be considered against the fact that there is not a single study I am aware of that supports bubble model / RD type emphasis on deep stops.
Simon M
