ajduplessis
Contributor
What would you do differently? Can you be a little more specific about how you would design a study on this subject ?
I would compare apples with apples and stop blowing smoke up others a$$es.
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What would you do differently? Can you be a little more specific about how you would design a study on this subject ?
I would compare apples with apples and stop blowing smoke up others a$$es.
Do you think the profiles generated represents typical deep stop within the VPM model?
"Blowing smoke up people's asses" is not a phrase usually associated with quality scientific discourse.
David Doolette on RBW:The Navy Experimental Diving Unit (NEDU) deep stops study (Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. Technical Report. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. Report No.: NEDU TR 11-06) was undertaken to determine if deep stops decompression schedules, such as those prescribed by bubble decompression models, are more efficient that the traditional shallow stops schedules prescribed by Haldanian models. More efficient in this context means a decompression schedule of the same or shorter total decompression time has less risk of decompression sickness (DCS) than an alternative schedule. Theoretical analysis at NEDU and by others had suggested this might be the case, and bubble models were being considered for calculating air decompression tables to replace the Standard Air Decompression Table that had been in the U. S. Navy Diving Manual since 1959, but this big change required a test.
To be clear about the purpose, methods, and outcome of the study, we need to be clear what is meant by decompression efficiency. The purpose of a decompression schedule is to reduce the risk of DCS to some acceptably low level. The cost of a low risk of DCS is time spent decompressing; efficiency relates to this cost/benefit trade off. In comparing two decompression schedules, if one could achieve the same target level of DCS risk with a shorter total decompression time than the other, the shorter schedule is more efficient.
With this definition in mind, one way to test if a deep stops schedule is more efficient than a shallow stops schedule would be to show that a deep stops schedule has the same (or less risk) than a longer shallow stops schedule. However, this is not a good scientific design because you are varying two things, stop depth distribution and total decompression time , and you will not know which was responsible if the result does not show deep stops to have lower risk. A better scientific design is to compare a deep stops schedule and a shallow stop schedule that have the same total decompression time and see which is riskier - only one thing is varied, the stop depth distribution, and any difference can be attributed to that alone. This latter is the method we used.
Remembering that the purpose of a decompression schedule is to reduce the risk of DCS, the definitive way to evaluate a schedule is to conduct many man-dives, following the schedule exactly, and count the incidence of DCS; the incidence of DCS is an estimate of the risk and the more man-dives the more confidence there is in this estimate. To compare two schedules, dive both, and count which results in more DCS. Contrary to what has been suggested in this thread, it is meaningless to compare the decompression efficiency of two schedules that are very unlikely to result in DCS imagine conducting a thousand man-dives on each of two schedules with no DCS occurring, all you have learnt is both schedules are very low risk, and probably quite inefficient.
So this is the experiment. In the wet pot of the NEDU Ocean Simulation Facility, where we can precisely control depth, time, water temperature, divers workload (all things that influence DCS risk), divers undertook two different profiles. Both profiles were to 170 fsw for 30 minutes during which time the divers exercised on cycle ergometers, followed by 174 minutes of decompression stops during which divers were at rest. The water temperature was 86 °F (30 °C) and dives wore only swimsuits and t-shirts and became cold during decompression. Divers were submerged and breathed surface supplied air throughout. More on all these conditions later. The only difference between the two profiles was the distribution of the total stop time among stop depths. The shallow stops schedule had stops of (fsw/minutes): 40/9; 30/20; 20/52; 10/93. The deep stops schedule had stops of (fsw/minutes): 70/12; 60/17; 50/15; 40/18; 30/23; 20/17; 10/72.
We planned to conduct 350 man-dives on each schedule, but to protect the diver-subjects from unnecessary risk, we also had several rules by which the experiment would stop early. We had stopping rules if both schedules had unexpectedly high or low risk, which were likely to result in severe DCS or an inconclusive result, respectively. We never came close to these (the figure presented in an earlier post is misinterpreted). We were also to stop if, at an interim analysis, we saw a statically significant higher incidence of DCS on the deep stops schedule than the shallow stops schedule, and this is what happened. At approximately the mid-point of the experiment we had 10 DCS out of 198 man-dives on the deep stops schedule and 3 DCS out of 192 man-dives on the shallow stop schedules. Incidentally, we also measured venous gas emboli (VGE) and these were higher on the deep stops than the shallow stops schedule. As Simon pointed out in a post, this is, in statistical terms, only moderately strong evidence that the deep stop schedule was riskier than the shallow stops schedule. So why did we stop? Because it is very strong evidence that the deep stops schedule is not better and, because deep stops better was the only result of any consequence to the U. S. Navy (shallow stops are the status quo).
I want to talk about the schedules we tested in some detail, because these have been the source of a lot of confusion and misdirection in various forums. Clearly they do not look like technical diving schedules - they are not, they are deep air decompression schedules. In selecting the test pair of schedules, there were two principal criteria. First, they had to result in some DCS so there was something to compare. Second, they had to be long, so that they could have substantially different stop depth distribution, i.e. the deep stops schedule should require a substantial amount of time at deep stops, so any deep stops effect (good or bad) can manifest. There is no point in testing, for instance, two 90-minute decompression schedules where one has five or ten minutes of time spent at deeper stops I would happily move five or ten minutes around in a 90-minute schedule and not expect it to make a any detectable change in my risk of DCS. Remember that the purpose of a decompression stop (deep or shallow): we stop to limit gas supersaturation and thereby limit bubble growth, and we stay to washout inert prior to moving to the next stop. The staying is important, the amount of gas washout that occurs in the course of one, two, or five minutes is relatively inconsequential.
The final test pair of schedules was the result of hundreds of hours of analysis and even a workshop attended by many people working in the field of decompression (acknowledged in NEDU TR 11-06). The shallow stops schedule was calculated using the VVal-18 Thalmann Algorithm. This algorithm was developed at NEDU for air and constant PO2-in-nitrogen rebreather diving and about 1500 man-dives were conducted during its development (NEDU TR 11-80, NEDU TR 1-84, NEDU TR 8-85). VVAl-18 Thalmann Algorithm is still very much in use, it runs in the U. S. Navy Dive Computers, desktop decompression software, and was used to calculate the MK 16 MOD 0 and MK 16 MOD 1 N2-O2 decompression tables in the U. S. Navy Diving Manual. For a 170 fsw / 30-minute bottom time air decompression dive VVal-18 requires the 174 minutes of decompression stops given above for the shallow stops schedule. Although this particular schedule was not tested during the development of Val-18, many deep, long air dives were, and lengthy air decompression was required. Many U. S. Navy dives are conducted with the diver working on the bottom and, because wet suits are often used, cold during decompression. This combination makes for a lot of required decompression because blood flow is increased, and therefore inert gas uptake is relatively fast, during the working bottom time, and blood flow is decreased, and therefore inert gas washout is relatively slow, when divers are at rest and cold during decompression. U. S. Navy decompression algorithms are designed to account for this worst case situation and tested under these condtions. Just to clarify some comments in this thread about the effects of cold, cold can increase the required decompression time, but cold does not cause DCS.
It has been suggested in this thread that 174 minutes decompression is 100 minutes too long for a 170 fsw / 30-minute dive well, of course, you can do 74 minutes of decompression if you want, if you accept a high risk of DCS. In fact, 74 minutes is close to the time required in the new Air Decompression Table in the U. S. Navy Diving Manual Revision 6 (2008): 170 fsw / 30 minutes requires 88 minutes of air decompression stops, which has an estimated risk of DCS of about 6% (NEDU TR 09-05), but this exceptional exposure schedule is for emergency use only (this dive is required to be planned using the lower risk oxygen decompression schedule). Why didnt we test this schedule? It is not long enough to allow meaningful redistribution of time to deep stops.
Now the deep stops schedule. This was calculated using a model called BVM(3) (Gerth & Vann Undersea Hyperb Med 1997;24:275-292). BVM(3) is a Bubble Volume Model, and models in this class (Mike Gernhardts TBDM is another example) model the growth and dissolution of bubbles using the equations that describe exchange of gas between tissue and blood (a feature of most decompression models) and the equations that describe diffusion of gas between spherical bubbles and surrounding tissue (the characteristic of this class). BVM(3) output is the estimated risk of DCS for a dive profile, and this risk is a function of bubble volume and duration in each compartment. BVM(3) is used in conjunction with an exhaustive search algorithm to find the optimum decompression schedule (under the model). This can be done two ways. First, you can specify a total decompression stop time, and an exhaustive combinations of stop depths and times (that add to the total) are tested to find the schedule that gives the minimum estimated risk. Second, you can specify a target risk, and the first step is repeated with different total stop times, searching for the shortest schedule that just reaches the target risk. We used the first step, and specified 174 minutes total stop time (the VVal-18 total stop time) and had the model find the optimum distribution of that time, which resulted in the deep stops schedule specified above. Actually, we examined hundreds of candidate schedule pairs until we decided on the 170 fsw / 30-minute dive.
To interpret our results, I have to describe some Decompression 101 theory, so this may a bit basic for a lot of you, and for brevity I am going to confine the description to diving on a single gas (e.g. air diving, as in the experiment) although it is possible to extend to multiple gas. The purpose of a decompression stop is to limit bubble formation and allow washout of tissue inert gas. Deeper stops are generally controlled by faster exchanging (short half time) compartments and shallower stops by relatively slower exchanging (long half time) compartments. Bubbles form and grow only while tissue is supersaturated and shrink when tissue is undersaturated. In a supersaturated tissue, at a deeper stop (compared to a shallower stop) less bubbles form, they will grow less rapidly, they will dissolve more quickly, and in some circumstances inert gas washout can be faster. This is all good stuff and the motivation for deep stops. However, the NEDU results indicate that emphasizing these effects in fast tissues by doing deep stops is not as important as previously thought, because our shallow stops schedule, in which fast tissues had substantial supersaturation, resulted in very few cases of DCS. So why did the deep stops schedule result in more DCS? We looked at the supersaturation predicted in a range of half-time compartments. In fast compartments, the deep stops schedule resulted in less, and less prolonged supersaturation than the shallow stops schedule. However, iIn slow compartments, gas washed out slowly or continued to be taken up during deep stops, so that later in decompression, the deep stop schedule resulted in more, and more prolonged, supersaturation than the shallow stops schedule. The increase in supersaturation in the slow compartments was greater than the decrease in fast compartment. There is a principal, Occams Razor, that roughly means the simplest answer is the preferred one. The simplest answer here is that the greater supersaturation (and by extension greater bubble formation and growth) is responsible for the greater incidence of DCS on the deep stop schedule. In other words, the cost of doing the deep stops outweighed any benefit. And remember, any benefit was slim, because there was very few DCS in the shallow stops schedule.
So an important question is how relevant is this result to other deep stops schedules, or put another way, is there another deep stops schedule that would have given the reverse result. Accepting the explanation that greater supersaturation is the culprit, we modeled the gas supersaturation in a range of half-time compartments for half a million different schedules, each comprising 170 fsw / 30 minutes followed by 174 minutes of decompression stops, but with different combinations of stop depths (deepest stop 100 fsw) and times. At the level of granularity we chose (5-minute blocks of time was the shortest we moved) we looked at all reasonable shapes of decompression schedule. As it turned out, the VVal-18 shallow stops schedule resulted in near the least combined (adding together the fast and slow compartments) supersaturation. Moving a small amount of time to deeper stops resulted in no improvement, and moving any substantial amount of time to deeper stops resulted in more combined supersaturation. This would suggest that there are some schedules with a little bit of time at deep stops that are no worse than the shallow stops schedule, but most deep stops schedules will be worse. Clearly, this theoretical analysis is not proof, but it is a compelling hypothesis, and I am very confident we would not have gotten the reverse result (deep stops better) if we had tested another schedule.
So what is the relevance of this to technical diving? For that I have to speculate a bit because I am moving away from the facts of the study. First let us deal with the issue of whether this applies to helium-based breathing mixtures. Probably. Blatteau and colleagues have done a small comparison of deep stops versus shallow stops open circuit trimix decompression profiles, using VGE as an endpoint and found more VGE with the deeps stops (Proceedings of the Decompression and the Deep Shop Workshop) and there is another, as yet unpublished, similar study with similar results, although using algorithms familiar to technical divers. The more important issue is that technical divers do not do air decompression dives, they use oxygen-accelerated decompression. If decompression stops are conducted using a breathing mixture with a low inert gas fraction, then, of course, there is less gas uptake into the relatively slow compartments. The effect of this is to increase the depth at which stops become bad deep stops.
Ross,
Re:
1. "shooting my foot off". I feel I am at very low risk of shooting my foot off in an argument about decompression and DCS. Having treated a number of the sort of cases (fulminant DCS) you refer to after omitted decompression I am well aware that extremely high tissue supersaturations can produce bubbling and symptoms with almost no delay. I am also aware that very low levels of supersaturation may be sustained for very long periods without the development of problems. However, these facts do not constitute a valid argument against time being important at levels of supersaturation between these extremes. Indeed, since you raised the example of surface decompression on oxygen, would you care to explain why the various tables instruct you to get the diver across the deck and recompressed within 5 - 7 minutes? If the integral of supersaturation and time didn't matter, why not come up, have a coffee and a scone, and recompress later (or even at all?). Since you're "more intelligent that to get suckered into this "integral of supersaturation and time" nonsense" I'm sure you can explain this to me.
Simon M
This is especially so when it comes at the cost of increased supersaturation (both in terms of peak levels and duration) in slower tissues later in the ascent.
and ...
However, these facts do not constitute a valid argument against time being important at levels of supersaturation between these extremes.
and...
3. The Haldane and Schreiner equations. A complete red herring. There is nothing wrong with them. I have no idea why you raised this.
Simon M
4. The DCIEM tables. I am well aware of the pedigree of these tables. However, I challenge you to provide any evidence that they tested 170ft / 30 minute air decompression dives with human subjects working at at the same intensity as the NEDU divers, in 30 degree water and no thermal protection, with sufficient repetitions to derive an accurate probability of DCS for the 77 minute decompression profile that their tables prescribe. In other words, do they really know the risk of DCS for their decompression following a 170/30 dive performed under the conditions of the NEDU study?
Simon M
5. "these are two shallow stop profiles": no they are not. They are profiles that generated tissue supersaturation patterns typical of one that emphasizes deep stops (like your own algorithm) and a one that does not emphasize deep stops. The prevention of fast tissue supersaturation early in the decompression at the expense of greater slower tissue supersaturation later in the decompression was shown to produce more DCS. Sorry Ross. That is the reality.
Simon M
2. "to trick the public into a false belief": Why on earth would I or any of the other scientists involved in this debate want to do that?
Simon M
Hello Kev,
The bubble models and the deep stop approach were originally promoted on the basis that they were more successful at controlling bubble formation. The attempts to evaluate this notion in decompression dives in humans that I am aware of have shown that gas content models (or decompression procedures that have backed off deep stops to some extent) actually produce less bubbles when measured after surfacing. Neal Pollock presented some fascinating work they have been doing at the inner space event at a NOAA / AAUS rebreather diving forum I attended last week. Hopefully this will find its way into the literature at some point soon. In any event, the more we investigate it, the more the "control bubbles by deep stopping" concept appears to need reconsideration. What this is suggesting is that the bubbles are coming from the slower tissues that absorb more inert gas during the deep stops. It also implies that the faster tissues that deep stops attempt to protect from supersaturation are less prone to bubble formation when they become supersaturated. You are seeking a physiological explanation for this, and while I can't be definitive, I would suggest that it makes sense that a tissue washing inert gas out quickly might be less prone to bubble formation and growth than a tissue with slower inert gas kinetics where the supersaturation persists for longer (there's that time integral again).
Simon M
1/ You can read all about SurD decompression procedures in the USN dive manual (sect 9.8.3), including contingencies for missed procedures.
The point of bringing up the SurD supersaturation, is that it demonstrates there are limits to high initial supersaturation. Yes its tolerable for a brief period(3 1/2 mins), but it must be addressed immediately with O2 treatment. They are basically doing part of a re-compression treatment. They have taken a great (life threatening) risk, and then patch it up quickly with a lot of serious O2 treatment.
So your contention that the Fast tissues can be ignored, does not seem to hold true.
ZHL has more gas pressure stress in the dive, and less on the surface. VPM has less gas pressure stress in the dive and more on the surface. They both attempt to balance up the gas pressure stresses across the dive and surface periods.
Because you don't seem to understand what or where this "integral of supersaturation and time" detail comes from:
You do realize that Haldane and Schreiner equations, include a time component? They are tracking pressure over time. The duration of a pressure level is accounted for within the formula. The period and sustained levels of supersaturation can be retrieved from Haldane and Schreiner equations, and all our currently used deco models address this in the same way.
But you seem to want to add a second layer of time components on top of that with "integral of supersaturation and time"? Your trying to apply a time component twice, and one is magnified against to the other. That's not right.
Here is yet another example of why "integral of supersaturation and time" is an ambiguous nonsense.
3000 kPa/mins = riding the elevator to the 30th floor and staying there 50 hours.
3000 kPa/mins = a 32% EAN dive within the NDL range.
3000 kPa/mins = a small deco dive.
3000 kPa/mins = 150 mins of normal flight in pressurized aircraft.
3000 kPa/mins = 15 mins surface transfer times for a SurD diver - serious injury / possible death
3000 kPa/mins = 40 mins of blood boiling total pressure loss for a space walker - death.
Your "integral of supersaturation and time" needs more work. In its current state it's an ambiguous nonsense.
I'm sure that the old DRDC team will be very happy to see you hold their work on DCIEM tables in such low regard.
I forgot to mention, the USN when it published it latest tables, it too ignored the nedu test outcomes. The standard USN table for this dive calls for 93 mins deco.
It seems that the Naval dive command (who decide on the table sets and publishes them), they didn't take any cues from the nedu test either. The published USN tables for this dive is consistent with times from DCIEM, ZHL, VPM-B, RGM, and just about everything else.
Here we have two table sets - the DCIEM and the USN rev6, that give realistic deco times, while the nedu test used 2x (double) the required time.
You wrote above "performed under the conditions of the NEDU study". So now you have changed positions. All throughout the RBW thread you claimed the nedu test conditions were normal to regular diving. But now they have been elevated that to special conditions.
I think the astute scientist would have looked into the reasons for the nedu test anomaly, and explored and explained why the test outcomes are so far outside the normal experience. But no one involved with the test, or in recent commentary has done that. In fact all we have seen so far, is a concerted effort to hide the anomaly, to pretend it's not there, to play games with descriptions and language, and omission and Half-Truths.
Simon wrote: "The prevention of fast tissue supersaturation early in the decompression... "
Ahh... no. The A2 profile has a high supersaturation in the fast cells. The premise you made, was not tested.(with more commentary and graphs)
Here we go again.
VGE (Venous Gas Embolii: circulatory micro-bubbles detected with Doppler) is NOT DCS
VGE in NOT an indicator of impending DCS.
DCS is normally from tissue micro-bubbles,which are not VGE
Deco models work to prevent tissue microbubble growth, not VGE.
VGE has been with us since Spencer first documented them in early 70's. Virtually every diver, including NDL dives, will experience some form of VGE, because VGE is supersaturation generated - something that is present in every dive we do.
But still you try to link these two aspects of deco on a 1:1 basis. More trickery... again.