Michael Guerrero
Contributor
Sorry, just searched again and I'm looking for and commenting on the "Altering blood flow" paper, which was not a manned tests, but was the study the OP referenced.
Will http://www.ncbi.nlm.nih.gov/pubmed/25525213 change deco procedures?
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Ross,
It is a matter of clear, unassailable, public record (on this thread) that YOU raised the allegation that there were "paid trolls" operating on the RBW thread. I have asked you to provide evidence to support this allegation. Your various contorted responses, including this latest one...
...is simply bizarre. There is no other word for it. At least it puts some perspective on things for people looking on.
Simon M
It is a matter of clear, unassailable, public record (on this thread) that YOU raised the allegation that there were "paid trolls" operating on the RBW thread. I have asked you to provide evidence to support this allegation. Your various contorted responses, including this latest one...
...is simply bizarre. There is no other word for it. At least it puts some perspective on things for people looking on.
Simon M
Got it. Actually, when you back off from all of the push-and-shove about this then the result really isn't the least bit surprising. Going back as far as 2003 there has been criticism of RGBM among the deep crowd because it appears to give far too much credit for deep stops as dives get deeper or longer. On the surface of it, the NEDU study appears to confirm what the deep crowd have been saying all along.
That said I can't make heads or tails of the profiles they were testing. They used some model that calculated 174 minutes of deco for a dive to 52m for 30 min. The navy tables prescribe 93 minutes of deco if you grind it out on air and frankly, the other model (the deep one) they tested gives -- to say the very least -- an utterly bizarre ascent profile when spreading that 174 minutes around. NO technical diver anywhere would ascend like that on *any* real-world decompression model.
I guess I must be missing something but what's interesting to me is that on the bubble model they tested the last three stops are in the same ball park as what the Navy tables would have said you needed using Buhlmann except they kept those divers under higher pressures than Buhlmann would have accepted for a FULL HOUR before doing those stops. Therefore, it's not surprising that this profile would give higher rates of DCS because the exposure is skewed to be very deep and very long at those depths, essentially extending the bottom time significantly before doing the required shallow stops. Likewise, the "shallow" profile they tested involved 80 minutes MORE deco than the Navy tables would prescribe in the shallow zone including a last stop which is equivalent in duration to the TOTAL deco time the Navy tables prescribe.
Either I don't get it or the study was engineered (by fault or by design) to stack the deck in favour of the shallow profile and against the deep profile. If they had tested this profile using the Navy tables and either RGBM or VPM then the results would be something divers could understand and use. As it is, I have no idea *what* they were trying to test, but it doesn't appear to be a profile that any diver would use.
That's why I've been fishing to get feedback about the utility of their results. I tried asking the question a couple of times without saying why I was asking, but this is the reason. When I read that study there seems to be literally nothing there that I can apply to the real world except what we already knew which is that certain models give too much credit for deep stops..... again, I presume this is because I'm missing something.
Well... humour me then. I'll be ok with the cliffs notes version if you don't want to go through it all again. I actually don't follow RBW and I'm just catching up with this discussion here because I've been up to my eyebrows in work. I am glad, however, to hear that I'm not the only one that read the report and couldn't interpret what the heck they were actually doing.
R..
R..
The RBW links are blocked in my post 101 on this thread, as the "epic RBW thread" is locked.
Simon Mitchell's cliff notes are here:
Deep Stops (rebreather dive charts) - Page 14
"Ross simultaneously turned his attention to the NEDU deep stops study; a unique piece of work conducted by the only group of full time professional decompression modellers in the world, and amazingly, using DCS in humans as the primary outcome measure. In a comparison of shallow and deep stop profiles of identical duration, conducted in identical carefully controlled conditions, the deep stop profile was associated with a greater incidence of DCS. The most plausible explanation for this finding was that protection of fast tissues from supersaturation early in the ascent (by imposing deeper stops) resulted in greater supersaturation in slower tissues later in the ascent, and that this distribution (or “pattern”
of supersaturation is disadvantageous compared to the opposite one generated by a shallow stops profile. "
And here is David Doolette's post 139 from RBW verbatim regarding the NEDU study:
Deep stops debate (split from ascent rate thread) - Page 14
"I have been reading this thread with some interest, since it has been largely a criticism of my work. Since this is my first post, a quick introduction: my name is David Doolette and I am, among other things, a co-author of the NEDU deep stops study around which much of this thread has revolved. Of course, I think much of the criticism of this report in this thread is misguided, but it would seem self-serving if I debated the criticisms point-by-point, and Simon has done a good job of that. However, I wonder how many readers have seen the NEDU deep stops report. It is not on Rubicon yet (delay at the NEDU end). I see that Simon has offered to send it to a few people, which is great, but that said, it is a technical report, with an intended audience of scientists, so perhaps not very accessible to some. So I thought I might have crack at describing it (for those who do not wish to read the full report) in the context of why the study was conducted and some of the comments on this thread.
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 didn’t 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 Gernhardt’s 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, Occam’s 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.
David Doolette"
Simon Mitchell's cliff notes are here:
Deep Stops (rebreather dive charts) - Page 14
"Ross simultaneously turned his attention to the NEDU deep stops study; a unique piece of work conducted by the only group of full time professional decompression modellers in the world, and amazingly, using DCS in humans as the primary outcome measure. In a comparison of shallow and deep stop profiles of identical duration, conducted in identical carefully controlled conditions, the deep stop profile was associated with a greater incidence of DCS. The most plausible explanation for this finding was that protection of fast tissues from supersaturation early in the ascent (by imposing deeper stops) resulted in greater supersaturation in slower tissues later in the ascent, and that this distribution (or “pattern”
of supersaturation is disadvantageous compared to the opposite one generated by a shallow stops profile. "And here is David Doolette's post 139 from RBW verbatim regarding the NEDU study:
Deep stops debate (split from ascent rate thread) - Page 14
"I have been reading this thread with some interest, since it has been largely a criticism of my work. Since this is my first post, a quick introduction: my name is David Doolette and I am, among other things, a co-author of the NEDU deep stops study around which much of this thread has revolved. Of course, I think much of the criticism of this report in this thread is misguided, but it would seem self-serving if I debated the criticisms point-by-point, and Simon has done a good job of that. However, I wonder how many readers have seen the NEDU deep stops report. It is not on Rubicon yet (delay at the NEDU end). I see that Simon has offered to send it to a few people, which is great, but that said, it is a technical report, with an intended audience of scientists, so perhaps not very accessible to some. So I thought I might have crack at describing it (for those who do not wish to read the full report) in the context of why the study was conducted and some of the comments on this thread.
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 didn’t 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 Gernhardt’s 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, Occam’s 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.
David Doolette"
The issue is and will continue to be the validity of this study in the context that applies to technical divers. There are different ways that study could have been done, but for some reason they never challenged their own findings. The folks that believe this study means that deep stops are crap like to assume that the run times had to be identical, or that the deco time overall had to be identical...therefore they came up with two profiles that they could test. The problem is that both test profiles are ****. Nobody, including the US Navy would dive either of those profiles.
If they really want to make a breakthrough in this area of deco theory, they need to test the variables that they claim are too difficult. Those would include profiles without significant increases in DCS risk, raw profiles based on depth/time that may differ in overall deco time and runtime etc. Until that is done, there will be two trains of thought....those that believe the NEDU study is undeniable evidence denouncing deep stops, and those that can see through the BS and realize the NEDU study is highly inapplicable to technical divers.
For me, the NEDU study did show that "Conservatism factors" are generally BS. They are ok I guess as it applies to NDL diving, but does not translate very well into deco diving.
If they really want to make a breakthrough in this area of deco theory, they need to test the variables that they claim are too difficult. Those would include profiles without significant increases in DCS risk, raw profiles based on depth/time that may differ in overall deco time and runtime etc. Until that is done, there will be two trains of thought....those that believe the NEDU study is undeniable evidence denouncing deep stops, and those that can see through the BS and realize the NEDU study is highly inapplicable to technical divers.
For me, the NEDU study did show that "Conservatism factors" are generally BS. They are ok I guess as it applies to NDL diving, but does not translate very well into deco diving.
Yeah, that's mostly a reiteration of what's in the report. I get that.
I'll just preface what I'm about to say next by being very clear that I don't have a horse in this race. What follows are simply my observations and my interpretations as a diver and nothing more.
First of all I haven't read anything yet that explains why they decided to use two apparently eccentric models for this test that resulted in a Jekel and Hyde of dive profiles that can't be plotted on the map of any realistic possibilities. Why those models? Why this profile? No real world diver would dive either of those profiles. I'm no scientist but I can read a dive profile and one of those profiles is clearly too long and the other is clearly too short. They try explaining the reasoning behind this choice in the article (to make it clear that the results had to do with the efficiency of the models and not something else) but they overdid it, in my opinion, by pushing it to the extreme -- and beyond -- which has undermined their ability to draw any practical conclusions. If they had run that profile using any mainstream deco model the deep part of that ascent would have been much MUCH shorter and shallower.
The reason for the test was to advise the Navy about different ascent strategies but here they have chosen two ascent strategies for this profile that are so extreme that it makes it impossible to draw any useful conclusions whatsoever about the utility of deep stops and/or bubble models. IN fact, I think they knew this, because the only practical conclusion they could reach was that, "controlling bubble formation in fast compartments with deep stops is unwarranted for air decompression dives". Seems to me that the entire report is based upon a faulty premise, namely that these two particular models are somehow representative of real world decompression diving. The choice of models would appear, to my way of thinking, to invalidate the entire study. They are simply not studying the animal that they are trying to describe. It's like saying that you're going to compare the efficiency of brakes on a car with and without ABS but then your test involves comparing a formula-1 car to a go-cart. That would appear to be what has happened here.... At least, that's how it looks to me.
R..
UWSojourner
Contributor
I understand the basis of your frustration. I posted sentiments very similar to yours back in 2007 on theDecoStop when I first saw the study. I was in about the same place when the deep stop thread started. I knew what smart people (the NEDU scientists) were saying, but it was just in a language that I hadn't learned. I was losing a lot in translation.
As we hunted down all of Ross's objections to the study (e.g. "the profiles are miles apart", "the on/off-gassing characteristics are completely different", etc.) I started seeing that A2 and VPM-B+7 lined up very closely in many respects. And as we looked at more, it just kept coming. Ross raised an objection, we produced an analysis and found yet another way in which A2/VPM were similar. The thread was an education for me. Bruce Partridge voices a similar experience here.
Take a look at these similarities:
Profile similarity
Compares shapes of the A2 and VPM profiles
Heat map similarity
Visually compares patterns of supersaturation in the A2,VPM, A1 and GF profiles
Deep stop skew similarity
Compares and index of how much a profile "skews" toward deeper stops
Risk function similarity
Compares a profile risk function for each of A2,A1, VPM and GF.
...
Take a look at why solely looking at peak supersaturations might not be best … the charts here tell the story if you'll take the time to try to understand them. In short, you HAVE TO REFLECT EXPOSURE TIME in your analysis. Look at how the top chart, which looks so alarming when you compare peak supersaturations, really amounts to very little supersaturation exposure as shown in the bottom chart. And the bottom chart shows how smaller supersaturations sustained over a long time can really dominate the picture.
Mitchell's post here about the efficacy of "protecting the fast tissues" is very good.
Just take the time to go over what's there.
Once you understand those charts, you can move onto the thread where we showed a similar analysis for a "real-world" rebreather dive. The similarities are easily seen and the pattern of deep stop models accepting longer sustained supersaturation upon surfacing in exchange for "protecting the fast compartments" is repeated. It's just that in a very carefully planned test conducted by the NEDU, that exchange failed.
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