Kevrumbo
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Hi Simon!
Can you comment or expand on practical applications on finding the best compromise "Sweet Spot" Gradient Factors now, with regard to the NEDU Deep Stop & He/N2 Kinetics? And what GF's are you now using for a typical multi-week expedition like we did at Bikini Atoll and Truk (especially over consecutive days/weeks of deep "bounce" deco dives)?
Some cross posts from the other boards for reference:
Originally Posted by teddy2005 
Probalby not reading well, but it looks to me that to the contrary, a lot are saying that VPM is pure s..t.
Hello Teddy,
I don't think this is true and it is certainly not my message. Andrew has summed it up pretty well in his last post. VPM works, and many people have been (and still are) using it over a longish period now. However, there is some human evidence from (the NEDU study) which is consistent with underlying theory (Dr Doolette's original integral supersaturation analyses and the various enhancements produced by Kevin) that VPM may emphasise deep stops more than necessary to produce optimal decompression. Unfortunately, as Andrew has pointed out, Ross's complete refusal to even acknowledge this possibility and his frequently baseless criticisms of a valuable and unique investigation have produced a polarized argument which at times has resulted in it sounding as though people on one side of the debate think VPM is "pure s..t" as you put it. The real message is that deep stops may have been oversold to the technical diving community, but by exactly how much and where the optimum lies is not known.
As I have pointed out before, it is difficult to recommend a definitive practical application of this conclusion to divers for all sorts of reasons. However, the use of GFs does give the diver an opportunity to "de-emphasise" deep stops to an extent. In my own experience, this means that whereas I might have used a GF-Lo of 10 or 20 at the height of the belief in deep stops, I would now use 40 or 50. Given I am frequently the only diving physician in remote locations I usually also dive a conservative GF-Hi (70 - 75) but that is a personal choice. There is nothing definitive in these recommendations. It just seems like a sensible response to the data that we have as of February 2014. As Andrew has pointed out, in the world of science things can change as new data emerge.
Simon M
Originally Posted by Mark Chase
So the BIG question in my mind at the moment is why do we feel its OK to end a deco on 90GF but not to start deco on 90GF?
Is the gradual build up of body stress somehow beneficial or is it all in our minds.
I am not clever enough to understand the finer points of deco but even to a dullard like me it makes sense that staying deep will on gas some tissue compartments which will inevitably increase the shallow decompression stop time.
Thats why I could never get my head around VPMB profiles which had more deep stops but less shallow stops?
So does any one have an opinion why we start of on low GFs and build to high GFs. Is there a progression benefit?
Mark
Hello Mark,
Cutting edge questions!
Indeed, I think it is a serious possibility that it could all be in our minds, but the issue deserves careful and cautious appraisal.
Perhaps the place to start is your question about why we start on low GFs and build to high GFs. My personal take is that it is the classic example of an unstoppable word of mouth / internet propagated fashion driven by the theoretical attraction of bubble models. This is not intended to sound disrespectful of the process. As I alluded to in an earlier post to Ross, the theory was all we had in the early days. It seemed compelling, and so most people came into line with the trend. By and large, and (I believe) after some early adjustments, it seemed to work OK. But if you go looking for data describing the efficacy of the approach you won't find it. You certainly won't find any sort of proof that it was better than anything it replaced.
Fast forward to today, and we have inherited a situation where, as Ross says, a majority of technical divers incorporate "deep stops" (that is, the deeper stops that are naturally prescribed by a bubble model, or deeper stops resulting from user manipulation of a gas content model like Buhlmann) in some form or another. In this milieu there is no denying that the majority of dives seem to have good outcomes. However, this does NOT, of itself, mean that our current approach is the best approach. There are plenty of bad outcomes too.
This paragraph contains a bit of a sidebar... but a relevant one. None of us know how safe the dives we do are. The incidence of DCS in technical diving has not been accurately measured. (WARNING: PERSONAL ANECDOTE HERE) However, on the basis of my participation in many technical expeditions and trips as medical officer, I think DCS, especially minor DCS (rashes and pain for example) is way more common in technical bounce dives than we might think. Many of these cases never get reported or sent to a hyperbaric unit. Thus, based on my personal observations I certainly don't think there is any room for complacency about the efficacy of our current practices. Choose to ignore this by all means.... just my observation. Back to science now....
This brings us to the fact that we have seen the recent emergence of data that suggest the deep stop approach might not be the best one. In particular, these data suggest that the logic of protecting fast tissues from supersaturation early in the ascent at the expense of increased supersaturation in slow tissues later in the ascent may be flawed. Whether one likes the relevant studies or not, they are all we have at the present time to illuminate this issue.
So, was the bubble model fashion a bad idea, and where does this leave the majority who have been using some degree of "deep stopping" with apparently good outcomes? Do we abandon deep stops entirely eg go to 90/90 as you hypothesised, do nothing, or do something in between?
I guess the first thing to acknowledge is that the studies indicating a disadvantage for deep stops are not the diving equivalent of multiple large multicentre randomized trials all showing that the risks of a widely used drug are greater than its benefits. On the basis of that sort of data you would probably stop using the drug overnight. The diving world data concerning deep stops are not as definitive as that. As more data emerge the situation may become clearer (or more blurred)! So, at the present time, as an informed commentator, I would not go so far as to recommend that the entire world dives 90/90 or 80/80 from tomorrow onward even though I personally would not be surprised if the outcomes were as good as (or better than) what we are getting now (if we were able to measure them!). Partly, this hesitancy to advocate substantial wholesale change arises from the certainty that every diver who subsequently got bent would inevitably blame it on the change in their decompression practice! The deep stop trend evolved over a substantial period, and if the data continue to be supportive, it may have to "de-evolve" over a substantial period.
Equally, I do believe the data are strong enough (and bear in mind they are the only data) to consider a change in practice if you are a strong "emphasizer" of deep stops. In practical terms, "de-emphasizing" deep stops (or lessening any potential disadvantage) would mean using bubble models on very high conservatism settings, and with gradient factors, avoiding very low GF-lo values. I have been evolving my own use of GFs and am currently around 50-80...sometimes as low as 70 for the high value when we are at places like Bikini and I am the only diving physician. Pre-NEDU study I was GF lo of 20. This is my personal perception of a sensible graduated response to the way the evidence is currently evolving. I may well go further in future (guided by the evidence).
Sorry about the long post. Hope it makes sense.
Simon M
Originally Posted by David Doolette
Yes, that is pretty much it. I think a more appropriate structure is to have the half-times for nitrogen and helium the same in fast-compartments and nitrogen slower than helium in slow compartments. This is not revolutionary, I think DCAP has the half-times for nitrogen and helium the same for compartments with a 25-minute half-time and faster, and has them different in slower compartments. I am not sure if 25 minutes is the best break-point, but it could be. However, as has been cautioned before, decompression algorithms with a "helium penalty" might be prescribing the right amount of decompression for deeper dives, where more helium is used, but for the wrong reason (i.e. you might need that extra decompression whether you are breathing heliox, trimix, or nitrox).
Can you comment or expand on practical applications on finding the best compromise "Sweet Spot" Gradient Factors now, with regard to the NEDU Deep Stop & He/N2 Kinetics? And what GF's are you now using for a typical multi-week expedition like we did at Bikini Atoll and Truk (especially over consecutive days/weeks of deep "bounce" deco dives)?
Some cross posts from the other boards for reference:
Probalby not reading well, but it looks to me that to the contrary, a lot are saying that VPM is pure s..t.
Hello Teddy,
I don't think this is true and it is certainly not my message. Andrew has summed it up pretty well in his last post. VPM works, and many people have been (and still are) using it over a longish period now. However, there is some human evidence from (the NEDU study) which is consistent with underlying theory (Dr Doolette's original integral supersaturation analyses and the various enhancements produced by Kevin) that VPM may emphasise deep stops more than necessary to produce optimal decompression. Unfortunately, as Andrew has pointed out, Ross's complete refusal to even acknowledge this possibility and his frequently baseless criticisms of a valuable and unique investigation have produced a polarized argument which at times has resulted in it sounding as though people on one side of the debate think VPM is "pure s..t" as you put it. The real message is that deep stops may have been oversold to the technical diving community, but by exactly how much and where the optimum lies is not known.
As I have pointed out before, it is difficult to recommend a definitive practical application of this conclusion to divers for all sorts of reasons. However, the use of GFs does give the diver an opportunity to "de-emphasise" deep stops to an extent. In my own experience, this means that whereas I might have used a GF-Lo of 10 or 20 at the height of the belief in deep stops, I would now use 40 or 50. Given I am frequently the only diving physician in remote locations I usually also dive a conservative GF-Hi (70 - 75) but that is a personal choice. There is nothing definitive in these recommendations. It just seems like a sensible response to the data that we have as of February 2014. As Andrew has pointed out, in the world of science things can change as new data emerge.
Simon M
So the BIG question in my mind at the moment is why do we feel its OK to end a deco on 90GF but not to start deco on 90GF?
Is the gradual build up of body stress somehow beneficial or is it all in our minds.
I am not clever enough to understand the finer points of deco but even to a dullard like me it makes sense that staying deep will on gas some tissue compartments which will inevitably increase the shallow decompression stop time.
Thats why I could never get my head around VPMB profiles which had more deep stops but less shallow stops?
So does any one have an opinion why we start of on low GFs and build to high GFs. Is there a progression benefit?
Mark
Hello Mark,
Cutting edge questions!
Indeed, I think it is a serious possibility that it could all be in our minds, but the issue deserves careful and cautious appraisal.
Perhaps the place to start is your question about why we start on low GFs and build to high GFs. My personal take is that it is the classic example of an unstoppable word of mouth / internet propagated fashion driven by the theoretical attraction of bubble models. This is not intended to sound disrespectful of the process. As I alluded to in an earlier post to Ross, the theory was all we had in the early days. It seemed compelling, and so most people came into line with the trend. By and large, and (I believe) after some early adjustments, it seemed to work OK. But if you go looking for data describing the efficacy of the approach you won't find it. You certainly won't find any sort of proof that it was better than anything it replaced.
Fast forward to today, and we have inherited a situation where, as Ross says, a majority of technical divers incorporate "deep stops" (that is, the deeper stops that are naturally prescribed by a bubble model, or deeper stops resulting from user manipulation of a gas content model like Buhlmann) in some form or another. In this milieu there is no denying that the majority of dives seem to have good outcomes. However, this does NOT, of itself, mean that our current approach is the best approach. There are plenty of bad outcomes too.
This paragraph contains a bit of a sidebar... but a relevant one. None of us know how safe the dives we do are. The incidence of DCS in technical diving has not been accurately measured. (WARNING: PERSONAL ANECDOTE HERE) However, on the basis of my participation in many technical expeditions and trips as medical officer, I think DCS, especially minor DCS (rashes and pain for example) is way more common in technical bounce dives than we might think. Many of these cases never get reported or sent to a hyperbaric unit. Thus, based on my personal observations I certainly don't think there is any room for complacency about the efficacy of our current practices. Choose to ignore this by all means.... just my observation. Back to science now....
This brings us to the fact that we have seen the recent emergence of data that suggest the deep stop approach might not be the best one. In particular, these data suggest that the logic of protecting fast tissues from supersaturation early in the ascent at the expense of increased supersaturation in slow tissues later in the ascent may be flawed. Whether one likes the relevant studies or not, they are all we have at the present time to illuminate this issue.
So, was the bubble model fashion a bad idea, and where does this leave the majority who have been using some degree of "deep stopping" with apparently good outcomes? Do we abandon deep stops entirely eg go to 90/90 as you hypothesised, do nothing, or do something in between?
I guess the first thing to acknowledge is that the studies indicating a disadvantage for deep stops are not the diving equivalent of multiple large multicentre randomized trials all showing that the risks of a widely used drug are greater than its benefits. On the basis of that sort of data you would probably stop using the drug overnight. The diving world data concerning deep stops are not as definitive as that. As more data emerge the situation may become clearer (or more blurred)! So, at the present time, as an informed commentator, I would not go so far as to recommend that the entire world dives 90/90 or 80/80 from tomorrow onward even though I personally would not be surprised if the outcomes were as good as (or better than) what we are getting now (if we were able to measure them!). Partly, this hesitancy to advocate substantial wholesale change arises from the certainty that every diver who subsequently got bent would inevitably blame it on the change in their decompression practice! The deep stop trend evolved over a substantial period, and if the data continue to be supportive, it may have to "de-evolve" over a substantial period.
Equally, I do believe the data are strong enough (and bear in mind they are the only data) to consider a change in practice if you are a strong "emphasizer" of deep stops. In practical terms, "de-emphasizing" deep stops (or lessening any potential disadvantage) would mean using bubble models on very high conservatism settings, and with gradient factors, avoiding very low GF-lo values. I have been evolving my own use of GFs and am currently around 50-80...sometimes as low as 70 for the high value when we are at places like Bikini and I am the only diving physician. Pre-NEDU study I was GF lo of 20. This is my personal perception of a sensible graduated response to the way the evidence is currently evolving. I may well go further in future (guided by the evidence).
Sorry about the long post. Hope it makes sense.
Simon M
Originally Posted by David Doolette
Yes, that is pretty much it. I think a more appropriate structure is to have the half-times for nitrogen and helium the same in fast-compartments and nitrogen slower than helium in slow compartments. This is not revolutionary, I think DCAP has the half-times for nitrogen and helium the same for compartments with a 25-minute half-time and faster, and has them different in slower compartments. I am not sure if 25 minutes is the best break-point, but it could be. However, as has been cautioned before, decompression algorithms with a "helium penalty" might be prescribing the right amount of decompression for deeper dives, where more helium is used, but for the wrong reason (i.e. you might need that extra decompression whether you are breathing heliox, trimix, or nitrox).
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