Gradient factors - deep stops thread in DIR forum

[dmaziuk]

Hello dmaziuk,
the whole point of the study was "for the same decompression time". The aim of the study was identify what was the most efficient decompression schedule. Redistributing the time toward deep stop would only increase the DCS risk.

As such if you do not limit the decompression time making it equal you would loose the point of the study.

That is exactly my point.

Erik Bakers paper on gradient factors states very clearly that they will increase total ascent time. I twitch every time I see "gradient factors" and "deep stops" in the same sentence exactly because of what you said.

 

[jgttrey]

That is exactly my point.

Erik Bakers paper on gradient factors states very clearly that they will increase total ascent time. I twitch every time I see "gradient factors" and "deep stops" in the same sentence exactly because of what you said.

I guess I'm still not understanding your point.

Of course, using GF will, by definition, increase total time vs straight M value deco. In the real world, efficient deco matters. It's easy not to get bent is you are willing to decompress like a saturation diver, or dive absurdly extended deco, but that isn't practical.

GFs are very useful to add conservatism and no one is arguing against that. The question is where you add it. As it turns out, past practice of using low GF low values to force deeper stops (in an effort to add conservatism) were likely misguided and actually increased risk. I'm all for adding conservatism, but without apples to apples comparison of deco strategies involving the same total deco time, it's hard to know the best place to add that conservatism.

My problem with the GUE current take is that it proposes to mask the increased risk of deep stops. It is implementing the very "bend and mend" approach that was criticised to support deep stops to begin with. If you want to add more total deco time, I'm all for it (within reason), but let's add time to make a good deco profile more conservative rather than to mitigate risks created by a deep stop profile that is unsound to begin with.

 

[CAPTAIN SINBAD]

Hello

I find that I cannot post in relevant thread in the DIR forum, so I thought I might put a post here. I don't want this to turn into a deep stops debate. We have been through enough of that already.

I have read the article by Jarrod Jablonski linked to in that thread and I just wanted to share an important perspective. My concern is that the article title puts the question "are deep stops dangerous"? Although the article is typically (for Jarrod) well written and thoughtful, I firmly believe that this title frames the issue in a way that risks inferring that there is a right or wrong answer to the question. It is far more nuanced than that.

The truth is that every decompression needs a deepest stop. The real question is "how deep should that deepest stop be"? Throughout the debate on deep stops, the position I and my colleagues have taken revolves entirely around the concept of decompression "efficiency". If you have X amount of time that you are going to spend decompressing, then what is the optimal distribution of decompression stop depths and times that will give you the safest outcome? Succinctly, the safest decompression for X amount of decompression time.

We have imperfect answers to that question of course, but the fact is that the only human evidence available at this time suggests that bubble models such s VPM and RGBM impose stops that are too deep for the most efficient decompression. In other words, if you spent the same time decompressing as proposed by those algorithms for a particular dive, but de-emphasized the deepest stops and spent more time shallow stopping (I reiterate, for the same total decompression time), then you would probably reduce the risk associated with the dive. This, of course, unveils the potential flaw in the argument that you can do bubble model style deeper stops but do longer shallower stops as well to make up for the extra uptake of gas during the deeper stops. Maybe, but it may also be that if you took this now longer decompression, did the same decompression time but less deep stops and decompressed shallower for longer, then that might be more efficient too.

But this does not mean that "deep stops" per se are bad (I reiterate every decompression has to have its deepest stop), or by inference that decompression plans should be moved shallower and shallower (eg if you believe the answer to "are deep stops dangerous" is yes). Indeed, if you progress too far down the latter path, then efficiency will start to fall and decompressions will become less safe. Unfortunately, the bitter nature of the debates on this matter, and the need to defend the human data against irrational attack, has tended to make the issue seem more polarized than it really. It has risked creating a belief that endlessly shallower is better. No one has ever said that, and the position that I and my colleagues have taken should not be interpreted that way.

Simon M

I am unable to thank you enough for making a very heavy point in such a short and precise manner. I was going to reach out to you and ask your views on this subject but you beat me to it. One questions remains unanswered, at least in my mind. What happens to silent bubbles? We know for a fact that we have bubbles in our bodies much earlier than acknowledged by traditional models. Science tells us that when you go up, you are definitely not making those bubbles smaller. You are causing them to grow and you are increasing their numbers. There can be no other outcome. In other words, should I see your views in contrast to what another scientist, Dr. Bruce Weinke is explaining in this 4 minute Suunto video?


Once again, many thanks.

 

[Dr Simon Mitchell]

I have been bothered lately by some long time divers that have had serious dcs events that have ended or limited their diving and perhaps worse. Trace Malinowski is in a serious situation. Steve Bogaerts has just announced that he can never dive again. The three events that ended his diving career were all conservative dives with little or no mandatory deco to shallow depths. John Adsit took a hit but I have no details about it. There are others. Is it possible that we are accumulating damage without knowing it that is creating later susceptibility to dcs? Will we later learn that deeper stops with longer deco schedules and less “efficiency” is what we should be planning for? What don’t we know?

Hello Ray.

Some great observations and questions. Unexpected DCS events have always been part of diving but it may be that we are seeing more in the age of technical diving. There are many potential reasons to explain them. But one is right in front of us and we often overlook it. Specifically we are diving very differently to the "style" utilized when decompression algorithms were adopted or tested (in those rare cases where they were actually tested). I am going to Truk later this month and there will be divers who do two decompression dives per day every day for two weeks. No decompression algorithm has been tested in this context, and basically, when we dive this way I believe that all bets are off in relation to what constitutes a "conservative dive with little mandatory deco". In addition, as inferred by others, there are many things that can influence an individual's risk independent of the dive and decompression performed. These include temperature changes, hydration, exercise during or after the dive, the presence of a PFO or an inducible pulmonary shunt and a number of others. Some of these may profoundly influence risk and so it should not be a massive surprise to us when someone suffers DCS even when they were compliant with a decompression algorithm.

Simon M

 

[dmaziuk]

GFs are very useful to add conservatism and no one is arguing against that. The question is where you add it. As it turns out, past practice of using low GF low values to force deeper stops (in an effort to add conservatism) were likely misguided and actually increased risk.

No. Gradient Factors as designed add conservatism across the board. They will compensate for more on-gassing during deep stop. Resulting schedule will never take you over the M-value, DCS will be "undeserved".

#14 is an example of why that is a very worthy discussion on its own. Edit: there's plenty others, e.g. this study has 8 out 320 DCS cases that exceeded M-value, and 10 that surfaced at or below .6 of it.

Extended deep stop with the same total ascent time will take you over the M-value at some point. DCS is expected.

Personally I think this difference is enough to never lump the two together in one discussion.

 

[johndiver999]

It seems reasonable to define "efficiency" as comparing various ascent strategies that all use the EXACT same ascent time.

Along those same lines, I have seen anecdotal evidence of some divers who use an accelerated ascent rate (maybe 60 - or more) fps for the initial portion of the ascent, maybe for the first 30 or 50 feet (depending on depth of course). I wonder how this practice could be examined?

 

[Dr Simon Mitchell]

IOne questions remains unanswered, at least in my mind. What happens to silent bubbles? We know for a fact that we have bubbles in our bodies much earlier than acknowledged by traditional models. Science tells us that when you go up, you are definitely not making those bubbles smaller. You are causing them to grow and you are increasing their numbers. There can be no other outcome. In other words, should I see your views in contrast to what another scientist, Dr. Bruce Weinke is explaining in this 4 minute Suunto video?

Hello Captain Sinbad.

Thank you for your question. I watched the video. I'm not sure when it was made so it is hard to put it in context with where we are now in the evolution of our knowledge on the subject. I find Bruce's points to be difficult to interpret at times, and there are some egregious inaccuracies. For example, at 2:29 he claims that gradient factors are used to "get you out of the water faster" or words to that effect. I have no idea where or how he has formed that view because it is clearly incorrect. I would also suggest that production of the video against the backdrop of the logo of a company to whom Bruce has sold RGBM mandates a degree of caution in interpreting the objectivity of the commentary.

Anyway, focusing more on your question: The video contains many allusions to the fundamental claim that bubble models address bubble formation whereas gas content models and GFs applied to them don't. This was one of the original claims that resulted in popular uptake of VPM and RGBM. However, as Neal Pollock is fond of pointing out in his presentations, "bubble models never measured a bubble" - not in humans at least. The original development of these models contained no testing of actual bubble formation in biological systems - it was all just physical theory.

In fact, the subsequent human studies in which bubble model decompressions have been compared to gas content model decompressions (so far) have shown greater bubble formation in the bubble model decompressions. Most of these cases are non-symptomatic and so these bubbles are the so-called "silent bubbles" you refer to in your post. If forming them, ascending, "causing them to grow and increasing their numbers" were truly a problem associated with gas content models, then you would expect the opposite result to that which we are seeing.

Thus, what the human data are telling us is that the original bubble model assumption that we need to protect the faster tissues early in an ascent with very deep stops so that we don't form bubbles early and then "cause them to grow and increase their numbers" later, is probably flawed; at least to some extent. The studies suggest that the tissues responsible for most bubbles and subsequent symptoms after diving are probably the slightly slower tissues that are disadvantaged by very deep stops because they continue to take up gas during those stops.

So, to address your question about how my views should be interpreted in relation to what Bruce says in the video. To be very clear, my "views" are based entirely on the evidence currently in front of us. Bruce's views articulated in that video are based on the original theoretical concepts around bubble models.

Simon M

 

[jgttrey]

No. Gradient Factors as designed add conservatism across the board. They will compensate for more on-gassing during deep stop. Resulting schedule will never take you over the M-value, DCS will be "undeserved".

#14 is an example of why that is a very worthy discussion on its own. Edit: there's plenty others, e.g. this study has 8 out 320 DCS cases that exceeded M-value, and 10 that surfaced at or below .6 of it.

Extended deep stop with the same total ascent time will take you over the M-value at some point. DCS is expected.

Personally I think this difference is enough to never lump the two together in one discussion.

I'm not as comfortable as you with the premise that staying within the deco schedule (whether straight ZHL or as modified with GF) will always perfectly compensate for ongassing during deep stops (so as long as you are willing to extend deco, as will be required). In theory, of course, that is a true statement -- I understand that -- but only if one assumes perfection of the model.

If we know nothing else about decompression, it is that our theories are imperfect. Witness your point about all the "undeserved" hits. The risks associated with deeper stops seem to outweigh the benefits for the same total deco time. Given that, why would I choose GF's to generate deeper stops only to then be required to extend shallow deco to make up for it? I'm doing more ongassing, on purpose, and counting on clearing out out later.

For many reasons, I'm not prepare to accept the statement that GF low is irrelevant because as you follow the schedule and stay below the line. For one thing, efficiency matters. For another, I see no reason to take on more gas just because I think I can get rid of it later. To me, this is just raising the stakes needlessly.

 

[Ayisha]

...Trace Malinowski is in a serious situation...

I spoke to two extremely good friends of Trace's a few months ago who kept in contact with him after his accident. They are convinced that he went into that 90 foot dive already bent from the day before from his 240 - 250 foot tech training dive with students. They believe the 90 foot dive and ascent(s) that preceded the neurological DCS hit pushed it from mild Type 1 DCS to severe Type 2 DCS.

He had gotten onto his own O2, then left it behind and walked into the hospital at the dive site. They gave him O2 nasally for hours instead of high flow and hooked him to an IV. He finally got high flow O2 with the paramedics, and finally got to the chamber 7 hours after the incident, where he underwent a few chamber rides over a few days. He is still in "very bad shape".

 

[tursiops]

They gave him O2 nasally for hours instead of high flow. He finally got high flow O2 with the paramedics, and finally got to the chamber 7 hours after the incident,

This is all too common a story.
How does one avoid this delay and get into a chamber sooner?