Gradient Factors and Deep Stops

[boulderjohn]

All compartments? None of them will still be ongassing?

The reason I asked this question is that if one is to use Buhlmann, the 9th of his 16 compartments has a halftime of just under 2 hours. The 16th has a halftime of just over 26 days. I am a little confused about the dives you are doing in which all compartments are offgassing on very deep decompression stops.

 

[PfcAJ]

The 16th has a halftime of just over 26 days.

Uhh...you sure about that?

 

[tursiops]

The reason I asked this question is that if one is to use Buhlmann, the 9th of his 16 compartments has a halftime of just under 2 hours. The 16th has a halftime of just over 26 days. I am a little confused about the dives you are doing in which all compartments are offgassing on very deep decompression stops.

I think you forgot to divide by 60. That 16th compartment is 635 minutes, so about 10.6h. But your comment still stands, just not as strongly.

 

[boulderjohn]

I think you forgot to divide by 60. That 16th compartment is 635 minutes, so about 10.6h. But your comment still stands, just not as strongly.

Yep--did some quick math and divided the minutes by 24 as if they were hours. Ooops!

 

[rossh]

You continue to make schoolboy inferential errors in relation to this issue. Yes, many hyperbaric services (including my own) have noted a decline in DCS case numbers over the last 20 years. But proper scientists recognise that you can make no sense of this trend in the absence of a denominator (number of divers or number of dives) which would allow calculation of a rate. When we looked at this in our own jurisdiction [1] using new entry level certification numbers as a denominator we found that the number of new divers fell in approximate proportion to the fall in numbers of DCS cases (see below).View attachment 437676


The publication of the UHMS remote DCI workshop proceedings in 2005 [2] also resulted in a reduction in recompression of milder cases globally. This will have contributed to the apparent reduction in recompressed cases reported by hyperbaric units.


Your notion that the downward trend in DCI case numbers can be attributed to the use of bubble models and deep stops is delusional, not least because all the directly comparative human studies in decompression diving show that deep stops approaches appear to be inferior. Why would you then conclude that a downward trend in DCI numbers is due to the use of an inferior approach? Moreover, the vast majority of the DCI cases that make up these data are from scuba air no decompression diving. Whatever is responsible for the trend, it provides us with little knowledge of relevance to decompression diving.




You have some strange notions about what constitutes "winning" or "losing" an argument Ross. You were claiming that perfusion of tissues does not play an important role in tissue gas kinetics. First me, then the world's foremost decompression modeller came on to the thread citing substantial bodies of published evidence demonstrating that you were completely wrong. In reply you cited nothing except your own flawed perceptions. And here you are again, claiming that you "won" the debate. Why would anyone believe anything you say? For those interested, the discussion is on this board here. It is worth a read.




Ross, you categorically stated, multiple times, that tissue gas kinetics do not depend on tissue perfusion, they depend on the tissue's half time or "tissue absorption rates". The thread is at the link above and is definitely worth a read, but here are some examples:




and this classic....




Your position that it is half times or "tissue absorption rates", not perfusion that limits gas uptake is like saying its not oxygen that keeps us alive it is air. The most important component of a tissue's half time / gas absorption is its perfusion. Whilst everyone has a right to speak on public forums, I believe you go beyond those rights in commentating in such an authoritative manner on something you clearly don't understand on a forum where divers come to get educated.


Simon M

So according to you...... there are less active divers in the world. And to inflate DCS issues, you imagine that there is wide spread (but secret) IWR going on.... without a scrap of data to support that.


**************

The other thread topic was about exercise and its affects on deco - not the physiology one you are trying to invent. I correctly described how models addressed this issue, and how the dive experience does not seem to justify any change.

You said I was wrong, so I challenged you to find a case report supporting your view - you failed to do this. You then trotted out David, but his summary on this subject also supports my view, that the effect of exercise on deco is too small to be noticed.

"...However, don’t expect the substantial differences seen in the experimental trials were extreme levels of risk factors were used....." D.Doolette/Richard D. Vann: p.129 Summary RISK FACTORS FOR DECOMPRESSION SICKNESS. DAN Decompression Workshop 2008.



Now, right under the nose of everyone here, you try to change the thread context, and invent more straw man arguments about me, so you can win a your self serving and phony argument. Those are the signs of a desperate looser, hell bent on ad homien and the ongoing deceptions.

.

 

[Scuba J7]

So according to you...... there are less active divers in the world. And to inflate DCS issues, you imagine that there is wide spread (but secret) IWR going on.... without a scrap of data to support that.


**************

The other thread topic was about exercise and its affects on deco - not the physiology one you are trying to invent. I correctly described how models addressed this issue, and how the dive experience does not seem to justify any change.

You said I was wrong, so I challenged you to find a case report supporting your view - you failed to do this. You then trotted out David, but his summary on this subject also supports my view, that the effect of exercise on deco is too small to be noticed.

"...However, don’t expect the substantial differences seen in the experimental trials were extreme levels of risk factors were used....." D.Doolette/Richard D. Vann: p.129 Summary RISK FACTORS FOR DECOMPRESSION SICKNESS. DAN Decompression Workshop 2008.



Now, right under the nose of everyone here, you try to change the thread context, and invent more straw man arguments about me, so you can win a your self serving and phony argument. Those are the signs of a desperate looser, hell bent on ad homien and the ongoing deceptions.

.

Why would he try to deceive us?
What is the deception?

I'm guessing the only reason to deceive someone about decompression would be financial gain, to "off" someone, or just generally be mischevious. I'm sure I'm missing something?

Could you also post sources for the info showing you are correct?

Cheers

 

[Dr Simon Mitchell]

So according to you...... there are less active divers in the world.

I cited data from a study in a peer reviewed scientific journal that clearly demonstrate the flaw in your assumption that decreased DCS case numbers = a reduced DCS case rate. Put another way, without knowing numbers of divers or dives, you can't tell whether an apparent improvement in diving safety (based on reduced case numbers alone) is even real, let alone wildly speculate about its potential causes.

And to inflate DCS issues, you imagine that there is wide spread (but secret) IWR going on.... without a scrap of data to support that.

The significance of the Remote DCI Workshop Proceedings has nothing to do with IWR as you have assumed. You should read documents and not simply assume you know what they say. The remote workshop developed a case definition of "mild DCI" and then reached a consensus that divers with DCI meeting that description could be managed without recompression. It follows that many divers who would have been recompressed in the past (and who would have shown up in recompression statistics) were subsequently managed with surface first aid procedures.

The other thread topic was about exercise and its affects on deco - not the physiology one you are trying to invent.

I don't have to invent anything Ross. People can and should read the thread for themselves to gain some insight into your propensity for providing authoritative-sounding commentary with little understanding of the issues you are discussing.

You admit that the topic was "exercise". Well, as was pointed out multiple times on that thread the significance of exercise is its effects on perfusion (blood flow) of tissues. You categorically stated multiple times that perfusion is not a significant influence on gas uptake and elimination from tissues, and instead claimed that it is half times that determine tissue gas kinetics. This betrays a massive hole in your knowledge about the central role tissue perfusion plays in determining a tissue half time.

I correctly described how models addressed this issue, and how the dive experience does not seem to justify any change.

There was nothing correct about suggesting that we don't see a signal that exercise is an important risk factor for DCS. It has been demonstrated in multiple studies, and we see cases of DCS where the diver has been working hard all the time. No one would think to report that as anything extraordinary because it has been part of the received wisdom of diving medicine for over 50 years. It seems it is only you that has not caught up with that.

You then trotted out David

David trotted himself out Ross.

but his summary on this subject also supports my view, that the effect of exercise on deco is too small to be noticed.

"...However, don’t expect the substantial differences seen in the experimental trials were extreme levels of risk factors were used....." D.Doolette/Richard D. Vann: p.129 Summary RISK FACTORS FOR DECOMPRESSION SICKNESS. DAN Decompression Workshop 2008.

This was not the "DAN Decompression Workshop 2008", but rather the DAN Technical Diving Conference 2009 [1].

Your selective quotation from the Vann / Doolette chapter is brazenly dishonest. The entire chapter is about risk factors that do change the risk of DCS. A more accurate quote would have been the first sentence of the second paragraph of the section on exercise during the dive which states:

Exercise during the bottom time has been shown to increase decompression requirements.

The sentence you cited is about all risk factors (not specifically exercise), and is an observation that the magnitude of increased risk seen in trials where a risk factor is intentionally exaggerated to investigate its effect would not be expected in real dives. This is very different to saying that the effect is too small to be noticed.

This is another example of your lack of respect for accuracy, and why readers should interpret anything you say very cautiously.

Now, right under the nose of everyone here, you try to change the thread context, and invent more straw man arguments about me, so you can win a your self serving and phony argument. Those are the signs of a desperate looser, hell bent on ad homien and the ongoing deceptions.

"Everyone here" can read the thread for themselves and form their own views.

See here

Simon M

1. VANN RD, MITCHELL SJ, DENOBLE PJ, ANTHONY TG (eds). Technical Diving. Proceedings of the Divers Alert Network 2008 January 18-19 Conference. Durham NC, Divers Alert Network, 401pp (ISBN 978 1 930536 53 1), 2009

 

[DevonDiver]

All compartments? None of them will still be ongassing?

My bad. What i meant to imply was relevant to OC/CC sports diving, not saturation.

Of course, there could be permutations caused by repetitive multi-day tech diving whereby those slowest compartments could still on-gas to a meaningful level whilst breathing an oxygen enriched blend at depth. I tend to forget those scenarios as I'd never choose to dive even close to that level of aggression.

The point being that if an o2 enriched deco mix was breathed, the slow-tissue decompression risk of a deeper stop is generally going to be nullified.

Obviously, the dive parameters whereby an enriched O2 blend is safely breathed on a 'deep stop' limits the range of diving under discussion.(forgot this was in 'Advanced Scuba' forum and not the tech or deco forums...)

 

[Neilwood]

Thank you @Dr Simon Mitchell for your input. Good to hear an experts point of view.

Interesting the discussion between you and Ross.

I am looking on from the viewpoint that you both have vested interests in this:
1) One has a medical degree and is working in deco medicine and research.
2) One has developed a deco planner and is interested in sales of it.

I know who's viewpoint I will be paying the most heed to - the one that has been recognised by his national diving centre, the Undersea & Hyperbaric Medical Society and who has the interest of minimising the amount of "customers" that require chamber rides.

 

[Akimbo]

It has been demonstrated in multiple studies, and we see cases of DCS where the diver has been working hard all the time. No one would think to report that as anything extraordinary because it has been part of the received wisdom of diving medicine for over 50 years.

(not debating the fact, supporting it)
Actually, it is closer to over 100 years. Both John Scott Haldane and Chief George D. Stillson addressed it. Guidelines for both of their early tables recommended that the diver be bumped to the next deeper or longer table to compensate for heavy work and/or cold. I can dig out the actual quote from the 1916 US Navy Diving Manual if anyone is interested.

In case any of you wiseguys are wondering, I've been researching a lot of diving history lately so my manual is a reprint instead of the one issued to me in diving school. :TongueInCheek: