Deep Stops Increases DCS

[UWSojourner]

As I follow this interesting thread, I keep struggling to figure out how it applies to the average diver, diving acceptable profiles. At the end of his presentation, “Decompression Controversies”, Dr Mitchell states that the data has resulting in him diving a profile of 50/70 or 50/75. He appears to put his money where his mouth is and has raised the GF lo above that many typically use

It is very interesting, that for the NEDU profile of 170 feet for 30 min on air, the decompression time for 50/70 is very close to that for VPM+4 and 50/75 is very close to that for VPM+3. The difference is in the distribution of the stops between deep and shallow, and is pretty striking. This has given me something tangible to think about. Now, if there was only some good data to compare these strategies... I'd love to see @UWSojourner's analysis of supersaturation associated with these profiles.

The heat map and some related charts are shown below. I chose the VPM-B+3 run time of 136 minutes and then matched 3 gradient factor settings with increasing low gradient factors.

The overall pattern remains the same -- VPM's deep stops cause the continued on gassing that results in a higher overall supersaturation upon surfacing and in total (see the integral supersaturation chart). To compare the pattern with other profiles see these links (270ft 20min CCR, NEDU study, 240ft 20min CCR). The gradient factor chart shows each profile's overall preference for emphasizing the faster or slower compartments.

Clearly VPM exits the dive with the most decompression stress remaining. Notice that with GF25/83 (supposedly very similar to VPM ), about half the benefit in reduced ISS is gained just from switching from VPM; more benefit is gained of course if the low GF is raised a bit..

 

[Macan]

UWSojourner, thank you for your inputs. I do understand that you need to use GF settings that will give the same RT as VPM-B+3 for a 170 ft 30 min profile.

However, similar to Scubadada's question, and approaching this from a typical Tec Diver standpoint, what would the decompression stress look like if the diver selected a GF of 40/85 or 50/70 as compared to VPM-B+3 for the same 170ft/30 min profile?

VPM-B+3 is showing an Integral Supersaturation of 1,036 mb-min for 170 ft/30min. What would the ISS be for a GF setting of 40/85 or 50/70? If the ISS are lower then it stands to reason that they present a lower DCS risk than VPM-B+3, right?

If ISS is a good indicator of DCS risk, what tools are available to a typical Tec Diver to compare ISS across different algorithms/settings?

 

[Dr Simon Mitchell]

Ross,

We have had quite a long debate about the significance of VGE in this thread after I pointed out that the Split group recorded consistently high VGE grades after decompressions from trimix dives using VPM. In an attempt to lessen the negative implications for VPM your position has been that VGE don't matter because DCS is caused by tissue bubbles and that the propensity to form tissue bubbles is somehow unlinked from the propensity to form VGE. You have claimed that VPM selectively protects from tissue bubble formation. Let me remind you of some of your statements:

"We have all collectively ignored VGE in our deco planning, as a cause of concern.. either in total, or with the knowledge that VGE are not harmful". Post 1050

"VPM does not count or pretend to count or monitor VGE. Dissolved models do not do it either. Deco models deal with tissue microbubble growth (extra vascular microbubble growth). VGE is something different. Don't get tricked by the wishful thinking of some around here to bundle it all together". Post 1022

"What the new fuss over VGE really means, is its yet another fake problem to justify more fake answers". Post 1025

"This reverse stress condition that you are implying (VGE is equal to tissue stress), is not true, and simply cannot be relied upon". Post 1013

Can I remind you of one of the things I said to you?

The other more important and irrefutable issue is that the capillaries acquire their supersaturated gas from the tissues. Thus, what is going on in the tissue capillaries is a direct reflection of what is going on the tissues. If a diver is forming greater numbers of VGE then there must be a concomitant propensity for greater numbers of bubbles to be forming in tissues. Post 1010

You might want to look the following (just published) paper up... For clarity, "autochthonous bubbles" are tissue bubbles and the pulmonary artery actually carries venous blood so the bubbles in it are VGE.

Bernaldo de Quirós, Møllerløkken A, Havnes MB, Brubakk AO, González-Díaz O, Fernández A. Bubbles Quantified In vivo by Ultrasound Relates to Amount of Gas Detected Post-mortem in Rabbits Decompressed from High Pressure. Front Physiol. 2016 Jul 21;7:310. doi: 10.3389/fphys.2016.00310. eCollection 2016.

Abstract: The pathophysiological mechanism of decompression sickness is not fully understood but there is evidence that it can be caused by intravascular and autochthonous bubbles. Doppler ultrasound at a given circulatory location is used to detect and quantify the presence of intravascular gas bubbles as an indicator of decompression stress. In this manuscript we studied the relationship between presence and quantity of gas bubbles by echosonography of the pulmonary artery of anesthetized, air-breathing New Zealand White rabbits that were compressed and decompressed. Mortality rate, presence, quantity, and distribution of gas bubbles elsewhere in the body was examined postmortem. We found a strong positive relationship between high ultrasound bubble grades in the pulmonary artery, sudden death, and high amount of intra and extra vascular gas bubbles widespread throughout the entire organism. In contrast, animals with lower bubble grades survived for 1 h after decompression until sacrificed, and showed no gas bubbles during dissection.

The pubmed link is here: Bubbles Quantified In vivo by Ultrasound Relates to Amount of Gas Detected Post-mortem in Rabbits Decompressed from High Pressure. - PubMed - NCBI

"wishful thinking of some around here to bundle it all together"?

"another fake problem to justify more fake answers"?

Simon M

 

[UWSojourner]

UWSojourner, thank you for your inputs. I do understand that you need to use GF settings that will give the same RT as VPM-B+3 for a 170 ft 30 min profile.

However, similar to Scubadada's question, and approaching this from a typical Tec Diver standpoint, what would the decompression stress look like if the diver selected a GF of 40/85 or 50/70 as compared to VPM-B+3 for the same 170ft/30 min profile?

VPM-B+3 is showing an Integral Supersaturation of 1,036 mb-min for 170 ft/30min. What would the ISS be for a GF setting of 40/85 or 50/70? If the ISS are lower then it stands to reason that they present a lower DCS risk than VPM-B+3, right?

If ISS is a good indicator of DCS risk, what tools are available to a typical Tec Diver to compare ISS across different algorithms/settings?

Hi Macan

Comparing decompression profiles using the same total dive time might be similar to comparing how fast a runner runs by using the same distance. What possible information would be gained, for example, in comparing how "fast" runners run if you find out the distances over which that comparison was done are 1) a marathon, and 2) a 100m dash? The same goes for decompression.

If we had a properly developed probabilistic model, then comparing the relative probability of a DCS hit in dives of different run times, depths, gases, etc., might be possible. I don't have that available to me. The best I can do is compare profiles that are pretty close -- same depth, total dive time, etc. When comparing dives that are closely related like this, scientists of the stature of Doolette confirm that ISS is a reasonable "index of decompression stress". If you review the literature I'm sure you'd find what I did -- that ISS is used extensively in decompression model development including in VPM's development as well.

Especially for this thread which is attempting to understand the NEDU study and its implications for tech diving, what exactly do you believe comparing dives that vary in total run time will reveal? How would you know if any differences you see are due to 1) the differences in time, or 2) the relative effectiveness of the deco method in allocating that time? I don't see how I could distinguish between the two since both time and stop distribution are moving. But if you see a way, please let me know . I could then adjust my comparative model and would be happy to do the comparison you suggest.

 

[UWSojourner]

If ISS is a good indicator of DCS risk, what tools are available to a typical Tec Diver to compare ISS across different algorithms/settings?

ISS is a good "index of decompression stress" when used to compare similar dives. It's not particularly good if you want to compare extremes like the chance of decompression problems for sudden loss of cabin pressure of an airliner and a CCR dive to 280ft. As things get relatively closer, the comparisons might get generally better.

Perhaps in the future a probabilistic model that can reliably predict the P(DCS) for an air dive to 80ft for 60 minutes and a 500ft dive for 15 minutes will be available. I join you in wishing we had it now. To my knowledge we don't.

So we're stuck trying to interpret the best scientific information we have and applying it as best we can. It seems to me the best scientific information we have clearly says that deep stops have been oversold to tech divers. That signal is not solely due to the NEDU study although that study was a landmark. It's also due to several other consistent signals in studies described in the various presentations Mitchell has been so kind to share. The key is that to date we're not getting conflicting signals. The signals are enough IMO to reflect in your dive planning even though we don't yet have the full probabilistic model that we wish we could access.

 

[Storker]

The deep stops crush or limit bubbles to a small size hence they can tolerate greater levels of SS at the more limited shallow stops.

You say this as if it's a fact, not an unproven hypothesis

 

[rossh]

Ross,

I want to keep this one in front of you because I think you need to be accountable for some of your statements:

Please see this post:

Can you provide evidence that my ideas (presumably you mean the ones I am articulating on this thread) were rejected by my "peers" in 2008?

You asked for a discussion about you. but it breaks the TOS rules - I'll try to be gentle.

The peer position, and its discussion is quite clear - a balanced - unbiased - no preference - cautious approach.


Did you keep to that balanced approach? NO. see below. You seem to make every effort to push one side of this argument to the exclusion and detriment of all else. And have actively downplayed the opposing views and test that were included the Workshop. You display a one sided view on deep stop efficacy - contrary to the in the workshop position, with no new evidence to support that..

The workshop also had a section on VGE and how to interpret them, but you don't follow that advise either. Your agenda to make VGE the primary measure (in the public view), is not accepted by expert researchers in the field.


Not to mention all the reliance on fake profiles, phony supersaturation graphs with no dimensions and drawing errors, provably invalid explanations of nedu test implications, hyperbole of unrelated science, youtube one sided con jobs. I could go on....


You asked for this discussion on you, but it breaks the TOS rules, so I'll stop here.



A simple search on "evidence" or "growing", in this thread alone, turns up this, and 20+ more.


# 896 "..... There is now a growing body of evidence from studies conducting direct comparisons that..."
# 367 " ....But be aware that there are no (and never have been) human data that support this approach. In contrast, there is a growing evidence base suggesting that emphasizing deep stops...:"
#188 "..... , and ignoring an increasing body of theoretical, pathophysiological, and outcome-based evidence that suggest...."
#723 "... the emerging evidence which indicates that we need to rethink the deep stop mantra..."
#781 ".... but it is the best evidence we currently have to base our "opinions" on. ..."
#810 "...some sort of counter to the evidence that bubble models..."


And and on it goes on and on..... in other forums, where the message is often a lot more direct.

 

[rossh]

Hey EFX

What I meant is that if the bubbles are so important why is it that the VPM conservancy settings all end up padding shallow time almost exclusively? In your example, the difference between +1 and +5 settings is a DT of 53 min. Of that time 44 min or 83 % is allocated at 40 feet or shallower. If being conservative in a VPM context means spending a higher proportion of your allocated DT shallower, then I think that seems to follow what everyones been saying anyway, or have I gone mad?

VPM conservancy is applied across the whole ascent. Its simply that applying a margin will be more needed / effective at the shallow end, because the shallow end is much slower / takes longer to achieve a change.

You can see the same in GF, if applied evenly. Compare 100/100 to 90/90 or 80/80.

 

[rossh]

Are you just ignoring the fact that when compared to the suggested gradient factors listed on this thread that vpm produces a shorter deco with a shorter time on oxygen? Additionally that vpm's dcs risk goes up as deco time goes up?

That's not "working". It's broke. Straight up broke. At least gf allows you to get rid of worthless stops and lengthen your shallow stop times to reduce the risk.

Your assertion that GFs like 40/70 produce some unreasonably long deco is downright absurd.

The ZHL-C risk goes up with exposure, like all models in use - just the same. To patch that ( a little bit) , you use GF.... ie. all broke.

Also The VPM risk goes up with exposure - like all models in use - we have VPM-B/E to address that. We added that back in 2004. See, VPM-B is smarter than you realize.



40/70 is much like VPM-B+3, in many ways. Its not quite the same but close enough to not matter. Factor in real world ascent rates. One thing is for sure - 40/70 is a lot closer to VPM-B, than the ZHL where it came from.

So yes, all this noise and fuss about DIR practices, DIR classroom theory, (conveniently blamed onto VPM), and when the expert smoke clears, the recommendation is almost,but not quite the same as a VPM-B+3 in many situations.


Simple Definition of hypocrisy

• : the behavior of people who do things that they tell other people not to do : behavior that does not agree with what someone claims to believe or feel

 

[RainPilot]

No you're not going mad but if you keep reading this thread you will. I completely agree that VPM will pad (biased) the shallow stops with more time. It does this because it is adding time at deep stops. But so what? Let's change the terminology from VPM style deep stops to very brief deep stays of a multilevel dive. Would you not expect a B-GF algorithm to pad, oops sorry, schedule longer stops at the shallower end of the dive when your staying briefly at deeper depths? Hell yes. Well , according to UWSojouner because VPM uses the same algorithm as Buhlmann this is exactly what VPM does exercising its dissolved gas phase portion of its algorithm.

My point was, VPM and bubble models in general, are not just looking at dissolved gas rates like its bubble blind cousin B-GF, but is looking out for that bubble that's going to whack you when you're not looking!

I see where you are coming from, but to my mind there is a big difference between an algorithm compensating for your choice to stop at deeper levels for a bit, and it REQUIRING you to do so.

Of course any "shallow-preferential" model you use will add time to penalize for dallying at depth, in order to compensate for the additional slow tissue on-gassing you have experienced. Im happy to accept that if that was part of my bottom time ( swimming up the wall and stopping to check out a fun fish or in the case of the Pyle stop, sticking a needle in a fun fish) but for me to actively plan to stop due to my algorithm, blowing expensive bubbles at depth, only to spend even longer at the shallow end for the privilege?

If there was ANY proof that the deep stops prevented DCS from any study at all, I would immediately switch. Unfortunately there is no such proof. All we have is some indications from multiple studies that the opposite may be true and that stopping shallower may be better for me so I have to go with the (limited) evidence for now and spread my deco time accordingly. For me (like Dr Mitchell and some/many others) that means switching my GF Lo up and my GF Hi down ( for overall conservancy)