ICD and gas swaps

[TheSnake]

Do you carry your 22% He reduction limitation all the way to the surface? At what point does reduced ambient pressure limit the risk of ICD?
In other words, by your logic, what next gasses did you use above 60m for your dive in the article? At what point do you consider it safe to just go to 50% O2 for accelerated deco?

Hi,
in PastoDeco "Flash deco gas" selection is made by maintaining PpO2 higher as possible ( between 1 and 1.6 ) to optimize decompression and limiting N2 raise to a percentage of He drop to limit ICD risks.
This means, slow N2 fraction raise in deeper swaps and higher N2 raise going up.
Depending on bottom gas, you reach a Nitrox > 40% between about 30m and 20m ( 98f ~ 65f )

 

[Pullnglide]

@Pullnglide, you went down on air to what depth? Up you did not use air? Where did you use the 21/30 for? Just curious.

Air was only used for travel although I kept it with me for the duration. I changed to bottom mix shortly after beginning descent, I do not recall the actual depth. Went to 21/30 at 170' and 50/10 at 70'.

 

[Dr Simon Mitchell]

Hello Antonio,

I think that there is no theory universally accepted by the scientific community and scientifically validated by a serious test protocol with a significant number of samples.

I don't agree. The relevant theories on ICD that are "accepted by the scientific community" are reasonably clearly articulated in the Lambertson and Idicula paper you cited. Those theories have, however, been misinterpreted by the technical diving community. There has also been one "serious test protocol with a significant number of samples" in a recent study by NEDU in which they conducted 100 dives with what you would consider to be extremely provocative heliox to air switches and demonstrated that even the form of DCS (inner ear) we link with gas switches is at least relatively uncommon (no cases in 100 dives with radical gas switches). You can read that study here.

If I'm not wrong, there was test made by Duke university in Washington DC, that shown that divers breathing Heliox at 7 ata had problems immediately after gas switches.
The same thing was experienced by Comex divers in Hydra program

You need to be much more precise about this before presenting it as evidence that what you say is correct. For example:

What gas switches, and what environment were the divers in? I have already pointed out that ICD can occur in environments relatively unique to certain saturation scenarios (such as helium to nitrogen switches when the diver occupies a helium filled environment). But these are not relevant to what technical divers do (unless, for example, you put helium in your drysuit).

What "problems"?. I have pointed out that inner ear DCS can occur after helium to nitrogen switches and may be partly due to augmented supersaturation of the inner ear due to counter diffusion occurring after the switch. Inner ear DCS would therefore not be unexpected, but your narrative essentially implies that helium to nitrogen switches may provoke bubble formation in a wide range of tissues.

They noticed that changing gas mix during decompression caused unexpected bubble formation and it was necessary to recompress them.
They changed decompression protocol, making gas changes more progressive and they got correct decompression.

I would suggest that you find the references, read them, and quote them very precisely if you are going to use them to support your position.

So, So I'm aware that there is not a widely accepted theory that can exactly describe and modelize ICD issues

Again, I do not agree with that. David Doolette and I did exactly that for the one form of DCS thought associated with helium to nitrogen switches in bounce diving decompressions. [1] Importantly, the explanation for the vulnerability of the inner ear in this setting is not related to the mechanisms you describe in your article.

That said, what is your position about ICD risks prevention? Do not worry at all about the limitation of N2 raise to He fall? Do you think this is useless or dangerous? If yes, why?

In my personal opinion this can reduce ICD risks and in any case does not add any additional risk. So why do not apply this protocol?

These are valid points. Avoiding gas switches or making them less dramatic may reduce the risk of inner ear DCS. Ross and I have previously argued about what tissues are affected by ICD, but I have not had a fundamental objection to his inclusion of an ICD warning in his deco software because it is probably harmless, and may reduce inner ear DCS risk; possibly not by much in view of the Doolette NEDU data cited above, but maybe it helps. The same would be true of your approach. My main objection to your article is that it presents a detailed mechanistic view of ICD that is both unreferenced to supporting data and inaccurate.

Simon M

1. Doolette DJ, Mitchell SJ. Biophysical basis for inner ear decompression sickness. J Appl Physiol 2003;94(6):2145-50

 

[TrimixToo]

Add me to the list of people to warmly thank Dr. Mitchell for his tireless contributions here and his work on behalf of all of us:

Thanks, Simon. Your efforts are vastly appreciated.

 

[TheSnake]

Hi Dr. Mitchell,
thank you for your reply.

There has also been one "serious test protocol with a significant number of samples" in a recent study by NEDU in which they conducted 100 dives with what you would consider to be extremely provocative heliox to air switches and demonstrated that even the form of DCS (inner ear) we link with gas switches is at least relatively uncommon (no cases in 100 dives with radical gas switches).

This confirms that we must care of gas switches and evitate brutals changes in mix composition.

What gas switches, and what environment were the divers in? I have already pointed out that ICD can occur in environments relatively unique to certain saturation scenarios (such as helium to nitrogen switches when the diver occupies a helium filled environment). But these are not relevant to what technical divers do (unless, for example, you put helium in your drysuit).

Effectively, using bottom gas to inflate drysuit can cause ICD when ascending and changing breathing gas mix.

What "problems"?

. I have pointed out that inner ear DCS can occur after helium to nitrogen switches and may be partly due to augmented supersaturation of the inner ear due to counter diffusion occurring after the switch. Inner ear DCS would therefore not be unexpected, but your narrative essentially implies that helium to nitrogen switches may provoke bubble formation in a wide range of tissues.

Heliox to Nitrox gas swaps, it seems. I will try to get more accurate data ....

In my personal opinion this can reduce ICD risks and in any case does not add any additional risk. So why do not apply this protocol?
These are valid points. Avoiding gas switches or making them less dramatic may reduce the risk of inner ear DCS. Ross and I have previously argued about what tissues are affected by ICD, but I have not had a fundamental objection to his inclusion of an ICD warning in his deco software because it is probably harmless, and may reduce inner ear DCS risk; possibly not by much in view of the Doolette NEDU data cited above, but maybe it helps. The same would be true of your approach. My main objection to your article is that it presents a detailed mechanistic view of ICD that is both unreferenced to supporting data and inaccurate.

I can retain that smooth gas changes may reduce "inner ear DCS risks", which are a consequence of Isobaric Counterdiffusion, but you not agree calculation method as unreferenced.
Do you think however that a ratio of about 20% ( 22.42% according to my calculations ) is reasonably correct and can be applied to reduce IEDCS risks?

 

[Dr Simon Mitchell]

Hello again Antonio,

This confirms that we must care of gas switches and evitate brutals changes in mix composition.

No, did you read the study? It implies the opposite. They did 100 decompression dives involving switches from pure heliox to air which according to your predictive algorithm should be disastrous. They did not see any counter-diffusion problems.

Effectively, using bottom gas to inflate drysuit can cause ICD when ascending and changing breathing gas mix.

Yes, potentially, I acknowledged that. But I have never known a technical diver to put helium in their drysuit.

Heliox to Nitrox gas swaps, it seems. I will try to get more accurate data ....

I would be most interested. All the theory is against this (except for the inner ear). The best data I am aware of was the Doolette study cited above involving heliox to air swaps (worse than nitrox). No relevant problems.

I can retain that smooth gas changes may reduce "inner ear DCS risks", which are a consequence of Isobaric Counterdiffusion, but you not agree calculation method as unreferenced.
Do you think however that a ratio of about 20% ( 22.42% according to my calculations ) is reasonably correct and can be applied to reduce IEDCS risks?

This is a good question. The 20% notion is almost certainly too conservative but as I implied in an earlier post the provision of guidelines is probably relatively harmless, unless it drives divers to substitute one risk for another like unnecessarily carrying too many cylinders of 'intermediate trimix' to smooth out the transition. The simplest solution on a rebreather is not to make diluent switches.

Simon

 

[TheSnake]

I would be most interested. All the theory is against this (except for the inner ear). The best data I am aware of was the Doolette study cited above involving heliox to air swaps (worse than nitrox). No relevant problems.

Yes, essentially we focus on the IEDCS problem.

This is a good question. The 20% notion is almost certainly too conservative but as I implied in an earlier post the provision of guidelines is probably relatively harmless, unless it drives divers to substitute one risk for another like unnecessarily carrying too many cylinders of 'intermediate trimix' to smooth out the transition. The simplest solution on a rebreather is not to make diluent switches.

I am convinced that 20% ( or 23% ) maybe too conservative, that's why I put in my deco software the possibility for the users to raise it, if they wishes.
Certainly, in CCR dives, as you mentioned, best solution is not to make diluent switches.
Concerning OC dives, this not adds extra tanks as it is calculated, so no additional risks are added.

I want to thank you for the contribution in this discussion and for the elements of reflection that you raised.

 

[Germie]

If you always use the 'icd ration' in your helium to higher oxygen mixes it can give you the need to take extra gas.
If you put some 'DIR' gases in your decoplanner, let's say 10/70 to 100m/330 ft, bottomtime between 15 and 17 minutes. What reductions are needed according to your planner? Can you do this dive in your planner with 3 other gases (travelgas and decogas) with the volume of 80cft per stage? Or do you need 4? What is the amount per gas needed with an rmv of 20l/min
What happens if you use a 10/50 as bottomgas (Ignore END of bottomgas at 100m)? Can you then plan with 3 gases (can be different from the 10/70)?
AND is there a difference in decotime (most planners will give you some more decotime with higher helium mixtures, yes I know this is another discussion)?
If I would do this dive OC I think I take a 12/60, but don't let the PO2 at the bottom play a decorole

 

[TrimixToo]

Yes, potentially, I acknowledged that. But I have never known a technical diver to put helium in their drysuit.
Simon

I did that in the St. Lawrence River once when the water temperature was 77F. The shivering at the 20' stop convinced me never to do it again...that it might have implications from a decompression theory standpoint did not occur to me until years later.

 

[Germie]

If you are interested, this demodive from another divecomputer/planner shows how ICD warnings are implemented in software for example. Who takes a 50/10 decogas with a 18/45 to a depth of 50m? I get in Suunto's planner (DM5) a ICD warning if not using the 1:5 rule (maybe they changed it in the last releases, but in the DM from 1 year ago there was the warning implemented as 1:5).
Suunto EON Steel - Virtuele duik naar een diepte van 50 m