ICD and gas swaps

[TheSnake]

Hi,
I wrote a short article to explain ICD and how to calculate gas swaps to prevent it.
PDF format.

Downloadable at:
http://pastodeco.antoniopastorelli.com/down/icd.pdf

 

[Pullnglide]

I know some people that are diving high mixes and going straight to 50% rather than reducing the helium beforehand. They were actually taught that way. To each their own however I was instructed to reduce the helium slowly. My last “big” dive we carried air as a travel and safety, 14/50 bottom gas, 21/30, 50/10, and o2 for deco. That may be a bit excessive for some but it’s worked for me and how I was taught.

 

[TheSnake]

Hi,
I wrote this article because some divers aked me "WHY" and "ON WHICH BASIS" is a good practice the increase of N2 to 20% of He.
This explains gas physics and theory on the basis of which this ratio is calculated.

Every divers knows that decompression is not an exact law. In fact we speak of "decompression theory" or "decompression models".
Human body is an extremely complex "engine" to be extacly reproduced, so it is ( hugely ) simplified in a "model".
This implies an "interpolation" that tries to get closer to reality never being 100% accurate.

There were some cases of decompression sickness even respecting all protocols and, on the other side, there were cases of divers
having "non conventional" decompression procedures whithout having decompression sickness.

Finally, there are some "calculated and modelised" limits and procedures on which we apply a conservatism level.
If we use lower conservatism level we "accept" an higher risk, that does not automatically translates into an accident.

All diving acencies say the PpN2 must be limited, and globally a air dive is considered safe up to 40 meters/131f.
In France all CMAS*** divers ( recreational dives ) made air dives up to 60m./196f with a single tank of 15 liters @ 200 bars ( = 105 cuft ) used
for both bottom and deco phases.
This normally would be considered as "crazy" and "dangerous".
However, having made thousands of dives in France, many of that as air dives at 60m/196f and deeper, I can say that I never seen on my buddies narcosis problems.
Even so, I would not recommend it as good procedure.

The reason of this article is to explain the objective reason of limiting N2 raise and how many.
It does not pretends to say what to do but on this basis everyone can make their choices knowingly and in relation to their experience.

 

[Germie]

@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.

@TheSnake, have you read the articles where is written that ICD can only be an issue deeper than 70m and with saturation dives?
I know a lot is unknown, so a 1:5 rule (1:4.46) is used a lot and seems to work. So there is not directly a need to change it. But in some planners (I don't know yours) the mathematic procedure only 1:5 is implemented and not a minimum depth and a bottomtime. Then you get for example a warning if you go to 55m for 20 minutes on 18/45 and switch back to ean50. A good planner will not give a warning then.
In other discussions some people mentioned icd is never an issue in normal recreational technical diving. I will not say this is true,but how is your icd warning implemented?

 

[TheSnake]

@TheSnake, have you read the articles where is written that ICD can only be an issue deeper than 70m and with saturation dives?

Nowhere.
ICD is related to mix changes when using mixes with high He fraction, which is not the case in normoxic dives.

 

[TheSnake]

Then you get for example a warning if you go to 55m for 20 minutes on 18/45 and switch back to ean50. A good planner will not give a warning then.
In other discussions some people mentioned icd is never an issue in normal recreational technical diving. I will not say this is true,but how is your icd warning implemented?

I agree that ICD is not an issue in normal recreational / normoxic tech dives.
If you're referring to PastoDeco, you can set ICD warnings OFF, or you can set a percentage between 20% to 32%. ( if you want a little less conservatism than 1:5 ratio ).
This only displays a warning icon on runtime row when gas switch is over the limit. ( non blocking ).

To to avoid tedious calculations, PastoDeco have a "Flash deco" function that automatically generates a deco gases list, maintaining the ideal theoretical ratio to avoid ( reduce to minimum ) the ICD risk.
As said in the article, this ratio it's not arbitrary, but related to difference on gas "solubility" between He and N2 ( or any other gases ) and it's application seems to confirm its validity.
Effectively this is a bit restrictive in gas mixes choice but is a safer practice ( more conservative ).

 

[Miyaru]

Antonio,
In your article, you mention superficial ICD caused by using trimix for drysuit inflation. There's not a single course teaching this.

Finally, you mention inner ear DCS caused by ICD, without further touching the subject. What I learned is that ICD is not causing any problems in your body during offgassing, except for 1 area: the inner ear.
The model you describe is based on lipid tissues, just as Stephen Burton initially did, but is that correct? This was discussed earlier:

...
The inner ear is the only tissue thought to be injured by IBCD processes and its composition is understood. As you know, we have published a physiological model of the inner ear based on its known anatomy and tissue composition. I don't know how Steve Burton would justify his choices / predictions, but I would be skeptical about their physiological provenance.
...

 

[rsingler]

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?

 

[Dr Simon Mitchell]

Hello Antonio,

I would like to pick up on related issues to those raised by Miyaru.

There is no doubt that isobaric counter-diffusion can occur under some very specific circumstances. One, for example, is changing from helium breathing to nitrogen breathing whilst surrounded by helium at high ambient pressures. This has been shown to precipitate bubble formation in the skin. However this scenario is not relevant to real diving.

The isobaric counter-diffusion principle has been incorrectly reinterpreted by some technical divers (and your article is an example) to mean that any gas switch is bad and will result in an isobaric supersaturation of tissues (that is, an increase in tissue dissolved gas pressure above ambient pressure without a change in depth). In particular, it is often inferred (as you do in your article) that helium to nitrogen switches during decompression (which are the only relevant circumstance to technical diving) may cause "deep tissue counterdiffusion" that is potentially harmful. You cite Lambertson and Idicula as supporting the concept you promote, but in fact their paper contradicts you if you read it carefully. Look on page 441 and you will find the following:

The opposite effect, accelerated desaturation, is accomplished by substituting the breathing of nitrogen or another relatively slowly saturating gas for helium.

As they imply, in theory, a helium to nitrogen switch during ascent should cause a transient undersaturation in tissues and accelerate decompression. Based on some of David Doolette's work, this advantage may not be real, but in most tissues the switch will not be a disadvantage.

To the best of our knowledge there is only one tissue in which a helium to nitrogen switch during decompression may cause an increase in supersaturation. That is the inner ear, and it is not for the reasons you describe.

If you have any evidence you believe supports your view that helium to nitrogen switches need to be regulated in the manner you describe, then please provide it. But I believe your article is based on a false premise.

Simon M

 

[TheSnake]

Hi Dr. Mitchell,
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 ( as for other aspects and phenomenons involving scuba diving ) and beside I am not a doctor.

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.
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 know some case of sickness in dives changing bottom gas directly to Nitrox 40 or more.
If I'm not wrong, these problems are more common in OC dives than CC dives, in which changes in breathing gas, because of specific constant PpO2 working, are very smooth.

So, So I'm aware that there is not a widely accepted theory that can exactly describe and modelize ICD issues, but it seems, from available cases observations, that a progressive ( slow ) change in breathing mix can likely reduce the risks associated to ICD.

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?