Something to Consider when Diving Trimix

[jonnyb]

   Something I became aware of whilst reading an excellent article by Johnny E. Brian highlighting the misconceptions of the Oxygen Window. It clarified xactly what the gasses were doing during your dive and decompression, made me aware of some points which where not part of any course I've done to date, TDI/IANTD Adv. Nitrox and Trimix. I highly recommend reading : http://www.noviz.co.uk/deco/Oxygen_Window.pdf

   The issue I became aware of was to do with the gas diffusion of the inert gasses inspired during the dive and the switch to your deco gas/gasses... What I understand from the article is that when diving with multiple inerts (trimix, He/N2) the Pp (partial pressures) of the gasses are related to the depth and F% of each gas in the mix. Basic teaching on any course. Also that the gasses diffuse independently of each other. The gas He will begin to diffuse when the inspired tension of He is less than that in solution (as you come shallow) and the same for N2. The important thing here is that inspired N2 has no affect on the off gassing of He and vice versa.

   The impact this has on decompression I will try and explain now as best I can, but hopefully you have read the article and understand it as I have. During a Trimix dive using the gas 18/35, a common mix for a dive down to 60 ish metre's, the FN2 will be .47. Commonly used for decompression is the gas 50/50, at least taught to me on my TDI course's. The problem we have when switchin to this gas at 22m (1.6 Ppo2 MOD) is the introduction of a greater % of FN2. Although it may be considered nominal, PpN2 of back gas at switch was 1.5 to the new PpN2 of deco mix 1.6, it should be consider during the decompression. What has happened, is that the absence of He in the deco gas will speed up the diffusion of the He in solution, however the increase in FN2 has slowed down the release of N2 in solution, as now the tension of inspired N2 is greater than that in solution. I dont believe that computer models such as Buhlmann consider this in the calculation of there decompression obligation, given that they are perfusion based.

   In summary, something which has been eluded from the Trimix course I have completed, is that when diving with multiple inerts and switching to a deco gas, ensure the deco gas/gasses contain less FN2/FHe than the back gas you are switching from. In the instance above switching from 18/35 to 60% O2 at 16m will create less of a complication with your bodys tissue's and in theory a safer decompression.

  If this is wrong or if anyone has any thoughts let me know,

 Jonny

[Scubadec / Dec Hanniffy]

Have you read Mark Powells book? Deco for divers, it gives a very good breakdown of this too, well worth € 38 i think i paid for it!

[LiamM]

I get the point you are making - in going from back gas (18/35) to 50/50 you have a massive helium gradient and it will off gas rapidly but that the N2 off gassing will be inefficient by comparison. However the Nitrogen fraction will only be raising itself slightly and for the brief period at 21m it'll more than likely be minimal on gassing. Your main goal will be to off gas the inerts (which by switching from 35% He to )% will be rapid) then i think you will gain more by getting off the helium at 21 rather than waiting untill 16m for 60%, after an easy 4min more of on gassing of Helium.

Its a personal thing and none of it come close to a definite idea of whats actually happening in your body, more of a best guess.

I try to avoid a jump in Nitrogen of more than 5% due to possible counterdiffusion issues but id only be concerned if id been subject to a very high helium loading due to extreme depths and a very high Helium fraction.

I like the idea of partial pressure vacancy, and as a result i pad my 21m 50% gas switch by a few mins, but the advantage it gives is IMHO minimal compared to slowing the ascent rates right down once you are in an off gassing depth. I'll take it 9m a min ish up to 21m and then 3m a min from there up to 6m. By this time my computer will probably have shit itself .........

Id second Mark Powells book, its a very striaght forward intro to deco theory.

[jonnyb]

I get the point you are making - in going from back gas (18/35) to 50/50 you have a massive helium gradient and it will off gas rapidly but that the N2 off gassing will be inefficient by comparison. However the Nitrogen fraction will only be raising itself slightly and for the brief period at 21m it'll more than likely be minimal on gassing. Your main goal will be to off gas the inerts (which by switching from 35% He to )% will be rapid) then i think you will gain more by getting off the helium at 21 rather than waiting untill 16m for 60%, after an easy 4min more of on gassing of Helium.

   Even though the FN2 is raised only slightly, it will be at the new fraction for the reminder of the dive (unless of course you also have 80% or 100%). The PpN2 will drop again as you ascend, and you will be at your previous PpN2 before the deco gas at 20m. The point I'am trying to emphasis is we are complicating our normal routine of decompressing (continually reducing inerts) and adding another variable into our dive scenario.


[/quote]I like the idea of partial pressure vacancy, and as a result i pad my 21m 50% gas switch by a few mins, but the advantage it gives is IMHO minimal compared to slowing the ascent rates right down once you are in an off gassing depth.[/quote]

   Partial pressure vacancy is ideal, however your He tissue content has already begun off gassing since about 37m, so at 22m during your ascent, the tissue's containing He are at an advanced stage of decompressing. I'am of the opinion that waiting till you can safely eradicate both inerts with minimal complication is best, even if this means continuing to breath 18/35 for a few more minutes. Padding your 21m stop will only increase your deco obligation due to the spike in FN2, it would make greater sense to pad your stops from 18m up, but here you would be better breathing your 18/35 till switching to 60/40 at 16.[/quote]

[/quote] Id second Mark Powells book, its a very striaght forward intro to deco theory.
[/quote]  Must have a look for it, although I doubt I want to spend 38 quid on it, any loaners...

   Jonny

[Stewart Andrews]

Hi Guys,

Interesting topic. The one I wory about is the potentially huge ppN2 spike if I have to bail out from depth from Trimix (LHS) on to air (RHS) at about 60m. This will 'bully' the He out of solution in the inner ear  (which is slow region in the bodies tissues to offgas)  and create a massive IEDCS (Inner Ear DCS) problem.

I firmly believe that He on a rebreather is your friend and therefore use a max of 15% to 20% N2 in my DIL...for all my trimix diving. Helium rich mixes give a reduced WOB (work of breathing), better scrubber life and increased off-gassing rate. Due to the high cost of He, this really only applies to rebreather diving.

One request, if I may - I have been asked to review Mark Powell's book and would welcome any good or bad things you have to say about it, including any issues that may have been 'overlooked'.

Thanks,
Stewie

[jonnyb]

Hi Guys,

Interesting topic. The one I wory about is the potentially huge ppN2 spike if I have to bail out from depth from Trimix (LHS) on to air (RHS) at about 60m. This will 'bully' the He out of solution in the inner ear  (which is slow region in the bodies tissues to offgas)  and create a massive IEDCS (Inner Ear DCS) problem.

  Hi Stewie,

      One of the interesting points which I gathered from the article linked above and one of the misconceptions of the Oxygen Window, is that gasses contained in the new mix you switch to will flush out other gasses in solution, or in your words above 'bully'. The article states (this is not a quote), FiN2 effects the off gassing of the N2 tension in solution, and the FiHe effects the He tension in solution. What this I believe means is that when you switch to air at 60m from a trimix gas, it is not the FN2 in air that bullies the He out of solution, but rather the complete absence of He in the gas and thus the rapid off gassing from tissues which have not begun to off gas at all.

    Somebody please correct me if I am wrong, but if this is correct then switching to a weak/weaker trimix at 60m will help reduce the rapid off gassing of the He in solution in slow tissues. This will cause complications to your N2 on gassing to a lesser extent also than switching to air...

    Jonny
 

[Stewart Andrews]

Hi Jonny,

I have not read the book yet - however I did attend a fantastically detailed presentation bt Dr. John Mitchell at EuroTek 2008 in Brum. I shall listen to the IEDCS debate again later this year at EuroTek 2010!

In relation to this, my understanding (subject to change!) is that large spikes in ppN2 means that the N2 now wants to get into equilibrium with the tissues (obviously the gradient is then INTO the tissues). My amateur theory... the gas exchange is the inner ear is 'hampered' by it's physical structure, blood flow, etc. and as the N2 comes in rapidly with its spike, the He can not get out fast enough to make room for it and the He ends up supersaturated and out of solution - ie. a bubble.

Think of it if a load of rugby supporters who get on at the Lansdowne bus stop (there having been few rugby guys on the bus already) - and they were in such a rush to the next stop for their pints - the smaller school kids (He) could not push their way off before the doors close........ Kids, eh!

I agree that the fraction of any individual gas does determine the inflow/absorbtion/ dissolution of that gas. By way of example if you were changing mixes (at constant depth) and the  FN2 was the same in both mixes, the amount of N2 would remain the same/the rate of offgassing would remain the same. If the He percentage changes to a weaker % in the new mix, then it would offgas and try to reach equilibrium.

Switching to a weaker mix WOULD be better - and this is exactly what happens in a rebreather by staying on the original DIL until you surface (no spikes)!
One way or the other

[scuba steve]

hi guys i had to do alot pf research into this 4 or 5 years ago for my instructor non modual lacture although i am not an expert nor ever will be but my understanding. (stewie i think you mean the bus to croc park . but great analogy )  . although i decided to only do the lecture on the air and nitrox gradient (ppo2) the easy side . compared to the He side which had so many endless possibilitys and complications .

the way the oxyegen window works is like havin a bad smell in a room opening a window to let out the smell which happens but also loosing the heat within the room , (thats the easy explination ) but when you add HE to the equation now there 3 different gases to contend with on inspiration and 4 on expiration .  not only do you have to think about external respiration (lungs to blood) but also internal respiration (blood to cells, where the prob really happens )

when you consider that the size of the membrane that the gas exchange happens across is tiny ,it cannot really deal with loads of different gasses goin in different ways . in science gasses alway seek equalilbirum . so if in fact you bail out of He to air . the N2 differential is bigger and seeks equality quicker than the he trying to get out . with the size of the mambrane the he is effectivly traped . when the reduced surface tension of the He in soultion is reduced (no He in the inspired gas ) the He indeed supersaturates and then bubbles out . now this is where it really gets interesting , if you start an asscent from say as stewie says 60m  you are then reduceing  the surface tension even furthur and the He goes crazy to get out and does some funny things .

[jonnyb]

   Hi Guys,

   I dont wish to undermine anyones knowledge or understanding of this, espeicially you Stewie with your vast experience in both OC and CCR decompression. It was, like yourselves my believe that the Fi inerts, whether He or N2 together or N2 alone, combined with the Pp effected the diffusion out of the inerts in solution i.e. N2 in the inspired gas would still restrict somewhat the off gassing of He. This concept of the interaction of gasses is wrong. This is what sent alarm bells ringing in my, unknownest, limited knowledge of decompression (I thought I had a grasp of it), and thought it important to post here about it and add the link to the article. If you ref. the article, under the heading Is the Oxygen Window Important? on the bottom of page 11 you will understand where I am coming from.

 I think the anology's and simile's (whether rugby players and kids to bad smells and a window) used to simplify what is happening have not helped us to understand exactly what the gasses are doing. Also when diving with He in the mix, and this is just a theory of mine, but when we have a bend or IEDCS we assume it is the He that has caused it. Are we sure about this and could it not posssibly be the N2, or maybe it is the rapid off gassing of He due to the partial pressure vacancy left by the switch in gas, while some tissues have yet to begin off gassing at all.

 I have not yet either read Mark Powell's book and look forward to reading and understanding more, but also if there is something at Eurotek about this I would be interested in reading about it when you's get back, I can't make it unfortunately.

   Jonny, I am not going to paint my colours to the mast here, as it is still theoretical presumptions....

[Stewart Andrews]

Hi Guys, - such an anal discussion really.

From my experience (of others with bends), I still believe that He is 'bullied' out in the case of a reasonable spike/increase in N2 and following intolerable super saturation.....this ends up as a bubble of He - especially in the inner ear - a vestibular bend. So, I'm sticking with MY analogy till I am convinced otherwise!

Hence almost nobody changes (anymore) to a nitrox mix at 45m on a CCR trimix dive - it is just too risky. People feel like crap and their body is warning them against this practice.

See below; my summary: A greater He deco obligation, when more N2 is coming in, may lead to a He bend when running a 'standard' deco table. The one thing I feel that I am sure of is that it is He which forms the bubble. It is the one which has reached super saturation on ascent (as it was the important constituent gas used for all of the dive up to then - before the high N2, low He gas came in).
Edit: I meant to say that I believe that the fact that He is a faster gas, has something to do with it as well - in this case trying to get out faster and not being able to do so, at times, before creating a bubble.

This extract is from the top of page 12 of the link at the top of this thread: esp from sentence two...
 "Decompression from an N2-based dive is longer with N2 containing deco mixes because
some N2 is continuously diffusing into tissue during deco. Decompression from a He-based dive
can be longer with N2 containing deco mixes because N2 is diffusing into tissue as He is diffusing
out of tissue. The decompression obligation of a tissue compartment is based on the sum of gas
partial pressures in the compartment. This means that if a tissue is loaded with N2 as He is being
removed, it tissue has a greater decompression obligation than when no N2 is added to tissue during
He off-gassing." (Johnny E. Brian, Jr., M.D.)

[jonnyb]

    I am fascinated by this topic, understanding what the gasses are doing in our bodies will give us a greater understanding of decompression theory, and how to carry it out safely. The idea of adding the article to the forum was to help enable a discussion as to what would be the best gasses for decompression, and what variables, certain gasses would carry if switched to during the decompression model.

  It would be great if when your at Eurotek, you might mention it to some and clarify it...

     Jonny

[Jack]

I posted about this on here about 6 years back. Have to say, there was some ridicule directed my way. Basically what Stewart posted reflects my original post on this. What is used in OC deco since the 90`s is He in the deco gas 50/25/25. CCR users already get the benefit of mix from surface to surface, this happened by default. A practice of switching to air dil @ 40m was used early on which could have and possibly did result in IEDCS. Anytime an inert gas is removed from the breathing source at depth, a huge off gassing spike is introduced for the removed gas.
Helium is your friend. Put a little in your oc deco gas

[Mister Mike]

What you need is play on the APD DIVE PLANNER / DIVESTORE.......... Not only can you plan dives , it will suggest the best Bailout gasses and give you tables to print off as well. A great device to get your teeth into whilst waiting for the weather to sort itself out......

[Stephen McMullan]

Just a few comments.

* Interesting article Jonny. It didn't add to or contradict anything I currently do but its about as good a description of the mechanism as I've seen.

* As far as I recall Rich did cover at least the practical aspects of how to plan CCR bailout gases to take this into account on the MOD2 Tx course. He wouldn't have gone to this level of theoretical detail at the time and nor should he have.

* You mention that you don't believe computer models take this into account. I would believe (without specific evidence) that some do although you're probably right that they didn't in the past. Mark Ellyatts Decocheck I believe was written to specifically check plans for this in the past. However modern planners like V-planner, APD Projection etc. have warned me off when choosing inappropriate bailout gases for the reasons we're discussing so I'm pretty confident they model the scenario fully.

*There's a suggestion that you can overload the body by performing a radical gas switch so that the pp of an inert goes to zero. Why is this a bad thing? Any evidence? I've read nothing to say so. How many of us have done deep air dives and deco'd on 02? See whats happening to the nitrogen!

* I was at the Simon Mitchell presentation with Stewie at Eurotek '08 and have a different recollection of the mechanism of how IEDCS was produced. But take it from the horses mouth so to speak: http://jap.physiology.org/cgi/reprint/106/1/298.pdf?ck=nck . I haven't had time to read re-read it as the missus is shouting at me to do the hoovering but I'm sure it will clear up the issue of bullies & rugby players on buses   BTW Simon is on RBW and Yorkshire Diver so you could always send him a message but I believe this whole area has been done on those boards before.

* Don't forget that most CCR divers aren't doing gas switches etc. as a matter of course so possibly non-expert in these matters. Personally I reckon OC divers would be a bit more up on it than us. We seem to talk a lot more about depths, runtimes and diluents than about appropriate gases and quantities for bailout (which is when we would be doing the switches). Maybe that is a slur against other CCR divers - I'm sure its just me and an ongoing learning process as my depth range is increasing   Stewie's been there done that and is quite right in saying in his own way that when you splash in its not so important as to why it happens compared to what you do about it to prevent it in the first place.

* CCR bailout is complicated by making sure you have enough gas to get to your next gas switch or gas source. So we're not necessarily carrying very rich deco mixes as we could well be out of gas before we reach the switch point

* Yeah adding some He to a 50% mix might smooth out the N2 ongassing spike but you're reducing the He offgassing so longer in the water for you. This may or may not be significant in the grand scheme of things.

Right I'm off to address the high gradient of filth between my apartment and a new hoover bag. Its a shame it won't diffuse on its own  

[emmbee]

Hi all.

To throw in my Saturday night, 10pm, cents...

I only skimmed the article (see above), but it seems to deal with a couple of basic problems in most decompression models.

To recap (sorry if this seems like well known facts), decompression models actually consist of two distinct models:
1) a model of the state of the body, that is how much gas dissolved where (the compartments);
2) a model of how shallow you can go before something bad happens (the decompression).

Note that the two mainline models, Buhlmann and VPM, only differ in 2). VPM uses the same compartment model as Buhlmann 16. These models are mainline because they've been published, either in a publication (Buhlmann), or as source code (VPM).

So what's wrong?

For 2), Bulhmann at least doesn't model inert mixes for decompression. There's a/b values (or M values, whichever you like) for Nitrogen and Helium separately, but nothing for mixes. And while it's okay to say that gases dissolve according to their own partial pressure only and thus independently of each other, it's not true at all when bubbles start to form. Bubbles formation still isn't so well understood. The "solution"(s) presented in M.Powell's book (which I highly recommend, BTW) are a kludge at best, with neither theoretical nor experimental support. I don't know what VPM does. That's the classic isobaric counter-diffusion (as I think it's called) problem: if you switch for an N2 (slow gas) rich to He (fast gas) rich gas, He will diffuse in faster than N2 will diffuse out, so the overall inert pP will increase. And the model doesn't tell you how high that can go. Yes, planners may warn you, but like I said, the warning algorithm isn't based on much.

For 1), they assume that compartments are directly linked to the lungs. That's a good first approximation, as blood is very fast (in that context), but it prevents modeling the ear thing that Stewart mentioned. If you have compartments in series, say fast-slow-lung, the slow compartment will continue to outgas to the fast one even tho the lungs have a lower pP because you ascended, with the same gas mix. That may lead to problems. The article seems to be saying that just compartment-blood-lungs changes things, because after a certain amount of bubble are formed, diffusion through the lungs goes down. That's not in the models either. And finally the gas composition and pressure of the blood is not constant. To wit, in arterial blood you have loads of O2 as well as N2 (especially if you're deep), but high pressure. In venous, you have much less O2 (or none), but more CO2 and N2 (on ascent), but less pressure. Again, not in the models.

Note thaht some models, DCIEM for one, have compartments in serie, not parallel like Buhlmann (DCIEM is serie only, which is going too far IMHO). But they're not published :-(

CU next week.

Matthieu