inner ear DCS

[thunderball5]

Inner ear DCS by way of inert gas counter diffusion is a risk in deep multi-gas diving.
However, switching from helium gas mixtures to nitrox is necessary to accelerate decompression and is the basis of decompression diving both on OC and CCR dives. What guidelines should one use when making gas switches during deco? How much of a gradient between helium and nitrogen-rich breathing gases is acceptable?

As you know, gas selection is based on:
1. END (~100fsw, for example)
2. ppO2 (1.4ata, for example)
3. availability and logistics of mixing the gases!
4. Training guidelines that advocate the use if Eanx50% at 70fsw and 100% O2 at 20fsw.

Should a diver use an intermediate trimix during ascent (deep stops) as the initial deco gas, to serve as a transition to Eanx 50%? Or just use the back gas (trimix) as long as possible?

What guidelines should a CCR diver use when forced to switch to OC bail-out gases?

 

[Duke Dive Medicine]

Thunderball,
If you're talking strictly about breathing gas, it's fine (and sometimes even beneficial) to switch from a helium-based bottom mix to an N2O2 decompression mix. Nitrogen and helium have different solubility coefficients, so the helium will diffuse out faster than the nitrogen can diffuse in, especially in the 70 foot range where the diver would be switching over to 50/50 nitrox.
You may have heard about case reports and studies where divers breathed a mix with a heavier inert gas like nitrogen while physically surrounded by helium in a dry hyperbaric environment. In that case, a phenomenon called superficial counterdiffusion can take place, where helium actually diffuses in through the skin and can cause cutaneous and inner ear DCS. This shouldn't happen in a tech diver, who's only breathing the gases through his or her lungs.
As far as switching to bailout gas on a CCR, that's more a function of the planned depth of the dive. The diver should plan to use a bailout gas that's compatible with the depth on the bottom. One thing that a diver WOULDN'T want to do is use an N2O2 diluent in the rebreather with HeO2 or trimix as a bailout gas. The diver's tissues would contain dissolved nitrogen (more or less depending on the depth/bottom time/residual N2 from other dives), and if he/she switched from the heavier inert gas (N2) to a lighter one (He), the higher solubility of the helium would cause it to dissolve into the tissues fairly rapidly and could result in tissue supersaturation and subsequent DCS.

 

[Kevrumbo]

.....

. . .It should be intrinsically obvious that removal of a gas from tissue can be speeded by elimination of the gas from the inspired mixture. . .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, its tissue has a greater decompression obligation than when no N2 is added to tissue during He off-gassing. . .
p.11-12, Gas Exchange, Partial Pressure Gradients, and the Oxygen Window, Johnny E. Brian, Jr., M.D.

. . .What this all adds up to is the following isobaric switch
prescription, which is, of course, outside of most dive
logistics. But useful on first principles -- and some places
DO have the support in place for just such logistics.

So, it goes like this.

Best deco strategy is to increase O2 on way up in same proportion
as He is reduced, while keeping N2 relatively constant. Switch to
a nitrox mix with less N2, and thus outgas both He and N2. . .
the lower N2 nitrox switches avoid gradient slams and further ingassing of
N2.

Practically, [and in actual usage] this means all the above plus EAN50 at the 70 fsw
level for long exposures, and/or He to the surface for shallow
exposures, plus O2 in the 20 fsw zone either way.

Cheers, and safe diving always,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
--------------------------------------------------------------------------------

Let me see if I can reduce this....

down to something a bit more practical.

Is the bottom line that from a bubble mechanics point of view, one should never increase the percentage of an inert gas as one "rides up" the decompression chain?

So, for instance, if you are using Trimix 18/45, the FN2 in the bottom mix is 37%. Thus, switching to a gas with more than 37% FN2 as a deco mix is inadvisable?! That precludes a 70' 50/50 bottle, but might allow a 60/40 bottle, which is pretty close to 37%.

Or, for example, if you're using 21/35, the FN2 is 44% in that mix; as a consequence the "maximum FN2 permitted" is 44%? That might permit 50/50, as that's pretty close, but even better might be that 60/40 mix again....

In both of the above cases you COULD ride the Trimix up to the 30' depth, in that neither would become hypoxic and, in fact, you could breathe the 18/45 on the surface (its a bit hypoxic but not significantly so.)

I think I understand what the prescription is here, in that increasing the Fx of an inert gas can cause counterdiffusion (actually increasing your inert gas load rather than decreasing it in that gas, even as the other gas vacates your body) but this is majorly counter to what I've understood about general deco strategy.

(The odd thing is that for a deco dive on Nitrox, for example, taking a single bottle of 50/50 might remain an optimum strategy, in that there is no counterdiffusion problem since the FN2 is always decreasing, and if something goes wrong on the shallow stops you've done some of the deco with an advantageous oxygen window .vs. no benefit from that, but that same deco gas option on a 21/35 dive would be inadvisable, with either a single bottle of 80/20 or 100% being preferred.)

Did my distillation end up correct?

--------------------------------------------------------------------------------

Optimal 70 fsw mix

BRW;

Sounds like you would advocate 50/25/25 at the 70 over 50/50 nitrox coming off, say, a 15/50 at 250 kind of dive. Is that correct?

Isobaric Countertransport And Mix Switch Strategies

Mark Powell in Deco For Divers says on p189 that the issue of ICD (Isobaric Counter Diffusion) is not a problem until you reach depths of around 80m. Taking literally his "straightforward rule of thumb" recommendation [for a deco gas]: Never let your partial pressure of nitrogen rise significantly. Powell also goes on to quote a less conservative approach by Steve Burton on p195, by "increasing the nitrogen percentage by no more than 1/5th of the reduction in the helium percentage. . ."

See also:
http://www.subpacific.cl/icd.pdf

 

[Duke Dive Medicine]

http://www.scubaboard.com/forums/as...c-countertransport-mix-switch-strategies.html

Thanks for the link, that's a good thread. Bruce's comment: "1) -- light-to-heavy switches are the ONLY safe switches, and once started, should never be reversed back the other way;" sums it up very well.
I think it's important to differentiate what Bruce said was ideal from what he said was practical. Ideally, increasing the pO2 while decreasing the pHe in the mix until you reach a depth where the pN2 in the back gas is equal to the pN2 in the decompression mix makes the most sense (provided you remain in safe territory with the pO2), but to implement that strategy would mean carrying multiple gas bottles and may not be practical. In practice, what's necessary is to avoid any additional on-gassing of N2 when switching mixes, which is dependent on the diver's N2 load and the pN2 in the deco mix, and there's more than one way to skin that cat.

Thunderball, did your question get answered here?

 

[thunderball5]

I read Doolette and Mitchell (2003) as well as the ad hoc info about the 1/5 rule on scubaengineer.com. All very interesting. Also, I am clear on NOT switching from nitrox to trimix on ascent (unless for some stupid reason I have no other gas to breath). I'm all over the no-trimix in the drysuit rule too.

Finally, obviuosly all gas switches should be done with close scrutiny to narcosis and oxygen toxicity.

However.

My question remains: when making an ascent from say 100meters on an OC dive with trimix. As I follow my deco schedule (using vplanner), making all my deep stops on my bottom gas - trimix. Is it really safe to then switch to 50% nitrox at 70fsw (this is the basis of what we all routinely do on many oc deco dives)? Or, conversely, when is it NOT safe to do so (for example, when my bottom time exceeds a certain number or when I got to 120meters)? you see, the main risk appears to be when the helium - to - nitrogen switch is made on ascent - and intermediate depths!

Are you aware of any models that advocate the use of an intermediate trimix - to use as a deco gas on ascent - inbetween the helium rich bottom gas and the nitrogen rich deco gas? Let's call it a "transition gas" (travel gas has other connotations).

I am going to run some simulations on vplanner this weekend with just such a transition trimix but keeping all other variables constant and see what it does to my profile. Unfortunately, there is no warning tool in any of the deco software programs I have to alert the diver when a helium to nitrogen switch is going to embarass my inner ear!

by the way. I am a physician, tec diver, instructor.

Dirk Peterson.

 

[TSandM]

Shoot -- I can't find the paper, but I was recently reading a paper where the authors did a model of gas transport in the inner ear (based on some animal studies) and came to the conclusion that switches off helium to nitrogen should either be done deep or shallow, but not at intermediate depths. The problem, IIRC, was that they really didn't define those terms.

I know that, for these very deep dives, GUE uses deep deco mixtures that contain helium.

 

[bvanant]

I was under the (apparently mistaken) impression that He was less soluble in both aqueuous and lipid tissues (from Chris Duer's paper) so Duke Docs comment on the higher solubility makes me want to understand the issue. I understand that He diffusion is much faster than N2 (aqueous diffusion coefficients are like 3x) so is that what the issue is?
We use helium for removing dissolved N2/O2 in much of our lab gear, precisely due to its lower solubility.
Bill

 

[Duke Dive Medicine]

I was under the (apparently mistaken) impression that He was less soluble in both aqueuous and lipid tissues (from Chris Duer's paper) so Duke Docs comment on the higher solubility makes me want to understand the issue. I understand that He diffusion is much faster than N2 (aqueous diffusion coefficients are like 3x) so is that what the issue is?
We use helium for removing dissolved N2/O2 in much of our lab gear, precisely due to its lower solubility.
Bill

Bill, sorry, you're not wrong, I didn't put it very clearly (but you probably already knew that!) Diffusibility vs solubility: Helium solubility coefficient is 0.0104, N2 solubility coefficient is 0.0158, so at saturation at equal ambient pressures, 52% more N2 is physically dissolved. Helium diffuses more rapidly than N2.
Thanks,
DDM

 

[Kevrumbo]

I read Doolette and Mitchell (2003) as well as the ad hoc info about the 1/5 rule on scubaengineer.com. All very interesting. Also, I am clear on NOT switching from nitrox to trimix on ascent (unless for some stupid reason I have no other gas to breath). I'm all over the no-trimix in the drysuit rule too.

Finally, obviuosly all gas switches should be done with close scrutiny to narcosis and oxygen toxicity.

However.

My question remains: when making an ascent from say 100meters on an OC dive with trimix. As I follow my deco schedule (using vplanner), making all my deep stops on my bottom gas - trimix. Is it really safe to then switch to 50% nitrox at 70fsw (this is the basis of what we all routinely do on many oc deco dives)? Or, conversely, when is it NOT safe to do so (for example, when my bottom time exceeds a certain number or when I got to 120meters)? you see, the main risk appears to be when the helium - to - nitrogen switch is made on ascent - and intermediate depths!

Are you aware of any models that advocate the use of an intermediate trimix - to use as a deco gas on ascent - inbetween the helium rich bottom gas and the nitrogen rich deco gas? Let's call it a "transition gas" (travel gas has other connotations).

I am going to run some simulations on vplanner this weekend with just such a transition trimix but keeping all other variables constant and see what it does to my profile. Unfortunately, there is no warning tool in any of the deco software programs I have to alert the diver when a helium to nitrogen switch is going to embarass my inner ear!

by the way. I am a physician, tec diver, instructor.

Dirk Peterson.

Here's an excerpt of a deco gas planning write-up for some of the deeper longer cave dives of the WKPP (utilizing a dry habitat for Oxygen deco), and an example of considerations regarding ICD:

A trimix of 10.5 percent oxygen/ 80 percent helium was selected owing to the average bottom depth of 280'/85m. Considerations in this selection were:

Since many tissue compartments will reach saturation and decompression will take longer than a few hours, the high helium content has advantages for off-gassing effficiently later in the dive. The amount of time helium takes to reduce its partial pressures in tissues by one-half are about 2.7 times faster than the half-times for nitrogen. . .

As decompressions times lengthen to two and a half hours or more, counterdiffusion of excessive amounts of nitrogen can become a real problem. It can have the effect of doing a deep air dive in the middle of decompression. As shallower stops are made near the end of deco, the diver's body can be loaded with enough nitrogen that it offsets any advantages gained in eliminating helium. Because of nitrogen's greater molecular weight, greater solubility in body tissues and slower half-times, it can take longer and be more difficult to eliminate than helium. This is a special concern at the final deco stop where oxygen is used to remove inert gas from the slowest tissue compartments. . .

[Non-standard, intermediate] decompression mixes that achieve an acceptable balance of these factors are a trimix of 19 percent oxygen / 50 percent helium at 240'/73m; trimix 25 / 35 at 190'/58m; trimix 35 / 25 at 120'/36m; trimix 50 / 15 at 70'/21m; 100 percent oxygen at 28'/8.6m [in a dry habitat], with periodic breaks using trimix 15 / 45.

This selection allows the fraction of helium to gradually taper off while the fraction of oxygen gradually increases and the fraction of nitrogen remains nearly constant. Helium off-gases efficiently with the reduction in pressure and the increasing oxygen fractions. Nitrogen loading during deco is kept below target limits upon arrival at the [oxygen] dry habitat stop. . .

From Erik C. Baker, Decompression Strategies Enable Deep, Long Explorations of Wakulla Springs, Immersed Magazine p.30, Fall 1999.
See also Erik Baker and the Varying Permeability Model: Technical VPM Publications

 

[rossh]

At my web site, I have some basic diagrams of the IBCD problem, and how it relates to deco times. Decompression myths and mistakes In V-Planner we suggest limiting the increase of inert pp to 0.5 between mixes. This is measured between the swap point depth - the inspired pp of each.

Thanks to KevRumbo for the referenced comments above. I concur with Mark Powell on this issue.

If you're talking strictly about breathing gas, it's fine (and sometimes even beneficial) to switch from a helium-based bottom mix to an N2O2 decompression mix. Nitrogen and helium have different solubility coefficients, so the helium will diffuse out faster than the nitrogen can diffuse in, especially in the 70 foot range where the diver would be switching over to 50/50 nitrox.

This is the standard argument listed in medical journals. In normal deco dives with modest deco times we can all "get away with it" and turn the effect into a reduction of deco time benefit.

But its actually the root cause of IBCD in deep dives. For very deep dives with longer decompression, then we must avoid any spikes of N2 pp in deco. Allowing a big N2 spike mid way through deco, will bring on a gas reversal in the tissues. Helium will diffuses out at an uncontrolled rate, whiles N2 diffuses in at an uncontrolled rate. This happens mid way through your "controlled" decompression. The end result is that the body can not fully perform the reversal, deco is compromised from the mid point onwards. The diver has created an IBCD injury half up way through the deco schedule, which now requires a lot of extra deco time to counter and correct for.

Regards