ICD - preventing Inner Ear DCS

[Miyaru]

In an earlier thread, ICD came up as a decompression risk. ICD is basically a description of physical facts which play a role when you change breathing mixes. Dr. Simon Mitchell explained in this post how ICD can lead to IEDCS when going from a high helium mix (heliox/trimix) to a high nitrogen mix, using OC but the same applies when you have to bail-out from CC at depth.

Two strategies or rules-of-thumb to minimize IEDCS are to

1. keep the rise of ppN2 within 0.5bar
2. keep the change of He:N2 within a ratio of 1:5.

However, there must be more to it. Depth and time are not taken into consideration looking at these two strategies.

Sheck Exley described his approach to prevent an ICD hit by taking 1 breath from his decogas, followed by 2 breaths of backgas, then 2 breaths of decogas followed by 1 breath of backgas and then breathing decogas. No vertigo, no vomiting, not narced.
Let's say that's a timespan of half a minute in which the partial pressures of all inert gasses in your lungs are more gradually changing than from one breath to the next. Is half a minute enough to prevent exceeding the critical supersaturation of the endolymph and the surrounding sensitive functional inner ear tissue?

Course material from agencies don't say much on preventing ICD either. It's limited to describing the mechanism that can lead to IEDCS and the two prevention stategies.
One organization ignores the ICD risks by stating that the use of standard gasses excludes this. Which I don't really understand since a change from standard gas 10/70 to standard gas 15/55 exceeds the limits from both strategies.

My experience sofar is all above 100m/330ft, but before passing that depth I want to know how to minimize the risks. There must be loads of succesful deco strategies from those depths and I'd like to better understand these upfront. So educate me from your experience!

• When do the risks kick in?
• After how much bottomtime at what depth is the risk becoming an issue?
• For how much time after a switch is there an IEDCS risk?
• From what depth can you exceed the limits set by the mentioned strategies?

 

[TrimixToo]

After a lot of reading about this several years ago, I adopted the very conservative strategy of planning gasses for deco that always have both decreasing PPN2 and PPHe.

 

[elmo]

This is an interesting topic. You mentioned in the other thread that Simon Mitchell was referring to research ~4 years ago and you were interested in more current information. As little as my understanding of the topic may be, 4 years ago is pretty current when it comes to decompression theory and I think that is about bleeding edge of the research (but far from precise). I am not aware of any work more recent than that by Mitchell and Doolette. Certainly not Sheck Exley or the rules of thumb referred to.

 

[w3dge]

I'd be interested in the outcome of this also. I've been taught about theory of ICD, and the mitigation strategies, but I'm left wondering about exactly when it becomes a risk factor.

Is it something to consider on all normoxic trimix dives? Only ones pushing limits? Or does it really become a factor when using hypoxic mixtures. Obviously tissue supersaturation has a huge impact here, so my initial gut feeling is that 'smaller' dives have less of a risk factor.

 

[Germie]

There is another theory and that is that it does not happen shallower than 60m as maximum depth. But of course the bottomtime is important. Ander never is stated when and where the problems will arise.
The 1:5 rule is most used.
Some divers state if you use the standardgases no icd risks. But standardgases are not always the best choice in decotime/strategies.
A lot is unknown.
If you use Multideco as planner there is a nice graphic option to see the PN2, PHe and PO2 differences after a gasswitch. It is just an illustration, but it shows the same things as in Mitchells picture.

 

[100days-a-year]

I'm not sure inner ear DCS is as easy to predict as some would think.No one I know that' had it got it diving deep or with gas switches and all were on easy dives with no ascent speed issues.
I'd be interested to see some documentation on how prevalent it is.

 

[Miyaru]

....
You mentioned in the other thread that Simon Mitchell was referring to research ~4 years ago and you were interested in more current information.
....

I was actually referring to the fact that Dr. Mitchell's post was a copy/paste from 2013. It's a very good explanation of what happens in the inner ear tissue during a gas switch. But it doesn't say when the conditions for this to happen are met.

....
Is it something to consider on all normoxic trimix dives?
...

Above 80m/260ft it obviously isn't an issue for shorter dives. Shorter meaning the dives I do on OC, where bottom time is limited to the amount of backgas available.
On such dives, switching from 18/45 to EAN50 at 21m keeps the ppN2 increase below 0.5bar, and although the increase exceeds the 5:1 strategie, it doesn't cause IEDCS. At least not for the OC divers I know of. In the other thread, people generally agreed that switching from 21/35 to EAN50 does not cause problems either.

Sheck Exley described his ICD avoidance technique for switches at a depth of around 80m/260ft, obviously a phased switching gave the fast tissues enough time to stay below critical supersaturation. And that is after coming up from much deeper dives.

So the grey area of possible ICD problems starts when you dive below 80m/260ft. The case report in the paper from Doolette and Mitchell (2003) describes a diver getting an ICD slam at 9m after a dive to 110m for 25min. That was a CC dive with more gradual changes in partial gas pressures on the ascent than OC, but anyway shallow enough to illustrate that the risk starts at depths just below 80m. The time you stay at that depth is an important factor I guess.

 

[rossh]

In an earlier thread, ICD came up as a decompression risk. ICD is basically a description of physical facts which play a role when you change breathing mixes. Dr. Simon Mitchell explained in this post how ICD can lead to IEDCS when going from a high helium mix (heliox/trimix) to a high nitrogen mix, using OC but the same applies when you have to bail-out from CC at depth.

Two strategies or rules-of-thumb to minimize IEDCS are to

1. keep the rise of ppN2 within 0.5bar
2. keep the change of He:N2 within a ratio of 1:5.

However, there must be more to it. Depth and time are not taken into consideration looking at these two strategies.

Sheck Exley described his approach to prevent an ICD hit by taking 1 breath from his decogas, followed by 2 breaths of backgas, then 2 breaths of decogas followed by 1 breath of backgas and then breathing decogas. No vertigo, no vomiting, not narced.
Let's say that's a timespan of half a minute in which the partial pressures of all inert gasses in your lungs are more gradually changing than from one breath to the next. Is half a minute enough to prevent exceeding the critical supersaturation of the endolymph and the surrounding sensitive functional inner ear tissue?

Course material from agencies don't say much on preventing ICD either. It's limited to describing the mechanism that can lead to IEDCS and the two prevention stategies.
One organization ignores the ICD risks by stating that the use of standard gasses excludes this. Which I don't really understand since a change from standard gas 10/70 to standard gas 15/55 exceeds the limits from both strategies.

My experience sofar is all above 100m/330ft, but before passing that depth I want to know how to minimize the risks. There must be loads of succesful deco strategies from those depths and I'd like to better understand these upfront. So educate me from your experience!

• When do the risks kick in?
• After how much bottomtime at what depth is the risk becoming an issue?
• For how much time after a switch is there an IEDCS risk?
• From what depth can you exceed the limits set by the mentioned strategies?

There are two apparent types or causes to IEDCS - deeper and shallow types.

The deep cause, as partially described in Simon's post, is also the type that is monitored for as IBCD warning in V-Planner and MultiDeco, since 2004. The (adjustable) 0.5 ppInert increase, is a fitted value that seems to align well with the experiences.

The deeper type applies to OC divers, and CCR divers doing bailout.

IBCD is accelerated deco, taken to the absolute maximum point possible. The He is made to leave the tissue ASAP, and the deco model takes advantage of the lull in pressures and ascends quickly through many adjacent levels/stops. While the math says its OK, can the body really keep up?


The deep IBCD is a question of proportions. We all experience the IBCD in small amounts on ordinary tech dives with virtually any gas switch, and have been using and enjoying the accelerated deco benefit from it. But the pressure differences and volumes of gas involved get bigger with depth and time of dive. Like most things in nature, a little of something is OK and lot will do you harm. Such appears to be the case with IBCD.


There is no hard cutoff point or exact depth where IEDCS / IBCD issues start. Where each individual fits within that scale of excess supersaturation tolerance will determine their outcome on the day. Experience shows that typical dives to 90m or so are tolerable, and beyond that, precautions are needed.


************

The second type of IEDCS is the shallow dive version. These are the common ones that occur at 20 and 30m dives. I am not aware of an explanation for these as yet, and no apparent supersaturation conditions exist to trigger it. Maybe its to do with regular ear equalization gone wrong??


************


Simon's description misses one key aspect of the deep IBCD/IEDCS issue. The cause has two parts - part a/ is a big gas switch causing a strong He off gas gradient (per Simon's post), and part b/ is the continued ascent which creates excessive the supersaturation in the inner ear fluids. Its both in combination that lead to injury, but each in isolation does not.

Cheers.

 

[Miyaru]

"There is no hard cutoff point or exact depth where IEDCS / IBCD issues start."

Yes, I can understand that. It will differ from diver to diver. But in V-Planner or MultiDeco, you must select a depth below which you take IBCD into consideration. I can change the IBCD settings for N2 and He, these values default to 0.5bar. From what I understand in your description, the software takes this setting into consideration when the dive goes deeper than 90m. Is there also a minimum bottomtime threshold?

The explanation from Simon has some strict boundaries which he described, staying at depth is one of them (which makes the gas switch isobaric). Within those boundaries, does the planning software consider the 1:5 rule-of-thumb?

Mitchell and Doolette use a water solubility approach, Stephen Burton used a lipid solubility approach. I cannot argue about those choices, since I lack the medical knowledge about tissue solubility and perfusion. Sheck Exley demonstrated that time is a factor as well, not backed up by a theoretical model, but still worth considering and investigating?

 

[rossh]

"There is no hard cutoff point or exact depth where IEDCS / IBCD issues start."

Yes, I can understand that. It will differ from diver to diver. But in V-Planner or MultiDeco, you must select a depth below which you take IBCD into consideration. I can change the IBCD settings for N2 and He, these values default to 0.5bar. From what I understand in your description, the software takes this setting into consideration when the dive goes deeper than 90m. Is there also a minimum bottomtime threshold?

The explanation from Simon has some strict boundaries which he described, staying at depth is one of them (which makes the gas switch isobaric). Within those boundaries, does the planning software consider the 1:5 rule-of-thumb?

Mitchell and Doolette use a water solubility approach, Stephen Burton used a lipid solubility approach. I cannot argue about those choices, since I lack the medical knowledge about tissue solubility and perfusion. Sheck Exley demonstrated that time is a factor as well, not backed up by a theoretical model, but still worth considering and investigating?

As I said, The MultiDeco and V-Planner IBCD alarm, is based on a 0.5 ppInert gas rise (inclusive of stops and ascents). You can adjust that to what ever you prefer, or remove it if you like. There is no fixed depth here.

To achieve this 0.5 ppInert rise or more, one has to either make some bad deco mix choices, or have enough absorbed inert gas to make rise to this level. Under normal planning conditions, that requires a dive around 80 to 100m or more. Its a lot less for CCR divers on bailout.

My own research on case reports from early 2000's, showed that a 0.5 level was a good fit into the data, to issue a warning. That what you see - a warning message that one should think twice about the choices made.

Cheers.