Rebreather Discussion from Brockville Incident

[gianaameri]

Many thanks for your kind offer Simon, PM sent. Regards Larry

You may find this of some interest as well:

RF3.0 - Knowing Your Limits - YouTube

 

[Larry7895]

You may find this of some interest as well:

RF3.0 - Knowing Your Limits - YouTube

Many thanks, this presentation combined with Simon's paper and presentation have given me a much clearer understanding of the risks. It does seem we are getting reasonably close to a system that will meet the standards that you have refered to but we are not quite there yet, though some experimental systems do seem to have been built.

What I am not so sure of yet is how significant CO2 retention is at recreational limits. It has also been confirmed to me that CO2 retention is a issue in OC as well as CCR, though my understanding is it more significant in a CCR because WOB is higher due to the diver having to maintain the gas flow in the loop themselves.

Given that CO2 removal in the diver seems to be directly linked to respiration capacity (ie the more CO2 you produce the more volume you have to breath to get rid of it), what I was really surprised to learn was that your maximum breathing capacity falls to half (if breathing oxygen/nitrogen mixes) at 30m even on OC. Given diving on to rec limits on OC is common without many noted CO2 retention issues, even give this large reduction in maximum breathing capacity, it therefore makes me wonder whether even on CCR with its extra WOB, at Rec limits, would CO2 retention really be an issue?

Simon any comments, have I got this correct? (I know I am conveniently ignoring that there is a wide disparity in an individual's predisposition to retain CO2).

 

[gianaameri]

Many thanks, this presentation combined with Simon's paper and presentation have given me a much clearer understanding of the risks. It does seem we are getting reasonably close to a system that will meet the standards that you have refered to but we are not quite there yet, though some experimental systems do seem to have been built.

We are not close at all to fully meet the standard, namely Clause 5.13.1 - Functional Safety.

Even if an End-tidal CO2 Monitor were not required (i.e. CO2 retention were non-existent), you would still need to satisfy all other requirements of Functional Safety (i.e. protect the diver to "SIL 1" level minimum).

It is a game of Russian roulette because it boils down to a game of Probability of Failure (being too high in current electronic rebreathers) and not a game of skill (diver ability).

However, you are right, the technology in the best of cases is incomplete because of the lack of a functional End-tidal CO2 Monitor.

It beggars belief from a moral and ethical perspective how such a system could be released to a category of divers who could quite happily just do the same dive as they currently do on a single tank of Air or Nitrox - but as Gordon Gecko says, "Greed is good" (and $$$ outweigh any other considerations).

The rebreather industry makes a banker look like the canary in a coal mine.

 

[Dr Simon Mitchell]

Given diving on to rec limits on OC is common without many noted CO2 retention issues, even give this large reduction in maximum breathing capacity, it therefore makes me wonder whether even on CCR with its extra WOB, at Rec limits, would CO2 retention really be an issue?

Simon any comments, have I got this correct? (I know I am conveniently ignoring that there is a wide disparity in an individual's predisposition to retain CO2).

Ah, the subtleties and nuances of physiology! What you describe can happen, but we think that it is very rare to get oneself into a situation where maximum breathing capacity (the ability to move gas in and out of the lungs) falls below that needed to keep CO2 normal. This cause of CO2 retention is probably reserved for scenarios involving things like extreme depth and very dense gas, equipment with high work of breathing, and other "ingredients". The David Shaw accident was probably an example of this (I will send you a paper on that too). The more common cause is a puzzling tendency of that part of the brain which controls breathing to become less focussed on keeping CO2 normal if the breathing work required to do so increases (as it does in diving). Thus, it is not so much that you can't breathe enough, but rather that your brain chooses (sub-consciously) not to breathe enough. Its as though the brain has to choose between allowing the CO2 to rise or performing harder respiratory work to keep it normal. Some people are more prone to making the "allow CO2 to rise" choice than others, so we call them CO2 retainers. This is why CO2 retention can be a problem even at rec limits.

Hope this makes sense,


Simon M

 

[gianaameri]

Ah, the subtleties and nuances of physiology! What you describe can happen, but we think that it is very rare to get oneself into a situation where maximum breathing capacity (the ability to move gas in and out of the lungs) falls below that needed to keep CO2 normal. This cause of CO2 retention is probably reserved for scenarios involving things like extreme depth and very dense gas, equipment with high work of breathing, and other "ingredients". The David Shaw accident was probably an example of this (I will send you a paper on that too). The more common cause is a puzzling tendency of that part of the brain which controls breathing to become less focussed on keeping CO2 normal if the breathing work required to do so increases (as it does in diving). Thus, it is not so much that you can't breathe enough, but rather that your brain chooses (sub-consciously) not to breathe enough. Its as though the brain has to choose between allowing the CO2 to rise or performing harder respiratory work to keep it normal. Some people are more prone to making the "allow CO2 to rise" choice than others, so we call them CO2 retainers. This is why CO2 retention can be a problem even at rec limits.

Hope this makes sense,


Simon M

As a scientist/physician, would you rule out CO2 retention on a 40 meter air dive with an apparatus producing 3 - 4 j/l WOB?

Sent from my HTC Desire C using Tapatalk 2

 

[Larry7895]

Ah, the subtleties and nuances of physiology! What you describe can happen, but we think that it is very rare to get oneself into a situation where maximum breathing capacity (the ability to move gas in and out of the lungs) falls below that needed to keep CO2 normal. This cause of CO2 retention is probably reserved for scenarios involving things like extreme depth and very dense gas, equipment with high work of breathing, and other "ingredients". The David Shaw accident was probably an example of this (I will send you a paper on that too). The more common cause is a puzzling tendency of that part of the brain which controls breathing to become less focussed on keeping CO2 normal if the breathing work required to do so increases (as it does in diving). Thus, it is not so much that you can't breathe enough, but rather that your brain chooses (sub-consciously) not to breathe enough. Its as though the brain has to choose between allowing the CO2 to rise or performing harder respiratory work to keep it normal. Some people are more prone to making the "allow CO2 to rise" choice than others, so we call them CO2 retainers. This is why CO2 retention can be a problem even at rec limits.

Hope this makes sense,


Simon M

Thanks Simon, yes it does make sense, but I'm still trying to understand, is CO2 retention at rec limits worse on CCR or the same as OC? Or am I still barking up the wrong tree? I am guessing that it is the same other than the additional WOB that a rebreather has and the consequent impact this may have on the brain deciding to get the lungs to work harder or accept a CO2 build up. Ideally I'd like to know if that means for all practical purposes CO2 retention is the same on CCR as it is on OC, 10% more likely, 20% more likely etc or am I asking a question that can't really be answered yet?. Happy to have my ignorance corrected Many Thanks.

 

[gianaameri]

Thanks Simon, yes it does make sense, but I'm still trying to understand, is CO2 retention at rec limits worse on CCR or the same as OC? Or am I still barking up the wrong tree? I am guessing that it is the same other than the additional WOB that a rebreather has and the consequent impact this may have on the brain deciding to get the lungs to work harder or accept a CO2 build up. Ideally I'd like to know if that means for all practical purposes CO2 retention is the same on CCR as it is on OC, 10% more likely, 20% more likely etc or am I asking a question that can't really be answered yet?. Happy to have my ignorance corrected Many Thanks.

CO2 Retention is a function of WOB.

Try Warkander and others for an honest answer here:

1. Comprehensive Performance Limits for Divers' Underwater Breathing Gear: Consequences of Adopting Diver-Focused Limits.
2. Work of Breathing Limits for Heliox Breathing
3. Physiologically and subjectively accepta... [Undersea Biomed Res. 1992] - PubMed - NCBI
4. Physiological and Human Engineering Aspects of Underwater Breathing Apparatus.

 

[Dr Simon Mitchell]

As a scientist/physician, would you rule out CO2 retention on a 40 meter air dive with an apparatus producing 3 - 4 j/l WOB?

Sent from my HTC Desire C using Tapatalk 2

Hi Gian,

Certainly not. I would not rule it out.

Simon M

---------- Post added July 19th, 2013 at 06:55 AM ----------

Thanks Simon, yes it does make sense, but I'm still trying to understand, is CO2 retention at rec limits worse on CCR or the same as OC? Or am I still barking up the wrong tree? I am guessing that it is the same other than the additional WOB that a rebreather has and the consequent impact this may have on the brain deciding to get the lungs to work harder or accept a CO2 build up. Ideally I'd like to know if that means for all practical purposes CO2 retention is the same on CCR as it is on OC, 10% more likely, 20% more likely etc or am I asking a question that can't really be answered yet?. Happy to have my ignorance corrected Many Thanks.

Hello Larry,

OK, I understand your question. Certainly possible on both. As to which is more likely... this probably does fall into the category of not possible to answer in a definitive sense. Moreover, merely specifying OC or CC is probably too coarse a definition of the circumstances to be useful. There is considerable variability in the breathing characteristics among both types of device depending on user mods and maintenance. How much exercise is being performed also plays a big role. If I was forced to choose I would say it is probably more likely on CC, but not to an extent that would influence my choice between CC and OC in any way. Again, hope this makes sense.

Simon M

 

[gianaameri]

Hi Gian,

Certainly not. I would not rule it out.

Simon M

The WOB tests are all done under ideal laboratory conditions.

An end-user can't do any better in actual use than the WOB tests done by the manufacturers under ideal conditions.

It is very easy for a rebreather with a WOB which is not so good to start with under ideal lab conditions to move in actual use to the upper range of 3.0 - 4.0 j/l.

Another "User Error" risk.

 

[mathauck0814]

The WOB tests are all done under ideal laboratory conditions.

An end-user can't do any better in actual use than the WOB tests done by the manufacturers under ideal conditions.

It is very easy for a rebreather with a WOB which is not so good to start with under ideal lab conditions to move in actual use to the upper range of 3.0 - 4.0 j/l.

Another "User Error" risk.

Survived yet another day of russian roulette Happy horse beating.