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Thread: Missed the talk on deep CCR at Eurotek =(

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    Re: Missed the talk on deep CCR at Eurotek =(

    Quote Originally Posted by sadave  View Original Post

    The full paper has been published, I know its been posted on here a couple of times, but for those who haven't seen it already, its a free download, and well worth the read.
    Thanks dave....

  2. #12
    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: Missed the talk on deep CCR at Eurotek =(

    Hello,

    Between Mark and Sutty most of the key concepts were there.

    A couple of clarifying points:

    I was definitely not saying that back-mounted counter-lungs are invariably bad. Indeed, they have their advantages (which is why the USN has been using them for decades). However, the diver with a BMCL swimming horizontally will have a negative static lung load (the pressure in the airways is slightly lower than the ambient pressure). It is reasonably well documented that a negative static lung load is less well tolerated by divers when exercising hard, especially at signfiicant depth. There are probably a number of reasons for this but one of them is that a negative static lung load almost certainly makes the airways more likely to collapse during forced exhalation when breathing a dense gas (because of the reasons articulated by Mark and Sutty). To summarise: The pressure generated inside the chest by contraction of the respiratory muscles during exhalation may actually collapse small airways inside the lung because the pressure inside those airways drops more quickly as gas flows out of alveoli and along them when the gas is dense and flow rates are high. Once this starts to happen it becomes almost impossible for the diver to increase lung ventiliation and therefore CO2 elimination which is entirely dependent on ventilation. Indeed, the extra effort to do so just produces more CO2, which itself drives more breathing effort, and so on into the spiral I described. Now, you must remember that the accident I described at Eurotek occurred at extreme depth with the inevitably dense gas, a loop that probably had higher resistance to flow than normal (see below) and a negative static lung load (that probably predisposes the airways to collapse). In other words, it was the result of a lot of factors coming into play simultaneously. The BMCL and negative static lung load was just one of these factors. Similarly, I would not deem the use of non-standard moisture pads in the rebreather loop to be the "key issue". The almost inevitable increase in resistance to gas flow in the loop was another one of several factors that would have resulted in impaired ventilation and the build-up of CO2.

    Poor David's tragic legacy to the rest of us is a stark reminder that when working at extreme depths whilst breathing dense gas there is a very real danger of inadequate ventilation and CO2 build-up. This CO2 build up may eventually drive levels of respiratory effort that cause airway collapse during exhalation. This in turn creates a spiral of increasing respiratory effort that produces more CO2 but does not achieve extra ventilation (which you need to get rid of the CO2).

    These are actually quite complicated physiological concepts that defy my attempts at accurate portrayal in an internet post. The DAN technical diving workshop proceedings will have a detailed account of these matters, and will soon be available for purchase though DAN (not very expensive I don't think).

    Hope this helps.

    By the way, I thought the Eurotek weekend was a fabulous event, and I offer my sincere congratulations to Leigh, Carl and Roz for making it happen so seamlessly.

    Warm regards,

    Simon M
    Last edited by Simon Mitchell; 23rd November 2008 at 02:45.

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    Re: Missed the talk on deep CCR at Eurotek =(

    Simon,

    If I am understanding this correctly - would a diver be able to cease the CO2 retention and get his lungs "flushed" by rolling onto his back if the work was stopped and he was resting (BMCL of course)?

    Just trying to figure out if there is a way to arrest this problem before it gets out of hand. Please assume that the mix is not too dense for the depth and that the WOB is within reasonable expections.
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  4. #14
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    Re: Missed the talk on deep CCR at Eurotek =(

    Quote Originally Posted by Dsix36  View Original Post
    Simon,

    If I am understanding this correctly - would a diver be able to cease the CO2 retention and get his lungs "flushed" by rolling onto his back if the work was stopped and he was resting (BMCL of course)?

    Just trying to figure out if there is a way to arrest this problem before it gets out of hand. Please assume that the mix is not too dense for the depth and that the WOB is within reasonable expections.
    Hello,

    The "rolling onto your back" thing has arisen at these presentations in response to the inevitable questions along the lines of "what could David have done to arrest / reverse the process".

    As you know, there are a number of responses to CO2 toxicity during a rebreather dive and to some extent their efficacy depends on the cause of the problem.

    If the problem is caused by failure of the scrubber and CO2 rebreathing, then flushing the loop with fresh gas or bailing out (or both) will usually solve the problem. Those who were at my presentation will recall me stating that David did flush the loop for quite some time during his dive but that it didn't seem to make any difference. The key point is that you would not expect it to if the problem is not the scrubber, but mainly one of not breathing enough to get rid of the CO2 your body is producing (which is what I have described in my earlier post).

    This brings us to how you deal with this latter situation. Either you have to increase ventilation of the lungs, or reduce CO2 production, or both. Now, up until some point in most of these scenarios the situation can be managed with the classical "PADI advice" of stopping, resting, breathing deeply which should lower CO2 production and may improve ventilation. Other options include changing to a breathing device with lower resistance, and breathing less dense gas (eg by getting shallower).

    The danger in very deep diving with dense gas is that the work of breathing itself may become so high that a large proportion of your CO2 production comes from that alone, especially if your breathing loop has high resistance. Thus, stopping and resting won't necessarily help. You still have to breathe. Hope you can see what I am getting at here. Now, this is most particularly true of any situation where you are in the spiral I describe in my earlier post. In this setting you are trying hard to move more gas in and out of your lungs to get rid of CO2, but because of the airway collapse I described above your efforts just result in production of more CO2 and little or no extra ventilation. Once you are in this situation it would be very hard to get out of it. In the case of a diver wearing a BMCL with a negative static lung load, there MIGHT be some merit in rolling onto your back to give yourself a positive static lung load (which MIGHT make the airways less likely to collapse, therefore allowing you to increase ventilation without increased effort). This intervention is completely speculative, and has not (to my knowledge) been tested in any objective way. But it does make sense in terms of what we know about back and front mounted counterlungs and exercise capacity underwater, and it is consistent with the way we sometimes treat patients with chronic lung disease who have this airway collapse problem even breathing air at atmospheric pressure.

    Please understand, this is just speculation about managing a scenario that is probably very rare (but has the potential to become more common as we keep pushing deeper).

    Warm regards,

    Simon M

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    Re: Missed the talk on deep CCR at Eurotek =(

    Simon

    Not sure if you know but is there a depth at which this issue become one of the major variables that should be reflected in gas planning (trimix, he % and/or heliox) decisions?

    I am thinking something here like when planning max Po2 you take into account and buffer for things like cold, work load etc.

    John

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    Re: Missed the talk on deep CCR at Eurotek =(

    Quote Originally Posted by Simon Mitchell  View Original Post
    Hello,

    The "rolling onto your back" thing ,,,,
    ,,,,,,to become more common as we keep pushing deeper).

    Warm regards,

    Simon M
    I think that I am understanding this much better now. Thanks for taking the time to explain it a little further.

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    Re: Missed the talk on deep CCR at Eurotek =(

    Quote Originally Posted by Simon Mitchell  View Original Post
    I was definitely not saying that back-mounted counter-lungs are invariably bad. Indeed, they have their advantages (which is why the USN has been using them for decades). However, the diver with a BMCL swimming horizontally will have a negative static lung load (the pressure in the airways is slightly lower than the ambient pressure). It is reasonably well documented that a negative static lung load is less well tolerated by divers when exercising hard, especially at signfiicant depth. There are probably a number of reasons for this but one of them is that a negative static lung load almost certainly makes the airways more likely to collapse during forced exhalation when breathing a dense gas (because of the reasons articulated by Mark and Sutty). To summarise: The pressure generated inside the chest by contraction of the respiratory muscles during exhalation may actually collapse small airways inside the lung because the pressure inside those airways drops more quickly as gas flows out of alveoli and along them when the gas is dense and flow rates are high. Once this starts to happen it becomes almost impossible for the diver to increase lung ventiliation and therefore CO2 elimination which is entirely dependent on ventilation. Indeed, the extra effort to do so just produces more CO2, which itself drives more breathing effort, and so on into the spiral I described. Now, you must remember that the accident I described at Eurotek occurred at extreme depth with the inevitably dense gas, a loop that probably had higher resistance to flow than normal (see below) and a negative static lung load (that probably predisposes the airways to collapse). In other words, it was the result of a lot of factors coming into play simultaneously. The BMCL and negative static lung load was just one of these factors. Similarly, I would not deem the use of non-standard moisture pads in the rebreather loop to be the "key issue". The almost inevitable increase in resistance to gas flow in the loop was another one of several factors that would have resulted in impaired ventilation and the build-up of CO2.
    Simon,

    I always understood that the CO2 retaining mechanics involved somehow a high exhalation work load (against a backpressure or if you like positive static load). This would shift most WOB towards the exhalation part of the cycle. If I get it correctly, normally (breathing air at 1 ATA) the inhalation part of the cycle requires more work as the diaphragm and ribcage muscles must contract to suck gas into the airways and alveoli. If there is no or little back pressure to exhale against there should be little exhalation WOB involved as the muscles only have to just relax and gas will exit the airways thru the mouth and nose. Now, residual lung volume has normally very high pCO2. If you breath in a positive static loading situation your residual lung volume increases. When you breath in the fresh gas will dilute the high CO2 content mix in alveoli and pCO2 will decrease, allowing more CO2 to be draw from the blood in capillary vessels. If you increase residual volume then the dilution decreases and alveoli pCO2 stays high making it more difficult to eliminate CO2 from blood. On the other hand if you exhale against a slight negative static loading residual volume will decrease and dilution increase thus maximizing the CO2 elimination gradient. Considering this how can positive static loading help get rid of CO2 even if it will lessens the “collapse effect” on bronchioli? Could you please comment and correct if I am wrong?

    Also, if one experiences ventilation difficulty at depth (CO2 retention) the “stopping, resting, breathing deeply” drill does NOT work. The only action one should take is start ascending immediately to decrease breathing gas density. Bring density to a manageable level and things will improve enormously.

    J Neves

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    New Member Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell has a reputation beyond repute Simon Mitchell's Avatar
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    Re: Missed the talk on deep CCR at Eurotek =(

    Quote Originally Posted by jneves  View Original Post
    Simon,

    Considering this how can positive static loading help get rid of CO2 even if it will lessens the “collapse effect” on bronchioli?
    Because your interpretation of the physiology is incorrect. In terms of getting rid of CO2 what matters is lung ventilation. At deep depths using dense gas a positive static lung load is likely to facilitate ventilation whereas a negative static lung load may impair it (for the reasons I have explained). This observation is supported by objective experiments demonstrating better tolerance of positive vs negative static lung loads during exercise underwater.

    Quote Originally Posted by jneves  View Original Post
    Also, if one experiences ventilation difficulty at depth (CO2 retention) the “stopping, resting, breathing deeply” drill does NOT work.
    Where do you get this from? Up to a certain point in the process (which is what I said) of course it will work. Anything that either decreases CO2 production (eg rest) or increases elimination (eg breathing deeply) will result in a fall in alveolar and arterial CO2. Obviously, this does not apply if the gas you are breathing is so dense (or your equipment so poor) that the only work you are capable of doing is the work of breathing but such situations would be vanishingly rare. It also would not apply (as I have previously mentioned) if you were in the "effort independent respiratory failure spiral" that I have described in my earlier posts. But to claim that resting and concentrating on breathing deeply is invariably futile when early symptoms of CO2 toxicity are noticed is wrong.

    Quote Originally Posted by jneves  View Original Post
    The only action one should take is start ascending immediately to decrease breathing gas density. Bring density to a manageable level and things will improve enormously.
    J Neves
    I mentioned this as an option, but I don't agree it is the only one.

    Warm regards,

    Simon M

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    Re: Missed the talk on deep CCR at Eurotek =(

    Quote Originally Posted by Simon Mitchell  View Original Post
    Because your interpretation of the physiology is incorrect. In terms of getting rid of CO2 what matters is lung ventilation. At deep depths using dense gas a positive static lung load is likely to facilitate ventilation whereas a negative static lung load may impair it (for the reasons I have explained). This observation is supported by objective experiments demonstrating better tolerance of positive vs negative static lung loads during exercise underwater.
    I obviously don’t know the testing and the results you mention but I would like the consult them… Would you be as kind as telling me where I could get those? Thanks.

    Where do you get this from? Up to a certain point in the process (which is what I said) of course it will work. Anything that either decreases CO2 production (eg rest) or increases elimination (eg breathing deeply) will result in a fall in alveolar and arterial CO2. Obviously, this does not apply if the gas you are breathing is so dense (or your equipment so poor) that the only work you are capable of doing is the work of breathing but such situations would be vanishingly rare. It also would not apply (as I have previously mentioned) if you were in the "effort independent respiratory failure spiral" that I have described in my earlier posts. But to claim that resting and concentrating on breathing deeply is invariably futile when early symptoms of CO2 toxicity are noticed is wrong.
    I was referring to the context of deep diving (100 m+) and when serious symptoms of hypercapnia have started to settled in. As you mentioned it is difficult to incur in this situation in shallow water unless something is seriously wrong with equipment (except if scrubber capacity is over or a breakthrough). I agree with you that in mild symptoms and in shallow water you can recover by stopping all activity, controlling and improving ventilation

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    Re: Missed the talk on deep CCR at Eurotek =(

    Gentlemen,

    In regard to the Dave Shaw incident, when the coughing begins, is it possible to ascertain his body position based on the height of the camera from the bottom?

    The reason for this question is to revisit the approach of reducing negative lung loading by modifying trim to 45 degree head-up from horizontal.

    Back-mounted counterlung units tend to exhibit reduced lung loading in this position. There is a reflexive response to get into this position with most units of this type. Simply wondering how effective this is and whether Dave had naturally gone into that position.

    Was gas density the dominant factor, contributing to a high resistance budget?

    Without dwelling on the tragedy, we can all appreciate the substantial contribution made by Dave in the advancement of extreme depth diving with RBs and paid so dearly.

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