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Thread: Off Oxygen Effect

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    RBW Member single tank numpty is on a distinguished road single tank numpty is on a distinguished road single tank numpty's Avatar
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    Off Oxygen Effect

    Quote Originally Posted by AM
    I knew that an open circuit bailout was out of the question since switching from elevated PPO2 to much lesser PPO2 gas can trigger full-bore CNS oxygen convulsions, followed by loss of consciousness.

    I cannot stress this point enough, there are circumstances where open circuit bailout is simply not appropriate and could kill you. Open circuit bailout is for immediately life threatening symptoms such as CO2. By the time CNS oxygen symptoms are present, a switch to a decreased PPO2 can trigger full scale grand mal seizure, followed by loss of consciousness, this is the tonic/clonic phase of a CNS oxygen toxicity seizure. The diver bites down on the mouthpiece first then when they lose consciousness, the jaw slackens and the mouthpiece falls out and floods the loop. If the diver then starts breathing again, they drown from water aspiration. The phenomenen known as the off effect is detailed in Kenneth Donald's book, Oxygen and the Diver and I have just remembered it is also detailed in Jeffrey Bozanic's excellent book, Mastering Rebreathers. These books should be supplied as mandatory reading in the CCR training framework.
    The above quote is from AM thread about her recent near incident.

    Kudos and respect to AM for posting all of the information.

    I'd like to discuss this Off Oxygen Effect (OOE) aspect of it further.

    From my reading (I haven't read Donald's book, but it is on order :) ), I have got the impression that the OOE is something that might happen. Therefore that it is something to watch out for.

    So in an OC situation, response is to switch to a known breatheable gas with an appropriate oxygen percentage. Then watch out for the off oxygen effect, and have buddy ready to take control.

    Therefore in a RB situation, the equivalent is to switch to bailout. Hypoxia concerns managed if they are an issue, depending on the DIL or Bailout gas.

    So this approach of staying on the loop as it is safer even though you don't know the gas in it, seems strange to my newbie self.

    Is what we are talking about here simply the "Bail in" vs "Bail out" argument. In which case, I'll learn more about that as my training progresses, as I always like to know all the views.

    Or do the bail outers also follow AM's line of thinking in certain circumstances?

    Please discuss, and point to further reading.
    Last edited by single tank numpty; 5th October 2008 at 02:27.

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    RBW Member PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2 is a name known to all PaulTG2's Avatar
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    Re: Off Oxygen Effect

    Hello,

    Thank you for starting this thread. I would very much like to hear from folks who have a solid background in this.

    Anything that suggests that bailout is dangerous is completely counter to all the training I have received on rebreathers. If there are situations where normal bailout is dangerous I'd like to understand in what situations this is true. Furthermore, what bailout gas should I plan to have to provide a safe bailout option.

    Sincerely,

    Paul
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    Re: Off Oxygen Effect

    I to wonder about the accuracy of that information.
    My limited and somewhat dated understanding of the subject leads me to believe that what was said is somewhat inaccurate/misinterpreted.

    But I don't want to endure the wrath of the CCR/diving "experts" so I'll keep out of this. But it should be an interesting thread.

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    Re: Off Oxygen Effect

    With the caveat that haven’t read Donald's book I question this “off” effect – doesn’t make any physiological (which I have some understanding) sense. I'll borrow Metro's book when he gets it but can someone say whether this is a seizure triggered by a lower O2 gas switch or whether it is just the inevitable happening following hyperoxic exposure (ie seizure would have happened regardless of what gas is being breathed)

    Graham

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    Re: Off Oxygen Effect

    What they are talking about is something that is not the norm. It is something that is related to each individuals physiology and physical conditions at the time. In recreational diving we are taught to never run a PO2 higher than a 1.6 and a CNS clock. In the military and for commercial divers who have 1000's to 10 of 1000's of hours they will run PO2 of 2.0 for deco and 1.6 for entire dives on heleOx then go to strait air. We used to dive the same way in the old days recreationally because the only standard we had was theirs. Each persons body handles gasses in a different way I can routinely blow my CNS clock apart most of the time. But I have taken hits before also. It is a variable that if you stay with what is considered recreational standards your chances of having a problem are slim to none. but you are not totally out of the water and safe flying their one day your physical condition may predispose you to a hit weather O2 hit or you get bent. That is part of pushing the limits. I have done long term sat diving were we are on high O2 for 90 days at a time on HeleOx. That does not fit into the realm of recreational diving and we all who have done this should be dead by recreational standards. I am not and neither are many of my friends who are out their still doing it to day and some of them have been doing that for over 20yrs.

    O2 and Deco is a massive variable that is not the same for everyone and no one person is the same as another. The standards that are taught and in place today for recreational are based on statistics of the least number of hits for 10 of thousands of hours of diving. thus giving us the best chance of statistical safety. But it is only statistics making guesses for a dynamic environment (your body).

    I have never read the book you are quoting in your posting. Technical divers routinely got form a high PO2 to air all the time. Take a typical day in San Diego or deep ocean diving with high swells. A tech diver OC or CCR will do all you deco below 20 feet most of it at 30 feet with a PO2 of 1.6 or a little higher. and after repetitive dives a CNS clock that is over 80%. you do not stop at 10 or 15 feet you go strait to the surface and guess what you are on air when you pull the DSV out of your mouth. 99% of the time you are fine. Some intellectual people try to put theory's based on real numbers and algorithms to things related to O2 and Deco and you cannot do that because we are not machines that are the same. Our bodies are a dynamic environment and you should not put too much into information that you read like that.

    Keep in mind decompression diving has been done for over 100yrs. It is not something that is new. People will try to put new spins on things and they are usually a grad student or someone looking for grants in many cases throwing out theory's. throes that yes can possibly have some impact on some individuals. But without knowing all the details in the original quote as to why that individual did not want to go to bail out one cannot make an informed opinion. All I can say is that their are millions of combined hours of bottom time out their with no problems going form high PO2 to a low one. So I would not take the info you read to heart all that much and stay with what you are taught. And keep in mind the folks who are making a living are pushing O2 limits a hell of a lot higher than you are and they are in the water every day.
    Last edited by Loanwolf; 5th October 2008 at 05:04.

  6. #6
    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: Off Oxygen Effect

    Hello,

    I have just posted a reply on this matter on the original thread (post 110 on page 11).

    If one of the mods wants to cross post it to here that is fine.

    Simon M

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    Re: Off Oxygen Effect

    See below. Subject closed I think.


    Quote Originally Posted by Simon Mitchell
    Re: CRITICAL: Near Fatal Mishap Sept 2
    Quote: (Originally Posted by satansma)

    Just got this from Annemarie:

    ...I ascended on the loop, whilst flushing air diluent for the full ascent to slowly lower my inspired PPO2. The reason for this choice and not an immediate open circuit air bailout, is that in the event of breathing elevated oxygen levels, a switch to a gas with a much lower PPO2, can trigger full CNS oxygen toxicity, with resulting convulsions, followed by unconsciousness. This is a phenomenon called the "off effect" and is documented in Kenneth Donald's book and some NEDU studies, both of which I am now glad I have read....

    AnneMarie
    Hello AnneMarie and list,

    Glad to hear this had a good outcome and like everyone else, I will be most interested in what is found on investigation.

    In respect of the above quote, I am concerned that you have taken a little understood phenomenon of uncertain significance, and over-interpreted it into a fairly strongly worded statement with implications for best practice in rebreather diving. Subsequent comments in this thread, and the new thread specifically on the topic suggest that this might become another classic piece of "received-wisdom-by-internet", and so some perspective is needed.

    The "off-oxygen effect" refers to an occasional observation of the onset of convulsions when a diver has been breathing a high PO2, and then switches to a lower PO2. I think it was first described by Donald, whose book I do not have here at home, but others have also described such events. IIRC there were cases in which the divers already had toxic symptoms (like AM) and a few where there were no premonitory symptoms. There has been debate over whether the phenomenon is real, or whether the timing of convulsions is just coincident, that is, the seizure was going to happen anyway, and the change to lower PO2 came too late to prevent it. Further skepticism arises from the recognition of an extensive body of literature that proves coming "off oxygen" intermittently during a long exposure (ie air breaks) significantly reduces the risk of a seizure.

    Nevertheless, if we ignore that debate and assume the "off-effect" is a real phenomenon, the important point for readers here to appreciate is that it is not an invariable phenomenon. I have seen many patients develop premonitory symptoms of oxygen toxicity in hyperbaric chambers. In accordance with standard practice the oxygen is immediately discontinued and the patients breathe air. I have never seen the "off-effect" manifest itself in this setting. The symptoms settle without progressing. In his paper on oxygen toxicity presented at the DAN technical diving conference, Richard Vann described two cases of definite premonitory symptoms in divers which included the sort of muscle twitching described by AM. In both cases the divers switched to air or an hypoxic trimix, and neither progressed to more serious oxygen toxicity. The symptoms settled. In one of the cases the scenario was played out a number of times.

    I think we all agree that if symptoms of oxygen toxicity occur, the PO2 must be lowered. AM's practice of lowering the PO2 more gradually than afforded by an instantaneous gas switch is logical for an "off-effect" afficionado, but the critical question is whether her suggestion (which prolongs exposure to toxic PO2s and is not what is usually recommended) actually increases or lowers the risk of symptoms progressing to seizures. I would like to see her either produce some data that answers this question, or rephrase her suggestion in terms that are much more compatible with the uncertainty that prevails.

    One of the problems with staying on the loop in these situations is that you are emphatically trusting your self diagnosis of oxygen toxicity which has been made under duress. AM can claim (and probably reasonably in my view) that her diagnosis was fairly certain, but there are symptoms of oxygen toxicity that are less specific than those she experienced. So, as a principle, the uncertainty that prevails in these situations is a good reason to get off the loop. Even if your diagnosis is correct, you are then in the situation of managing the PO2 in a rebreather which may not (as in AM's case it would seem) give you a sensible reading of what the PO2 actually is. I know you can't go too far wrong doing what she did (flushing dil into the loop), but that is still another thing to be thinking about. On balance, I would probably bail out under similar circumstances.

    I hope this helps clarify the concerns that are arising over this particular bit of the story.

    Warm regards,

    Simon M

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    Re: Off Oxygen Effect

    Quote Originally Posted by Simon Mitchell  View Original Post
    Hello,

    I have just posted a reply on this matter on the original thread (post 110 on page 11).

    If one of the mods wants to cross post it to here that is fine.

    Simon M
    Thanks Simon - as I suspected and makes physioligical sense.

    HOpe it doesn't get lost in the volume/noise ratio in the other thread.

    Graham

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    Re: Off Oxygen Effect

    Quote Originally Posted by Simon Mitchell  View Original Post
    Hello,

    I have just posted a reply on this matter on the original thread (post 110 on page 11).

    If one of the mods wants to cross post it to here that is fine.

    Simon M
    Hi Simon, thanks for taking the time to explain in the post copied below.

    Just out of interest what do you know or have you heard about completing lenghtly o2 deco in the shallows and then when back at the surface breathing air and passing out either climbing back on board the boat or while back on the boat.

    The above scenario was describe to me as an event of Vasoconstriction due to the high O2 exposure then rapid Vasodilation due to the switch.

    If any of this make any sense to you it would be good to here your thoughts.
    if its a possibility could this type of black out happen during a bailout switch?
    Last edited by tecdivertraining; 5th October 2008 at 12:24.

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    Re: Off Oxygen Effect

    I would look on the 'off effect' as a subset of my theory that a major trigger a CNS event is psychological. Realising you are on a high CNS and grabbing madly for the bailout predisposes you to a sudden onset of symptoms. AM reacted as coolly and as calmly as possible and, by my thinking, bought herself enough time to for the decreasing ppO2 to lessen the chemical problem.

    If you have low ppO2 by all means grab madly for the bailout. Here speed is off the essence.

    It's only a theory. I would prefer people didn't go test it. It just seems strange that Haas etc. Seemed to find O2 rebreathers safe to 30 feet and used tricks, that didn't reduce the ppO2 much, to go deeper. We tell beginners that 1.6 will kill them and they see 1.6 on the handsets and go all twitchy.

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