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Thread: Carbon Dioxide and the Rebreather Diver

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    Carbon Dioxide and the Rebreather Diver

    Howard has kindly written a article on Carbon Dioxide and the Rebreather Diver and has published it for peer review.

    You can read it here

    Please leave feedback and any comments in this thread



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    Re: Carbon Dioxide and the Rebreather Diver

    This would be useful as a taster for someone approaching Mod 1, though there is a lot in Mod 1 that is not considered here. The units are traditional Imperial units, rather than modern scientific units. I was surprised that the effects of CO2 are quoted as FiCO2, rather than as partial pressures, which would have been more appropriate to a diver.

    I have never really understood: when a diver dies of hypercapnia what kills him if not hypoxia? I always thought that CO2 takes up bonding sites on the RBC, preventing O2 bonding with the haemoglobin for O2 transport.

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    Re: Carbon Dioxide and the Rebreather Diver

    Quote Originally Posted by Abbo  View Original Post
    This would be useful as a taster for someone approaching Mod 1, though there is a lot in Mod 1 that is not considered here. The units are traditional Imperial units, rather than modern scientific units. I was surprised that the effects of CO2 are quoted as FiCO2, rather than as partial pressures, which would have been more appropriate to a diver.

    I agree in that it is introduction-type material, although it may be an overkill. Enthusiastically written.

    One very small point regarding CO2 and decompression mentioned in the article: I had also always believed that increased levels of CO2 might be harmful during decompression (the thought being that any deviation from the ideal scenario must have negative effects). However as David Sawatzky pointed out there is a line of thought that small doses of CO2 can be benefitial as it acts as vaso-dilator. A clear "don't try this at home" should be added though.


    Quote Originally Posted by Abbo  View Original Post
    I have never really understood: when a diver dies of hypercapnia what kills him if not hypoxia? I always thought that CO2 takes up bonding sites on the RBC, preventing O2 bonding with the haemoglobin for O2 transport.
    To the best of my knowledge (which is not very much - not a physician) the answer depends on the PCO2. Lower levels of CO2 act as narcotic, and the diver is at risk of drowning more than anything else. Such levels are not a concern in a clinical environment, as patients on a respirator are anaesthetised anyhow, and under supervision. A FFM would resolve this in a diving environment, contingent upon a dive partner present and capable of rescue.
    At higher levels (starting above 10% and higher) the CO2 starts inhibiting metabolism on a cellular level. As far as I remember, lab rats are killed using 20% CO2 gas, and death is instantaneous in a single breath - lack of O2 is not the cause. I'd be happy to learn something new here though!

    Joerg

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    Re: Carbon Dioxide and the Rebreather Diver

    Blood PH...

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    Re: Carbon Dioxide and the Rebreather Diver

    Rising CO2 levels cause gradual lack of consciousness, usually after a period of shortness of breath in healthy people (possibly not/less in high PO2 situations though), loss of consciousness is likely to be the cause of death in the diver (by leading to drowning). I've certainly seen a number of CO2 narcosed patients over the years, including one whose blood CO2 was >20KPa due to being connected to a breathing circuit with an inadequate fresh gas flow causing re-breathing of CO2, she remained well oxygenated, and woke up unharmed when I cranked up the gas flow and ventilated her down to a less extreme CO2. Another was unconscious due to high CO2 due to a rare muscular disease which prevented him from breathing enough to remove CO2 adequately. Other harmful effects include an increased likelyhood of cardiac rhythm problems, raised intra-cranial pressure, and low blood pressure due to vaso-dilatation. Eventually as CO2 rises even further and pH drops a host of other effects would occur as cellular enzymes work less well at low pH.

    I'm not sure about the lab rat thing - I once took a single breath of 30%CO2 in oxygen out of curiosity - I wouldn't recommend it!!

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    Re: Carbon Dioxide and the Rebreather Diver

    This is a well written and informative article. I have a couple of comments/questions.

    Percent CO2 may be useful at the surface but is it useful at depth. Should we not in a CO2 monitor for rebreathers use PPCO2 so it backtracks to surface percentage?

    If you were going to set an alarm at a particular PPCO2 where would you want it set?

    I will have a few more questions later but lets hash these out first.

    If nothing goes to hell, I will be showing a tested CO2 monitor at DEMA including a retrofit system for two hose rebreathers.

    Tom

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    Re: Carbon Dioxide and the Rebreather Diver

    Quote Originally Posted by Tom Rose  View Original Post
    Percent CO2 may be useful at the surface but is it useful at depth. Should we not in a CO2 monitor for rebreathers use PPCO2 so it backtracks to surface percentage?
    Tom
    The concept used here is SEV - surface equivalent value, effectively meaning partial pressure. If one talks about 0.5% CO2 it is then clear from the context that this is SEV (or 0.005 bar PCO2).

    Joerg

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    Re: Carbon Dioxide and the Rebreather Diver

    Quote Originally Posted by York  View Original Post
    The concept used here is SEV - surface equivalent value, effectively meaning partial pressure. If one talks about 0.5% CO2 it is then clear from the context that this is SEV (or 0.005 bar PCO2).

    Joerg
    Agree, but it is better to just come out and say it, remember the rebreather crowd is a bit simple.


    Myself included....

    Tom

    Now lets get to the alarm level question.....when should it go beep turn flashing red and display a get the hell out message.

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    Re: Carbon Dioxide and the Rebreather Diver

    Quote Originally Posted by Tom Rose  View Original Post
    Agree, but it is better to just come out and say it, remember the rebreather crowd is a bit simple.


    Myself included....

    Tom

    Now lets get to the alarm level question.....when should it go beep turn flashing red and display a get the hell out message.
    No disagreement here, however I found it even more confusing to figure out the number of zeros required to express e.g. 0.05% CO2 SEV in PCO2. ;)

    Are you asking: what level is "safe"?
    NOAA allows quite high levels in their habitat, but it did hit me pretty hard coming from outside into a CO2-rich environment. Slow onset appears to numb the susceptability (possibly merely the sensation?).
    There has been extensive critique on the Navy's 0.5%SEV - hrm, excuse me, 0.005 bar breakthrough test limit, as humans can clearly operate under higher levels. There are good reasons to stick to the 0.5%-level for testing, and
    I personally would really like to stay in zone I (see NOAA publication below)

    Joerg
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    Re: Carbon Dioxide and the Rebreather Diver

    Quote Originally Posted by York  View Original Post
    There are good reasons to stick to the 0.5%-level for testing, and
    I personally would really like to stay in zone I (see NOAA publication below)

    Joerg
    Great chart for discussion purposes...

    I have taken some liberties with the chart which could be wrong but are again for discussion.

    1. I extended the straight line at the bottom of the chart to a three hour dive. Extending the line may not be valid.

    2. The extention may give some food for thought for anyone doing long dive where the scrubber is starting to fail.

    3. It may provide some insight into the level the alarm is set.

    4. Perhaps a preliminary warnnig level?

    5.I woud sure hate to start a long decompression with a failing scrubber.


    Posted from Mountain Lake, Virginia

    Any comments

    Tom
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