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Pressure headache and prolonged narcosis (2)

Discussion in 'Diving Medicine' started by Juris Diver, May 26, 2002.

  1. Uncle Pug

    Uncle Pug Swims with Orca ScubaBoard Supporter

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    Hi Bill,
    I understood CO2 to be 25 times more soluble than N2 in lipids but be that as it may (and given the limitations of the Meyer-Overton rule) what is the different mechanism by which CO2 causes narcotic-like effects? And in what way do N2 and CO2 reinforce their individual narcotic effects? And can CO2 retention precipitate a CNS oxtox hit?

    Personally I consider anything >100fsw deep air (actually we use EAN32 on all dives to 100 fsw) and use a 30/30 mix to 120 fsw and 21/35 to 160fsw. Why? Several reasons but germane to this discussion I find that keeping my END <100 fsw I feel much better during and after a dive as well as clear headed enough to handle problems should they arise. I do recognize a narcotic effect from EAN32 at 100 fsw and if the dive we are doing calls for greater mental clarity I might use 30/30 >80 fsw. One benefit of using helium in the mix is decreased WOB which lessens the potential for CO2 retention.
     
  2. BillP

    BillP Senior Member

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    Well then, if your number is correct, the Meyer-Overton rule would only be off by half of an order of magnitude rather than a full order of magnitude (grin). In either case, it probably should not be quoted as the primary explanation for CO2 narcosis.

    Since no one seems to really understand how N2 causes narcosis, it would be hard to say just how CO2 is different. There are various tests the study designers use to measure the narcotic-like effects of gas under pressure in study subjects. Increased partial pressure of CO2 causes similar, yet different, results from N2. For example, increased pN2 causes study subjects to exhibit both decreased speed and accuracy on the tests, whereas CO2 only causes a decrease in speed. When subjects are exposed to increased CO2 they do get fewer test problems correct, but only because they don't complete the tests in the time allowed.

    Also, the researchers found effects from CO2 at lower partial pressures than would be explained by molecular CO2 dissolution in lipids. These differences suggest a different mechanism of action for CO2 compared to N2. What is that mechanism? One of the studies suggests CO2 causes its effect through hydrogen ions/pH. CO2 is not an inert gas. It creates carbonic acid when dissolved in water (creating hydrogen ions and decreasing pH). They claim that increased dissolved CO2 will change the pH well before molecular CO2 will have a Meyer-Overton-like effect.

    Again, who knows? I'm afraid I don't. One study found that the effects caused by increasing both N2 and CO2 was more than additive. (Again suggesting different mechanisms of action for the two gases.)

    This has indeed been proposed as a risk factor for CNS oxygen toxicity.

    I think your reasoning is sound, but logically if you want to keep your rationale consistent you should be aware that the very same study design and testing procedures that found CO2 so terribly narcotic also found O2 3-4 times more narcotic than N2. Seems to me if one "fact" shown by the studies is called into question so should be the other related "facts".

    HTH,

    Bill

    References

    Hesser, CM, et al: Role of CO2 in Compressed-Air Narcosis. Aerosp Med 1971; 42:163-168

    Hesser, CM, et al: Roles of nitrogen, oxygen, and carbon dioxide in compressed air narcosis. Undersea Biomed. Res. 1978; 5:391-400

    Fothergill, DM, et al: Effects of CO2 and N2 partial pressures on cognitive and psychomotor perfomance. Undersea Biomed. Res. 1991; 18:1-19
     
  3. Uncle Pug

    Uncle Pug Swims with Orca ScubaBoard Supporter

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    Hi Bill,
    Thanks for the references...
    While there are those who claim O2 is not narcotic and do not count it against the END when using nitrox I personally (from a purely subjective POV) consider the EAN32 we use at least as narcotic as air.

    How do you rate the significance of Nitrogen narcosis as compared to CO2 impairment with regards to diving accidents?
     
  4. BillP

    BillP Senior Member

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    I'm afraid that I don't have enough information to give a cogent answer to that question. I suspect that nitrogen narcosis is more significant in recreational diving, but I only think that because nitrogen narcosis has gotten far more attention from a bunch more people a lot smarter than me who "should" know.

    Just like nitrogen narcosis doesn't become an important factor in a dive until you reach a certain depth, CO2 doesn't seem to matter at all until you reach a partial pressure of end titdal (exhaled) CO2 of 40mm Hg (or more, depending on who you read). There is a small subset of "CO2 retaining" divers who might reach this level on every dive, and a "normal" recreational diver might reach this level on a dive with a higher workload or with a particulary bad regulator with increased work of breathing, but I haven't seen any data to tell me how often CO2 narcosis is really a factor in recreational diving. Nitrogen narcosis is often a factor in recreational diving, and occasionally a significant factor.

    Yeah, it certainly could be that the importance of CO2 narcosis has long gone under-recognized by the diving medical community, but I just don't know. Sorry.

    Bill
     
  5. Uncle Pug

    Uncle Pug Swims with Orca ScubaBoard Supporter

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    No need to be sorry Bill... this is exactly my point...

    I had two dogs once that would tag team the UPS driver...

    Mandy was a *grinner* not a barker nor a biter. She really loved people and would always come out to greet them grinning. But when she came up to the UPS driver and then grinned at him, he saw the teeth and focus on her alone.

    Bandit was not a barker either... at least not when he was working his plan... whenever the UPS driver came to deliver something, Bandit would take off in the other direction while Mandy went into her grinning routine. He would race around the house and arrive behind the UPS driver whose attention was now rivited on Mandy's grin.... and that is when he would take a bite outta the poor guy's butt.

    I believe that nitrogen narcosis gets all the attention while it is CO2 that sneaks up and bites us in the butt...

    Question: which is most likely to precipitate a panic response, nitrogen narcosis or elevated CO2 levels?
     
  6. BillP

    BillP Senior Member

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    You seem to be asking questions that you believe you already have the answers to, if I'm not mistaken. Why don't you answer the questions for us and reference the studies that support your answers? Then maybe we'll all learn something.

    Bill
     
  7. Uncle Pug

    Uncle Pug Swims with Orca ScubaBoard Supporter

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    Well, I've got some ideas alright, but...
    Referencing the studies that support the answers would be your department Bill...
    Subjective reflections, anecdotes, and illustrative tales are more my thing...
    Plus being somewhat lazy I was hoping you would do my homework for me :wink:
     
  8. The Iceni

    The Iceni Medical Moderator

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    This is all getting overly technical as I suspect the original problem can be attributed to inexperience and anxiety together with a major metabolic effect of our old enemy carbon dioxide rather than any narcotic effects of this gas.

    The Meyer-Overton principle applies only to the anaesthetic properties of INERT gasses but cardon dioxide, like oxygen, is far from inert as I discussed earlier on a thread concerned with oxygen narcosis;

    http://www.scubaboard.com/showthread.php?threadid=4425

    Indeed the body's ability to buffer CO2 is decreased in high pp O2 seen in ALL diving; air at 100 feet os sea water has a pp O2 of 0.84 bar.

    So, I suspect the initial "symptom" was Juris noticing his rapid heart rate which started him down the slippery slope into "the incident pit". Put this down to inexperience and anxiety not to any serious failings in equipment, technique or to medical problems. On the bottom on one of his deepest dives to date any inexperienced diver will be slightly anxious and once you become anxious you are unable to perform any task properly.

    I would put my money on that!

    There is of course the possibility of hyperventilation, which is a well recognised clinical syndrome commonly seen on the surface, although it is possible to my mind this is much, much more unlikely in the SCUBA environment.

    However, one thing of which I am certain, is that as a diver gains experience and confidence his gas consumption will fall remarkably.

    Do you know your current Respiratory Minute Volume, Juris?
     
  9. Juris Diver

    Juris Diver Angel Fish

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    No Doc, I don't know my Respiratory Minute Volume or my SAC rate. Could you explain (or provide a link) how to compute these figures and why knowing this information would be useful? Note that I usually have more air left after a dive than those with whom I dive and I usually keep my breathing slow and deep (although it's possible that on the 100' dive that I breathed more shallow and more often - we were haulin on the way down).

    By the way, the discussion above did go right "over my head." I suspect that as I understand more about the physiology of diving, I'll return to this post and read it again with new insight. :)

    Thanks to everyone!
     
  10. The Iceni

    The Iceni Medical Moderator

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    Hi juris,

    Apologies.

    My post was directed primarily at the experts Unkle Pug & Bill P. I'm just a doctor and was simply saying that I do not think you need look for sinister causes for your symptoms as I feel they were largely related to subliminal anxiety precipitated by CO2 retention. Psychologists will tell you that performance against anxiety takes the form of an inverted "U"curve.

    I think we are all agreed that it was not narcosis.

    If not the symptom of major problems a rapid pulse rate is most certainly a cause for anxiety!

    Respiratory Minute Volume is the same as Surface Air Consumption.

    I believe you guys tend to work in feet and psi so forgive me if I give you an example in SI units.

    You start your dive with a 12 litre cylinder filled to 232 bar.

    It contains 232 x 12 = 2,784 litres at the start.

    You do a dive at 20 metres for 30 minutes and surface with 130 bar so you have 12 x 130 = 1,560 litres left.

    2,784 - 1,560 = 1,224 litres used in 30 minutes = 40.8 litrs per minute.

    However this was MOSTLY at 20 meters where the ambient pressure is 3 bar.

    SAC= RMV = 40.8/3 = 13.6 (approximately) litres per minute.

    Please note I have made no allowance for the time taken to descend or to surface and, of course, I have not accounted for any deco stops or surface gas use and I have treated the gas as an ideal gas.

    Even if you do not use as much gas as the others, I am certain SAC will still reduce with time as you gain increasing confidence.

    Added as an afterthought. You asked why it is useful, Juris.

    If and when you start doing more adventurous diving you will need to plan your gas needs.

    Hope this helps!
     

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