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    Carbon Dioxide on ascent

    Hello clever people!

    I was wondering, do you off gas CO2 in the same manner as Nitrogen, as you ascend? I’ve not fully formed my thoughts on this question, but...

    Say you have a hypercapnic episode for what ever reason at depth, should ascent be at the forefront of your exit strategy (putting to side the narcosis/confusion/mental paralysis etc)? Taking this to the extreme, ascend before trying to bail off.

    Obviously getting shallow is important form a bail out gas usage point of view when breathing hard from CO2 build up, but what about the effect of ascent on partial pressure of CO2? Or are the amounts of CO2 so low, that it actually doesn’t make much difference.

    Also, the first question - does it off gas from your blood faster as you ascend??

    Looking forward to opinions!

    Chris

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    Established TDF Member nigel hewitt's Avatar
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    No.
    Remember CO2 is in your blood as H2C03, Carbonic Acid, and you can't carry more as you blood is going acidic so CO2 levels are not depth dependant.
    If you ever suspect you might be even a bit hypercapnic get off the damn loop for a couple of breaths.

    I taught myself to check my breathing rate/desire to breath every time I checked the ppO2.
    Then let your CO2 level control the effort you expend so you always stay well away from CO2.

    The same goes for OC diving. CO2 is the nightmare dive incident. It will take your brain away.
    Helium, because I'm worth it.
    Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.

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    Thanks Nigel, I was way off track.

    How do you check yourself? I know some guys check they can easily hold their breath for 10s
    or so.

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    Established TDF Member nigel hewitt's Avatar
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    Quote Originally Posted by mrmints View Post
    Thanks Nigel, I was way off track.

    How do you check yourself? I know some guys check they can easily hold their breath for 10s
    or so.
    Go for the direct readout: "How fast am I breathing?"
    Anything else is a subjective call.
    Helium, because I'm worth it.
    Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.

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    Prior Member Tim Digger's Avatar
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    Quote Originally Posted by nigel hewitt View Post
    Go for the direct readout: "How fast am I breathing?"
    Anything else is a subjective call.
    Absolutely, you have sensors in your brain stem that measure CO2, the output from these feeds directly into your respiratory centre which controls breathing depth and rate. Although the central processor(respiratory centre) has other inputs including the ability to override by input from conscious higher centres this should not be employed, particularly on a closed circuit where build up of CO2 due to a gradually failing absorber or other circuit problem will then produce the warning sign of an increased respiratory rate. Of course exercise will increase respiratory rate and depth and this is why SCUBA use (of all sorts) is an exercise in staying fit to do very little. If this is obeyed (IE don't exercise underwater) then the input from one's own CO2 sensors will produce a detectable increase in respiratory rate. In the event of an exercise induced increase in CO2 production and hence respiratory rate then the dictum in most training organisations early training holds good, STOP doing the exercise and if your resp rate is not falling within 30secs you have a problem!
    CO2 is carried in the body and produced by it it is a dynamic process involving a controlled balance between production and excretion, we have evolved exquisitely sensitive mechanisms to control this with little input from our higher centres, breathing underwater just adds another layer of complexity but is nothing to do with depth and increase in ambient pressure as with increasing dissolving of inert gases as CO2 level depends on the balance between production and removal not on partial pressure.
    Evolution is great at solving problems. It's the methods that concern me.
    Tim Digger

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    Thanks for the information Tim and Nigel, interesting stuff.

    It was this line "For instance, in cases of CO₂ elimination system failures during rebreather dives, elevated inspired partial pressure of carbon dioxide (PCO₂) can rapidly lead to dangerous levels of hypercapnia." in the abstract of "Hypercapnia in diving: a review of CO₂ retention in submersed exercise at depth" that had me confused [also note the next sentence "Elevations in PaCO₂ (arterial pressure of PCO₂) can also occur in divers without a change in inspired PCO₂."!!]. However, I think they're referring to the fact that if your scrubber isn't removing the CO2, then the PPCO2 is going to go up (pretty obvious). Honestly, I'm not trying to be stupid here (although probably succeeding), but if the partial pressure of CO2 in the loop has gone up because you're scrubber isn't removing it (say if you were over exerting), wouldn't it go down faster if you ascended and stopped exerting when compared to just stopping exerting? I realise this isn't necessarily of practical benefit to someone in the situation and bail out is always going to be the way to go, but I'm interested to attempt to understand the mechanisms at work here - which I currently don't by any stretch.

    This is a pretty interesting article: https://www.gue.com/carbon-dioxide-narcosis-and-diving

    I note that the only advantage of getting shallower is to reduce gas density in the GUE article.

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    Quote Originally Posted by mrmints View Post
    It was this line "For instance, in cases of CO₂ elimination system failures during rebreather dives, elevated inspired partial pressure of carbon dioxide (PCO₂) can rapidly lead to dangerous levels of hypercapnia." in the abstract of "Hypercapnia in diving: a review of CO₂ retention in submersed exercise at depth" that had me confused [also note the next sentence "Elevations in PaCO₂ (arterial pressure of PCO₂) can also occur in divers without a change in inspired PCO₂."!!]. However, I think they're referring to the fact that if your scrubber isn't removing the CO2, then the PPCO2 is going to go up (pretty obvious).
    Hello,

    No, they meant exactly what they said. You can have have a CO2 problem on open circuit too. Have a read of the below from an article I wrote a few years ago. Its longish, but if you are interested it should answer your questions about CO2 physiology.

    BASIC CO2 PHYSIOLOGY

    Carbon dioxide (CO2) is a product of metabolism of oxygen. It is constantly produced in the tissues and its elimination must equal its production otherwise it will accumulate and cause a variety of adverse effects which we will return to later. For elimination, CO2 enters the venous blood and is carried to the lungs where we breathe it out.
    Key point number one: The more gas you breathe in and out of the lungs, the more CO2 you eliminate, and vice versa. In other words, if you take a series of rapid deep breaths you can increase elimination of CO2, and this so-called “hyper-ventilation” is what some free divers do to intentionally lower their blood CO2 prior to a long breath-hold. Equally, if you take shallow breaths or breathe slowly you decrease elimination of CO2 and it will accumulate in the blood. This accumulation of CO2 is called CO2 retention.

    Under normal circumstances the CO2 dissolved in tissues and blood is carefully and automatically regulated by the body. The brain has what is effectively a CO2 sensor that indirectly monitors blood levels, and adjusts breathing accordingly. Thus, if the blood CO2 starts to rise, then your brain will drive you breathe more (either by increasing your breathing rate or breath size or both), and if your blood CO2 starts to fall, then your brain will cause you to breathe less so that the levels rise again. All of this is happening at a completely subconscious level as you sit there reading this. In most people the brain is “set” to maintain a dissolved PCO2 of about 5.2 kilopascals (kPa) (or 0.052 ATA or 39mmHg depending on what units you prefer to use). However, this control system is imperfect and under some circumstances it can become less precise.

    For example, if the work of breathing increases unnaturally (which occurs in diving for a variety of reasons) the controller in the brain appears predisposed to allowing the blood CO2 to rise rather than drive the extra work involved in maintaining sufficient gas flow in and out of the lungs to keep CO2 at normal levels. Think of it as though the brain is still driving “X” amount of work by the respiratory muscles in response to a given level of CO2, but because the work required to breathe has increased, “X” amount of work now achieves less gas flow in and out of the lungs, therefore less CO2 is eliminated, and CO2 is retained. The more the work of breathing increases (eg deeper, denser gas, hard work, poor equipment), and the more CO2 that is being produced (eg because of exercise) then the more likely CO2 retention is to occur. The increase in PO2 and PN2 that are also encountered in diving can also “depress” the respiratory controller and make CO2 retention more likely.

    Interestingly, there is also significant variability between individuals in the degree to which they retain CO2. Some peoples’ respiratory controller will adjust breathing to maintain their normal level of CO2 irrespective of how much the work of breathing increases, whilst others are very vulnerable to increasing CO2 because of this disturbance of normal control. The latter group are often referred to as “CO2 retainers”. It almost seems paradoxical, but the non-retainers are the ones who suffer the unpleasant symptoms of increasing CO2 most readily. Thus, they get the horrible feeling of shortness of breath early when CO2 is rising, but if you think about it, that explains why they don’t retain CO2; they feel short of breath, start to breathe more, and get rid of the rising CO2. In contrast CO2 retainers do not experience those unpleasant early symptoms. They don’t feel short of breath, they don’t increase their respiratory rate or the size of their breaths, and therefore they don’t eliminate the rising CO2.

    Hopefully it is clear to you that CO2 “retention” because of inadequate lung ventilation is one potential cause of CO2 toxicity in diving. You will note that this has nothing to do with CO2 breaking through a scrubber and consequent CO2 rebreathing. It is simply a failure to breathe sufficiently to eliminate all the CO2 that is being produced in the body. This can occur on open circuit and closed circuit, and I reiterate it has nothing to do with the scrubber. CO2 rebreathing can occur of course, if the scrubber fails during use of a rebreather, and this is a second cause for CO2 toxicity in diving. A normal blood CO2 can be maintained, despite a low level of CO2 breakthrough, by increasing the rate and depth of breathing. There are some complex considerations, but suffice to say that as the amount of inspired CO2 increases, the harder it is for the diver to maintain a normal blood CO2. Moreover, those who have a tendency to retain CO2 (as described above) are more prone to mount an inadequate respiratory response in the presence of inspired CO2.

    Rising blood CO2 is a problem in diving for several reasons. First it can cause unpleasant symptoms such as headache and shortness of breath. These can precipitate panic. Second, if the levels get high enough CO2 can cause incapacitation and unconsciousness. As mentioned above, those who tend to retain CO2 generally suffer fewer early unpleasant symptoms, and indeed, may not develop symptoms until they are close to the second tier of problems (incapacitation and unconsciousness). To give you some sense of the small changes in blood levels required for these phenomena, 5.2kPa is the average normal level, 6.2kPa is the upper limit of the normal range, and over 8.5kPa sudden incapacitation is likely. Experiments show that levels between 6.5 and 7.5 are not uncommon in divers working underwater (at least some of whom would have related symptoms). The point is that small changes in PCO2 of 1kPa or less can have very important implications for the safety of the diver. As a prelude to the monitoring discussion, this is why any monitoring system purporting to measure blood CO2 levels, and base safety management decisions around that measurement, must be very accurate. Finally, high CO2 worsens narcosis, and predisposes to cerebral oxygen toxicity. We can discuss the reasons for the latter at another time if you wish.

    Simon M

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    Quote Originally Posted by simon mitchell View Post
    Hello,

    No, they meant exactly what they said. You can have have a CO2 problem on open circuit too. Have a read of the below from an article I wrote a few years ago. Its longish, but if you are interested it should answer your questions about CO2 physiology.

    BASIC CO2 PHYSIOLOGY

    Carbon dioxide (CO2) is a product of metabolism of oxygen. It is constantly produced in the tissues and its elimination must equal its production otherwise it will accumulate and cause a variety of adverse effects which we will return to later. For elimination, CO2 enters the venous blood and is carried to the lungs where we breathe it out.
    Key point number one: The more gas you breathe in and out of the lungs, the more CO2 you eliminate, and vice versa. In other words, if you take a series of rapid deep breaths you can increase elimination of CO2, and this so-called “hyper-ventilation” is what some free divers do to intentionally lower their blood CO2 prior to a long breath-hold. Equally, if you take shallow breaths or breathe slowly you decrease elimination of CO2 and it will accumulate in the blood. This accumulation of CO2 is called CO2 retention.

    Under normal circumstances the CO2 dissolved in tissues and blood is carefully and automatically regulated by the body. The brain has what is effectively a CO2 sensor that indirectly monitors blood levels, and adjusts breathing accordingly. Thus, if the blood CO2 starts to rise, then your brain will drive you breathe more (either by increasing your breathing rate or breath size or both), and if your blood CO2 starts to fall, then your brain will cause you to breathe less so that the levels rise again. All of this is happening at a completely subconscious level as you sit there reading this. In most people the brain is “set” to maintain a dissolved PCO2 of about 5.2 kilopascals (kPa) (or 0.052 ATA or 39mmHg depending on what units you prefer to use). However, this control system is imperfect and under some circumstances it can become less precise.

    For example, if the work of breathing increases unnaturally (which occurs in diving for a variety of reasons) the controller in the brain appears predisposed to allowing the blood CO2 to rise rather than drive the extra work involved in maintaining sufficient gas flow in and out of the lungs to keep CO2 at normal levels. Think of it as though the brain is still driving “X” amount of work by the respiratory muscles in response to a given level of CO2, but because the work required to breathe has increased, “X” amount of work now achieves less gas flow in and out of the lungs, therefore less CO2 is eliminated, and CO2 is retained. The more the work of breathing increases (eg deeper, denser gas, hard work, poor equipment), and the more CO2 that is being produced (eg because of exercise) then the more likely CO2 retention is to occur. The increase in PO2 and PN2 that are also encountered in diving can also “depress” the respiratory controller and make CO2 retention more likely.

    Interestingly, there is also significant variability between individuals in the degree to which they retain CO2. Some peoples’ respiratory controller will adjust breathing to maintain their normal level of CO2 irrespective of how much the work of breathing increases, whilst others are very vulnerable to increasing CO2 because of this disturbance of normal control. The latter group are often referred to as “CO2 retainers”. It almost seems paradoxical, but the non-retainers are the ones who suffer the unpleasant symptoms of increasing CO2 most readily. Thus, they get the horrible feeling of shortness of breath early when CO2 is rising, but if you think about it, that explains why they don’t retain CO2; they feel short of breath, start to breathe more, and get rid of the rising CO2. In contrast CO2 retainers do not experience those unpleasant early symptoms. They don’t feel short of breath, they don’t increase their respiratory rate or the size of their breaths, and therefore they don’t eliminate the rising CO2.

    Hopefully it is clear to you that CO2 “retention” because of inadequate lung ventilation is one potential cause of CO2 toxicity in diving. You will note that this has nothing to do with CO2 breaking through a scrubber and consequent CO2 rebreathing. It is simply a failure to breathe sufficiently to eliminate all the CO2 that is being produced in the body. This can occur on open circuit and closed circuit, and I reiterate it has nothing to do with the scrubber. CO2 rebreathing can occur of course, if the scrubber fails during use of a rebreather, and this is a second cause for CO2 toxicity in diving. A normal blood CO2 can be maintained, despite a low level of CO2 breakthrough, by increasing the rate and depth of breathing. There are some complex considerations, but suffice to say that as the amount of inspired CO2 increases, the harder it is for the diver to maintain a normal blood CO2. Moreover, those who have a tendency to retain CO2 (as described above) are more prone to mount an inadequate respiratory response in the presence of inspired CO2.

    Rising blood CO2 is a problem in diving for several reasons. First it can cause unpleasant symptoms such as headache and shortness of breath. These can precipitate panic. Second, if the levels get high enough CO2 can cause incapacitation and unconsciousness. As mentioned above, those who tend to retain CO2 generally suffer fewer early unpleasant symptoms, and indeed, may not develop symptoms until they are close to the second tier of problems (incapacitation and unconsciousness). To give you some sense of the small changes in blood levels required for these phenomena, 5.2kPa is the average normal level, 6.2kPa is the upper limit of the normal range, and over 8.5kPa sudden incapacitation is likely. Experiments show that levels between 6.5 and 7.5 are not uncommon in divers working underwater (at least some of whom would have related symptoms). The point is that small changes in PCO2 of 1kPa or less can have very important implications for the safety of the diver. As a prelude to the monitoring discussion, this is why any monitoring system purporting to measure blood CO2 levels, and base safety management decisions around that measurement, must be very accurate. Finally, high CO2 worsens narcosis, and predisposes to cerebral oxygen toxicity. We can discuss the reasons for the latter at another time if you wish.

    Simon M
    Hello Simon - do you have a citation for the article as I'd like to use this information and cite properly.

    Thanks, Graham


 

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