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Thread: Is it Hypercapnia or Hypoxia?

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    RBW Member trimix is on a distinguished road trimix is on a distinguished road trimix's Avatar
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    Is it Hypercapnia or Hypoxia?

    Years back when I was doing freediving training, we learned to push ourselves to the point of blackout without actually blacking out...basically finding our threshold for what I then understood to be hypoxia. Everybody should have different symptoms, but I at least recognize what happens to me. I experience rapid perceptive narrowing, an increased ringing in my ears, muffling of outside noise, black spots in my vision - about 10 seconds of symptoms before... you know. Whether I would have the wits enough to recognize the symptoms and perform a quick flush and bailout in time is for another discussion...

    Fast forward a few years, and put a CCR on my back. We hear of guys walking to the waters edge, diligently breathing up the scrubber, but forgetting to turn on the O2 (talking COPIS here, with mass flow injector). Diver bends over to put on his fins, and whammo - blacks out. Hypoxia, right? I've never gone to the whammo stage, but I've been close...I feel the onset of those old freediving blackouts coming on around PO2 of 0.25 - blackout is theoretically around 0.16, right?

    Recent discussions I've read and heard have contradicted this, however. Some have argued that it is impossible to feel symptoms of hypoxia. So... what is it I'm feeling?

    They have suggested that it is Hypercapnia I am experiencing instead. Ok, I could buy that - maybe what I was experiencing in the pool as a freediver was CO2 buildup leading to blackout. But shouldn't there be an increased breathing rate? Other symptoms? Or am I not noticing the other symptoms because of the increased CO2.

    Can anybody (in the know) please enlighten me further? Which is it?

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    Re: Is it Hypercapnia or Hypoxia?

    Quote Originally Posted by trimix  View Original Post
    ...I feel the onset of those old freediving blackouts coming on around PO2 of 0.25 - blackout is theoretically around 0.16, right?
    So you feel the onset of blackout at a PO2 higher than normal air?
    Around here our air has a PO2 of .21 and most consider about .1 to be the lights out point.
    Cheers,

    Dave....

    www.wedivebc.com

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    Re: Is it Hypercapnia or Hypoxia?

    I'd go with the idea that hypercapnia is the freedive blackout because a samba is such an on-off effect. You don't have to breath to snap out of it. CO2 can trigger convulsions like high O2.

    Also I've seen sambas (I wasn't watching when I did it) and no blue lips. This is usually the measure of low oxygen in the head as, although your body will give the head preferential treatment for blood it doesn't distinguish what above the neck gets it. I've hit the blue lips stage and I was still very conscious but that was just a heart attack.

    But then I'm a physicist not a medical type so what do I know?

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    Re: Is it Hypercapnia or Hypoxia?

    Quote Originally Posted by nigelh  View Original Post
    I'd go with the idea that hypercapnia is the freedive blackout because a samba is such an on-off effect. You don't have to breath to snap out of it. CO2 can trigger convulsions like high O2.

    Also I've seen sambas (I wasn't watching when I did it) and no blue lips. This is usually the measure of low oxygen in the head as, although your body will give the head preferential treatment for blood it doesn't distinguish what above the neck gets it. I've hit the blue lips stage and I was still very conscious but that was just a heart attack.

    But then I'm a physicist not a medical type so what do I know?

    "Samba" is caused though by the muscles using lactose acid and therefore spasming because they ran out of O2 due to the blood shunt effect, not CO2...

    cheers

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    Re: Is it Hypercapnia or Hypoxia?

    As a former world record holder in freediving who has spent 10 years doing freediving related experiments, I can answer the above questions quite accurately:

    1. The symptoms of hypoxia are very consistent and they can be felt and detected very reliably by a person who has been trained to detect them. A person who has no training may not feel the onset of hypoxia and may black out without any warning signs. The exact symptoms vary slightly from one person to the next, but they are generally repeatable within the same person. Fading vision is one sign which is quite common. However, once the signs of hypoxia are felt (even by a trained person), you may have anywhere from two to ten seconds to do something about it before you black out.

    2. If you hold your breath until almost a black-out and measure the oxygen pressure in your lungs, you will find it to be around 3-5% (i.e. 0.03 to 0.05 bar). However in this case you benefit from the elevated CO2 level caused by a breath-hold. Without the elevated CO2, the blackout oxygen level is higher. The classic example is the Mount Everest one. At the top of Everest, the oxygen pressure is about 0.07 bar. A person suddenly placed in that atmosphere will usually black out (according to studies) in anywhere from 10 to 50 seconds.

    3. A blackout while freediving is caused by hypoxia, 99.99% of the time. Only a few people in history have had CO2 blackouts while freediving, and this always occurs while on the bottom, not while headed for the surface.

    4. A 'samba' or seizure will occur at the moment of blackout in 90% of people. This is caused by elevated glutamate levels in the brain during gradual energy failure. Glutamate is an excitatory neurotransmitter, which in elevated concentrations can cause epileptiform activity to 'spawn' in one area of the brain which quickly leads to other areas (= seizure).

    5. Although CCR divers often worry about PO2 values, in fact blood pressure is far more important. A sudden drop in cerebral blood pressure can cause you to black out within 3-8 seconds regardless of what you are breathing, even if it is PO2=1.0. This is sometimes felt if you are sitting for a long period and suddenly stand up -- especially if you are sick or dehydrated. You get suddenly light-headed, and you may faint. So, any activity which can cause a sudden drop in blood pressure to your head can be deadly -- even fear can cause it.

    6. People often look for 'blue lips' as a sign of hypoxia. This is not an accurate indicator. In the above example, if you stand up quickly, you can faint (i.e. black out), and you will never have blue lips in that case, even though your brain ran out of oxygen due to low blood pressure. If a freediver hyperventilates, he can easily black out without ever having blue lips. Blue lips will most often only occur after a PROLONGED period of hypoxia in the presence of high CO2.

    7. Most of the signs of extreme hypercapnia are different from hypoxia, but they too require some training to detect. Extreme hypercapnia often causes a dizzy, narcosis like feeling. The effects of hypercapnia are massively delayed if you are breathing an enriched O2 mixture (i.e. PO2 > 0.20 bar). Some signs of hypercapnia can include ringing in the ears, which can also occur during hypoxia -- which is why some people confuse the two.


    Eric Fattah
    Liquivision Products

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    Re: Is it Hypercapnia or Hypoxia?

    Great info Eric, some food for thought.

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    Re: Is it Hypercapnia or Hypoxia?

    Trimix,

    Basing my answer on some of Eric's responses:

    Quote Originally Posted by tdzao  View Original Post
    1. The symptoms of hypoxia are very consistent and they can be felt and detected very reliably by a person who has been trained to detect them. A person who has no training may not feel the onset of hypoxia and may black out without any warning signs. The exact symptoms vary slightly from one person to the next, but they are generally repeatable within the same person. Fading vision is one sign which is quite common. However, once the signs of hypoxia are felt (even by a trained person), you may have anywhere from two to ten seconds to do something about it before you black out.
    I agree with Eric. There is no "rule" that says you cannot detect symptoms of hypoxia. It would be more accurate to say that you cannot always or reliably detect them. In the scenarios you describe hypoxia is almost certainly the culprit. Put another way, if you breathe air from a loop with no oxygen supply and no CO2 scrubber, it is hypoxia that would cause you to black out first. The CO2 would rise, but not to a level that would cause unconsciousness before the oxygen fell dangerously. In fact, you would be unlikely to be able to do this intentionally, because the combination of rising CO2 and falling O2 would produce such unpleasant symptoms. In a related vein, it is the fact that CO2 levels are being kept normal by the scrubber that allows hypoxia to be the "silent killer" in a rebreather.

    Quote Originally Posted by tdzao  View Original Post
    2. If you hold your breath until almost a black-out and measure the oxygen pressure in your lungs, you will find it to be around 3-5% (i.e. 0.03 to 0.05 bar). However in this case you benefit from the elevated CO2 level caused by a breath-hold. Without the elevated CO2, the blackout oxygen level is higher. The classic example is the Mount Everest one. At the top of Everest, the oxygen pressure is about 0.07 bar. A person suddenly placed in that atmosphere will usually black out (according to studies) in anywhere from 10 to 50 seconds.
    Hmmm, Eric, since you introduced your answer by claiming that it was accurate, this deserves a response. The physiological response to extreme altitude is hyperventilation in order to lower CO2. A high CO2 does not provide benefit and would probably kill you at the summit of Everest (in the absence of supplemental oxygen). This is a simple matter of considering the alveolar gas equation. I can't remember how tall Everest is, but if you consider 5500m where the ambient pressure is about 0.5 (380mmHg) bar it provides a nice example. If you retain even a normal CO2 level at this altitude, the alveolar gas equation predicts an alveolar PO2 of 20mmHg!! (I can show the working if you wish to debate it). The response is to hyperventilate to lower PCO2, which increases PO2. This appears in any standard textbook of respiratory physiology.

    Quote Originally Posted by tdzao  View Original Post
    3. A blackout while freediving is caused by hypoxia, 99.99% of the time. Only a few people in history have had CO2 blackouts while freediving, and this always occurs while on the bottom, not while headed for the surface.
    I agree about the hypoxia, and would be interested to know how it has been established that any blackouts attributable to CO2 have ever occurred during free diving (on air).

    Quote Originally Posted by tdzao  View Original Post
    5. Although CCR divers often worry about PO2 values, in fact blood pressure is far more important. A sudden drop in cerebral blood pressure can cause you to black out within 3-8 seconds regardless of what you are breathing, even if it is PO2=1.0. This is sometimes felt if you are sitting for a long period and suddenly stand up -- especially if you are sick or dehydrated. You get suddenly light-headed, and you may faint. So, any activity which can cause a sudden drop in blood pressure to your head can be deadly -- even fear can cause it.
    Once again, in the interest of accuracy, I put it to you that this is rubbish in the context of CCR diving. I have no problem with the concept that precipitous hypotension can cause a blackout, but have you any evidence at all that sudden hypotension is a common problem in an immersed breathing diver, or that this problem is more important cause of blackout than hypoxia during a CCR dive? Your analogy to postural hypotension is completely irrelevant to diving (except where the diver leaves the water).

    Warm regards,

    Simon M

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    Re: Is it Hypercapnia or Hypoxia?

    Quote Originally Posted by wedivebc  View Original Post
    So you feel the onset of blackout at a PO2 higher than normal air?
    Around here our air has a PO2 of .21 and most consider about .1 to be the lights out point.
    Doh! My bad. It must have been around 0.17 when I was feeling symptoms. Can't remember - was too busy picking my ass up from the beach!

    .. and thanks for the input so far, guys!

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    Re: Is it Hypercapnia or Hypoxia?

    Simon,
    Are you the same Simon Mitchell that I was on a film lecture circut with in Australia 8 or 9 years ago? Ernie Brooks with the Brooks institute was with us?

    John McKenney

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    Re: Is it Hypercapnia or Hypoxia?

    Quote Originally Posted by McKenney  View Original Post
    Simon,
    Are you the same Simon Mitchell that I was on a film lecture circut with in Australia 8 or 9 years ago? Ernie Brooks with the Brooks institute was with us?

    John McKenney
    Hi John,

    Yes, that was me, and it was a really fun time. I have often wondered how things were going with you. I'm now in Auckland dividing my time between anesthesiology, diving medicine and related research work (and a bit of diving of course). I'll pm you with my email address.

    Trimix,

    On re-reading the first part of my answer above, I don't think I explained it very well. First, I meant to say that you can detect symptoms of hypoxia
    before blacking out (as pointed out by Eric Fattah). Unfortunately it is not reliable, and it is possible to black out with little or no warning. Second, in the scenario you are describing (breathing from a loop with a functioning scrubber but no oxygen supply) any symptoms experienced will almost certainly be caused by hypoxia, not CO2 toxicity. Moreover, if there was no oxygen supply and the scrubber was not functioning for some reason you would experience symptoms of hypercapnia (and hypoxia) which would be much more unpleasant, but it would almost certainly be the hypoxia that makes you black out (if you were silly and strong willed enough to stay on the loop).

    Hope this helps.

    Simon M

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