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Thread: Real limits of PO2

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    Real limits of PO2

    Dr Chris Pizzo measured his alveolar PO2 at the top of Everest at 0.05B during the American Medical Research expedition in 1981. I appreciate that acclimatisation plays a part but why doesn't the diving reflex allow me to endure such a low PO2 when diving?

    Comments on RBW suggest it is possible to endure significantly higher PO2s than 1.6 for a limited period, and that the oxygen clock has little to do with CNS toxicity. What are the real limits on the high end?

    No nanny comments please: I have no death wish - I merely seek knowledge!

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    Re: Real limits of PO2

    From my own (dry) hypoxic experiments on a KISS RB, I was sitting on the floor, and breathed the loop down to 0.05bar (5% O2) before things started to get a little, blurry, reckon this took 5-10 mins from the loop being full of air. I was not working at all, but also had done no acclimatisation, and was quite surprised by the results. I was expecting to feel something around he 0.1-0.15bar mark, based on the common understanding that <16% O2 is bad. I still have a very healthy respect for low PPO2, and don't use the results of this experiment to change my diving behaviours.

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    Re: Real limits of PO2

    At the other end of the scale, I've used a PO2 of 1.5-1.8 all the way up on some long deep dives. Many of us have sat through PO2's of 2-3 in the Pot without breakdancing too. CNS % goes through the roof.

    At the high PO2 end there is a thought that CO2 brings on the high O2 symptoms (quicker). So, sat in a (dry) decompression chamber where they make you sit still, you can tolerate a couple of hours of high PO2. If you are working very hard and retaining CO2 and underwater, then you may have problems. In other words, practice your zen-like trance if you're going to be messing with high PO2s, otherwise stick to the agency guidelines. And of course, just like undeserved DCI, you may get unlucky.

    Hope that wasn't too Nanny?

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    Re: Real limits of PO2

    Quote Originally Posted by Padowan  View Original Post
    .... I was expecting to feel something around he 0.1-0.15bar mark, based on the common understanding that <16% O2 is bad. I still have a very healthy respect for low PPO2, and don't use the results of this experiment to change my diving behaviours.
    I felt very exhausted when breathing 16/35 on surface while swimming against strong current. Exhaustion went away when I discarded the regulator and breathed air.

    You probably tolerated PPO2 0.05 because you were sitting still.

    -Pasi
    Last edited by plazma; 25th February 2007 at 14:27.

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    Re: Real limits of PO2

    Quote Originally Posted by plazma  View Original Post
    I felt very exhausted when breathing 16/35 on surface while swimming against strong current. Exhaustion went away when I discarded the regulator and breather air.

    You probably tolerated PPO2 0.05 because you were sitting still.

    -Pasi
    is'nt it so that you just tolerated 0.05 for a short time because your brain has residual oxigen from breathing before, and it is just a matter of time before you pass away??

    imagine breathing a 5% mix from a regulator at surface: you will not go unconcious after one breath, but it will not work for 10 minutes..

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    Re: Real limits of PO2

    Yes, I've breathed 16/35 at the surface in a current and it was an unpleasant experience till I reached 10 metres. People actually climb Everest, albeit pretty slowly for the last few hundred metres, at 0.05B. That's a long way below the 0.12 I was told will result in death for a diver.

    I'm intrigued about the oxygen clock: clearly for deep dives you can't help going to several hundred percent on the clock. It doesn't seem to have quite the relavence it had in training.

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    Re: Real limits of PO2

    Quote Originally Posted by Mdemon  View Original Post
    At the high PO2 end there is a thought that CO2 brings on the high O2 symptoms (quicker).
    Any references to support this? I am not saying I disagree; I would just like to see if anyone knows where I can find information about this. I have also read passing references to this in diving medical publications.
    Carl Edmonds, Christopher Lowry and John Pennefather, Diving and Subaquatic Medicine, 3rd ed. (Oxford: Butterworth-Heinemann, 1994), pp. 244 and 262.
    I was just wondering if anyone has anything with more specific material on the subject.

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    Re: Real limits of PO2

    Quote Originally Posted by ScubaDadMiami  View Original Post
    Any references to support this?
    I don't know references, but it seems to make sense that the Co2 makes carbonic acid and the resulting acidity dilates the arteries that increases the oxygen dose?

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    Re: Real limits of PO2

    Quote Originally Posted by Abbo  View Original Post
    I don't know references, but it seems to make sense that the Co2 makes carbonic acid and the resulting acidity dilates the arteries that increases the oxygen dose?
    That carbonic acid-induced vasodilation is why a little bit of CO2 in your loop during decompression should theoretically speed nitrogen off gassing, as well. But not sure how much empirical evidence there is with respect to this phenomenon.

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    Re: Real limits of PO2

    The first message said : Dr Chris Pizzo measured his ALVEOLAR PO2 ...
    And all the following threads speak about PO2 in INSPIRED mix

    So, you must compare these 0.058b with 0.14b (which is the alveolar PO2 with air at surface) and not with 0.21b.

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