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Accumulated 02 following a large number of repetitive Nitrox dives over 3 days.

Discussion in 'Advanced Scuba' started by Astran, Sep 30, 2019.

  1. doctormike

    doctormike Medical Moderator Staff Member

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    VENTID, as posted above, as well as myopia (different than the V in VENTID)

    You aren't wrong in saying that a seizure at a PO2 of 1.0 is not reported, but I would be careful about reading past the data. If you remember, the OP had concluded that O2 clock on the three computers used on a dive could be safely ignored. While that might be true in retrospect, for that specific profile, it's not a generalizable message.

    I understand that the thresholds for ox tox are hugely variable, and that the pathophysiology is incompletely understood. But I also understand that it's very hard to do good science with human data for something like this. Even chamber data might not correctly model dive experience, since immersion seems to be one of the things that significantly changes the threshold for CNS symptoms. Cumulative O2 cell injury at lower PO2s might not cause seizures, but it might change the threshold downstream when switching to a richer mix, going deeper or with recompression. Maybe a seizure in a dry chamber is harmless if you don't hit your head, but that's not the same thing as saying that ox tox can be ignored and has no long term issues as long as you don't drown.

    I'm far from an expert in this field, but since TDI, PADI and Shearwater all still use the time and PO2 model to track non-pulmonary exposure (NOAA or otherwise), I'm uncomfortable saying that time at PO2 under 1.4 is irrelevant outside of pulmonary issues.
     
    GJC, jgttrey, uncfnp and 1 other person like this.
  2. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    I'm more than uncomfortable; I absolutely would not say this!
     
  3. NAM001

    NAM001 Contributor

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    cant find the link again. I have been looking. Like I said it drew mi attention when I saw the labeling because I am used to ambient used instead. If I find it I will post the link. There are many other formulas that use strange labeling like Mo like this PO * MO = PO2 * 100% + PAIR * 20.9%

    OOPs I found it and it was P in stead of PO2 and PO2 was used instead of PPO2. My mistake

    Nitrox calculations
     
    chillyinCanada likes this.
  4. Madacub

    Madacub Contributor

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    Thanks to GJC and doctormike for your posts. (and also for your patience....)

    I had read the decodoppler blog post when it had been linked earlier. The truth is that I am not qualified to scrutinize the examples/numbers he puts forth, neither do I want to get lost in the weeds. But I had noted the to and fro with his poster carmine in the comments section. I do note where he stresses "... in other words, CNS toxicity is the issue, NOT Pulmonary". And in your link to the article authored by Dr Sawatzky in Dive Rite (OXYGEN TOXICITY AND CCR/REBREATHER DIVING), he does come out and say "CNS O2 toxicity is a result of cumulative damage in the cells".

    The thing is that in other articles etc, most discussion of time exposure is directed towards pulmonary toxicity, and not CNS. But of course, maybe these articles are just a function of more modern and better research, don't know. I'm not really advocating one thing or any other thing, just trying to understand.

    Here is another point that is blocking me from whole heartedly accepting this concept of a CNS clock. The PADI Encyclopedia of Recreational Diving (rev 2/08, version 3.02, page 5-19) provides the NOAA Oxygen Limits table and then says "This is the basis of ...... and for the misnamed "CNS clock" in common use in the dive community (NOAA limits aren't based on the prevention of CNS toxicity, but rather pulmonary toxicity)".

    I'm not trying to put forward PADI as an authority, but when they something that definitively, they deserve some attention. Any idea why they might make this statement?

    So I looked at the NOAA Diving Manual (4th edition, 2001) kindly linked by Tursiops earlier in the thread. It clearly refers to the "CNS Oxygen Exposure Table" (page 3-23). But then the two tables on that page also refer to ATM not ATA. All other reference to these tables that I have seen refer to ATA, with the same numbers.

    Tursiops, I have a feeling you have the 5th or even 6th edition. Any chance you could see if they have changed the reference to either CNS or the ATM?
     
  5. uncfnp

    uncfnp Solo Diver ScubaBoard Supporter

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    I’ve not kept up with the posts but I think some of your questions from reading this post might be found here (I don’t think it has been linked yet).

    Alert Diver | Understanding Oxygen Toxicity

    ALL manifestation of oxygen toxicity, CNS/pulmonary/ocular are time and oxygen partial pressure dependent.
     
    Jay likes this.
  6. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    I am neither a physiologist nor a oxygen-toxicity researcher, but NOAA's approach seems to say that CNS toxicity can come from either short exposures to high PPO2, or longer exposures to moderate PPO2, both causing CNS problems. They quantify the don't-go-too-deep with Maximum Operation Depths (MOD) based on 1.4 or 1.6, for example, and the don't go-too-long with their tables 4.4 and 4.5 (in the 5th edition, or 3.4 and 3.5 in the 4th on-line edition). Both are clearly CNS related. The "oxygen clock" is what they call the O2 Limit Fraction, a way of estimating how close you are to the don't-go-too-long exposure limits.

    Separately, section 4.3.3.3.8 (or 3.3.3.3.8 in the 4th edition) talks about whole-body, or lung (the most sensitive part of the body, other than CNS) toxicity, and they specifically say that this is an issue at PPO2 levels above 0.5 but below the levels causing CNS toxicity. Here they get into OTUs.
    They may have mispoken. I'd be curious what a more modern edition says. My 2008 edition says the same as your 2005 edition.
    ATA and ATM are the same thing, just sloppiness in editing. Atmospheres Absolute and Atmospheres.

    The 5th edition section on oxygen toxicity is almost identical to your 4th edition, except for the chapter numbering and section 4.3.3.3.8 about pulmonary toxicity. The newer edition is reworded, but has the same gist, and has more examples.
     
    Madacub likes this.
  7. doctormike

    doctormike Medical Moderator Staff Member

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    This is another area where you may find texts that don't stick to standard definitions of those two terms, but basically ATA stands for atmospheres absolute. That means it is measuring ambient pressure as compared to a vacuum.

    ATM is a unit of pressure (like PSI or bar), with one ATM being equal to around 14.7 PSI. So while it is often used interchangeably with ATA, it is a gauge pressure, which means it is measured against standard atmospheric pressure. An analog depth gauge measures ATM and is calibrated with zero being at the surface.

    I guess one way of thinking about the difference is that ATA is a measurement of pressure at any give depth (sum of the weight of the water and the atmosphere), while ATM is a unit of pressure. You could say that "I measured the ATA at 33 feet and it was 2 ATM".
     
    chillyinCanada and BackAfter30 like this.
  8. Madacub

    Madacub Contributor

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    Certainly seems that way, even if not all articles on the subject are very clear about it. (The article had been linked earlier, but thanks)
     
  9. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

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    I've thought that toxicity due to very high partial pressures is more idiosyncratic, not so tightly linked to time of exposure
     
  10. Madacub

    Madacub Contributor

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    Yes, I'm clear on ATM/ATA. Maybe I should have spelled out my point which is that when you are putting these numbers (1.2, 1.3, 1.4 etc) in a table, it makes a difference whether you label the column ATA or ATM. Going further along that thought, if they were careless about that, were they careless in labelling the table CNS toxicity? Why is table 3.5 labelled CNS while table 3.4 is not? But now that enters bean counter nitpickiness :eek:. So I'll stop.

    I do note that 3.3.3.3.4specifically mentions convulsions (which is CNS). while separately mentioning pulmonary. So that seems pretty clear. Which brings us to:

    It reads too definitive a statement to be a mispeak. Someone has put some thought into that particular wording. As you say, curious.
     

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