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OxTox (CNS) on Nitrox - what if you briefly exceed MOD significantly.

Discussion in 'Ask Dr. Decompression' started by Jay, Feb 17, 2019.

  1. Jay

    Jay Need to dive more!

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    There's a couple of what-if's scenario I want to sound out.

    I don't think there is a clear answer, but I would like to know if there's better analysis or knowledge than my 'research' below. I was split between posting this in Advanced or this section as aspects are relevant to both.

    Prelude ... I'm a Rec diver (RD) with Nitrox training (PADI). I am not Deep, Deco or Tec trained. My knowledge of OxTox was lacking. I found this decade old thread in which every post was informative: 02 at depth ?? and this more recent article from DAN that discusses common medications and OxTox risks. Alert Diver | Understanding Oxygen Toxicity

    The two what-if scenarios I had in my mind are similar. In both cases I'm on Nx32%. Max planned depth was 30m.
    A/ I get hit by a down-current, it takes me to a/40m b/45m c/50m before I can escape it even though I reacted quickly. How far up sh1t creek am I? (w.r.t. OxTox).
    B/ My regular buddy appears narc'd and is heading deeper rapidly (same depths as above). Same question. (excluding the 'I should have been closer' answer ... etc ... more about OxTox).

    Other info: Let's exclude the aspect of 'choice'; following vs. being caught in the current. And exclude my risk of getting narc'd as I'd like to focus on the OxTox question (I agree it's a very major question ... for another thread :). Both cases involve exertion at depth for 3-5mins to rectify the situation back to the MOD depth. The dive location is my typical warm water tropical 'vacation' dive spot and vertical currents are not common in the area. There is no viable bottom. Let's assume Nx32% was the only alternative mix available and also set-aside the reasons for diving that mix. What's the risk of catastrophe w.r.t. CNS OxTox convulsions across those depths?

    One answer for B/: if your buddy happened to be on air, would be to use his/her alt/occy when you reach him/her. If your buddy was on Nx32% ...:

    My initial thoughts: doing deco on 100% at 20ft/6.1m is a PO2 of 1.62 bar is common. 40m on 32% is a PO2 of 1.60 (noted that resting at deco for a long time is not the same as working for a short time). 1.60 also seems to be an acceptable contingency MOD level. So perhaps this might suggest that brief stint to 40m is extremely unlikely to have OxTox consequences and shouldn't be a concern.

    The DAN article (links section below) does highlight that there's a significant variability across individuals and also the same individual at different times, and research results were not clear.

    It goes on to say "The best that could be said is that a single 15-minute excursion to 40 fsw/12 msw [100% O2], or for five minutes at 50 fsw/15 msw, probably had no significant effect. This formed the basis of the current U.S. Navy recommendations." (last paragraph). The article seems to be written around '96.

    12m and 15m (100% O2) are PO2s of 2.21 and 2.51. 45m and 50m on Nx32% have PO2s of 1.76 and 1.92. It seems one would be very unlucky to have an OxTox event at those PO2s for 3-5mins.

    I came across a CNS % Tracking Table from @KWS in this thread (post#34) PPO2 maximum safe value: 1.4, 1.6 direct link to table:http://www.borrett.id.au/downloads/cns_percentage_tracking_v1-1.pdf

    Table: Screen Shot 2019-02-17 at 5.27.59 pm.png

    That table says that one would reach 100% CNS loading at 45mins, 4mins and less than 1minute (respectively) for the three depths (assuming the event is at the start of the dive). In my view, assuming the table's correct, it doesn't provide much comfort as one appears near a cliff. That cliff seems intuitively incorrect given the US Navy comments above and the history of a PO2 as per @tursiops's comment below - or perhaps it's just how a loading limit's table handles its end case.

    There are preventative measures such as another Nx mix with lower O2 providing more headroom (as per @Diving Dubai below).

    If the dive site was known for its vertical currents, and there wasn't a bottom at a reasonable depth, then I would stay away from Nx 32% if it was the only alternative. But this view is formed mainly on my intuition and not much science.

    So, given you found yourself in situation A/ or B/ what science / knowledge etc do you lean on for comfort re OxTox at those depths?

    ------------------------------------------------------------------------------------------------------------------

    Links and quotes

    DAN link: OXTOX: If You Dive Nitrox You Should Know About OXTOX — DAN | Divers Alert Network — Medical Dive Article

    Other good thread I found:Oxygen Toxicity Limits & Symptoms

    Interesting info / quote from another thread (post#48 - Should I get a Nitrox certification?):

     
  2. jale

    jale Nassau Grouper

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    Hi
    When i started diving Nx, the mod of the gas was 1.6 and we did use .5 point of decrease for layering factor as cold, exercice, ...
    For single tank recre diving in warm water, i still use 1.6 and dive regularly at 40M with N32.
    Now you can use, 1.4 as your limit and stay above 30M and if you find yourself in down current, don't worry about toxing as worrying about toxing will make you tox more easily :)
    Just be aware and know what you are doing and relax... :)
     
    Jay likes this.
  3. michael-fisch

    michael-fisch Manta Ray

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    Richard Pyle (Mr Deep Stops) was once told to immediately stop breathing from the O2 mask while in a chamber, later they told him that he was breathing pure O2 at 6.7Bar.
    A few breaths of O2 at 5 or 6 Bar are not going to kill you all by themselves, and many of us have breathed pure O2 in a chamber for hours on end, and lived because we did that. In Water Recompression, depending on whose plan you use, also involves 2.8Bar O2.
    Of course if you had been planning on 1.4Bar O2 for decompression, and suddenly find yourself at 10M breathing O2, the safest thing is to just stop breathing that dangerous gas and die. Otherwise just ascend a few meters untill you are at what you think is a safe depth and carry on.

    Michael
     
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  4. RyanT

    RyanT Solo Diver

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    For scenario A, I'm not going to lean on anything for comfort.. I'm going to immediately begin working to fix the problem and minimize any increase in depth. For scenario B, it's not going to happen. My buddy and I are going to plan the dive and use appropriate gases to ensure that we don't go so narced that one of us is headed down to meet Davy Jones. If those gases aren't available, then we wouldn't make the dive.
     
  5. wetb4igetinthewater

    wetb4igetinthewater Instructor, Scuba

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    There is Bret Gilliam's deepest dive on air: https://www.bretgilliam.com/assets/gilliam-world-record-deep-dive.pdf to 452 feet/137 meters. So a ppO2 of 3.

    I wouldn't recommend it though.

    I know of a group of divers who don't switch to trimix until 100 meters. But the group is getting smaller as members are slowly dying in accidents. Surprise, surprise.
     
  6. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    Your Ox tox issues are a joint effort of PPO2 and time. It takes time for any change in mix to take effect. There is a word for that but I forget what it is. Basically it refers to a changeover period between gasses. Think of this you do a dive and at 20 ft you go to O2 how long before your lungs have O2 in them as opposed to nitrox in them. Putting the O2 reg in your mouth and taking one breath does not exchange all teh gas in your lungs from 32 to 100 O2. If I can not get to teh lungs it can not get to the body. once it gets to teh body then the exposure time is a factor.
     
  7. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

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    O2 toxicity is dependent on several things. The two most important are partial pressure of oxygen and length of exposure. Pulmonary O2 toxicity can happen on the surface in someone breathing a high percentage of O2 for more than a day or so. CNS O2 toxicity only happens at O2 partial pressures greater than atmospheric, but the exposure period required is much less.

    There's a big difference between breathing oxygen in a chamber and breathing it under water. In a chamber, under controlled conditions and with the individual warm and dry, it's generally safe to breathe O2 at 60 feet/18 meters (2.82 atmospheres absolute) for short periods of time. On the other hand, doing IWR on 100% O2 at 60 feet/18 meters under water is highly inadvisable and extremely dangerous, despite some of the stuff you see circulating on the internet about it being safe.

    The maximum advisable PO2 for underwater use has dropped over the years. Now, you generally hear 1.4-1.6 ATA on decompression and 1.2 ATA while swimming. Brief, small excursions past that as in your 45 (pO2 1.76 ATA) and 50 meter (1.92 ATA) examples will probably not be harmful, but the key word is probably. Another major factor in CNS O2 toxicity is circulating carbon dioxide level, which tends to increase while diving and increase even more under stress and exercise conditions under water. Susceptibility to O2 toxicity is also highly individual, and varies in the same individual from day to day.

    The best medicine is prevention, as @RyanT alluded to. But, if you find yourself in a down-current or rescue situation where you go beyond what's considered safe, don't panic,and get yourself back to a safe depth as soon as possible.

    Best regards,
    DDM
     
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  8. wetb4igetinthewater

    wetb4igetinthewater Instructor, Scuba

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    Is the reason for this due to body temperature? Or ....?
     
  9. Duke Dive Medicine

    Duke Dive Medicine Medical Moderator Staff Member

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    Increased CO2 levels, exercise, and immersion all lead to increased cerebral perfusion.

    Best regards,
    DDM
     
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