Oxtox and fatality at 1.4 PPO2

[dsteding]

Hey all-

Posting this here not to be inflammatory but because I often hear some cite a constant 1.4 PPO2 as one of the advantages of a rebreather-if someone was sensitive to hyperbaric oxygen like this victim apparently was then it would most likely manifest itself on extended exposures to a 1.4 PPO2, like one can get on a rebreather (I recognize that others set their setpoint at a lower level).

Detailed accident analysis is here.

A sobering read.

[Sounder]

Wow. Tragic.

I have several questions for the rebreather folks:

What is the drawback, or is there one, of using a full-face mask (keeping a traditional mask in a pocket for use in the event of an OC bail-out)? Seems a full-face mask might have made the difference in this situation. She could have seized and still breathed, and on a CCR presumably (please allow me to disclaim my ignorance here) her buddies could have decreased the ppO2 in the loop.

Would being on CCR have made a difference here if she was on a full-face mask?

What about if she was using a full-face mask with on OC?

If the whole team was on CCR, would they have had more time at depth (presumably not limited by gas volume) to manage her seizure while decreasing her ppO2 and keeping her at the same depth (preventing expansion injury) until the seizure was over and they could see her breathing again?

Thanks in advance.

[Joshua Smith]

Most rebreather divers use a setpoint between 1.0- 1.2, I believe. I'm pretty sure running a constant 1.4 is not reccomended.

[airsix]

Wow.
That gives me a little chill. How many divers are out there diving with much less safety margin than they think?

-Ben

[Joshua Smith]

Wow. The link wasn't working at first- just checked back and read the article. Really sobering. Makes me think people ought to expose themselves to higher P02 levels on the surface, just to find out how they react. Overall, it seems like he's right, and this "shouldn't have happened."

[CaptnJack]

First point: Immersion exacerbates O2 toxicity. In a dry chamber, almost nobody toxes at 60ft on 100% O2 even after 20 mins. In water and your chances of toxing are pretty high at 3 ATA. Not 100% by any means but high.

Second: Day to day variability in O2 toxicity within a diver is incredibly high.
http://thedecostop.com/forums/attachmen ... 1165275703

[Dmitchell]

I usually run 1.2-1.3 mostly in the 1.3 range.

I wonder what her previous dive profile was? She had a 5hr SI and it sounds like they knew that she was susceptible to O2 issues. If the previous dive was a deep deco she would have had some clock time built up and while she was ok on the NOAA exposure chart she must have pushed herself beyond her own limits.

It's a sad story and I amazed that the husband is able to write it. I doubt that I could.

Dave

[Curt McNamee]

Wow. Tragic.

I have several questions for the rebreather folks:

What is the drawback, or is there one, of using a full-face mask (keeping a traditional mask in a pocket for use in the event of an OC bail-out)? Seems a full-face mask might have made the difference in this situation. She could have seized and still breathed, and on a CCR presumably (please allow me to disclaim my ignorance here) her buddies could have decreased the ppO2 in the loop.

Would being on CCR have made a difference here if she was on a full-face mask?

What about if she was using a full-face mask with on OC?

If the whole team was on CCR, would they have had more time at depth (presumably not limited by gas volume) to manage her seizure while decreasing her ppO2 and keeping her at the same depth (preventing expansion injury) until the seizure was over and they could see her breathing again?

Thanks in advance.

Using a full face mask while having a seizure can save your life, whether you are on OC or a CCR.

If you have a full face mask on and have a seizure, you will start breathing again at some point when the event is over. If your buddy can just keep your bouyancy under control until the seizure is over you might survive.

When you have an O2 seizure, usually you will spit out your regulator and then swallow water and drown. By having a full face mask on, the swallowing water problem does not take place.

Running a constant PO2 in the 1.2 range on a CCR gives you a huge advantage over OC that might have a planned max PO2 of 1.4-1.6.

The reason this is true is because on OC, your higher PO2, 1.4-1.6 can only be maintained at certain points during your dive and your average PO2 will be considerably lower. You cannot maintain a reasonable average PO2 without pushing it high at your optimum depth.

On OC you push your PO2 to minimize deco. On a rebreather you don't have to push your PO2 so high and you get the benefit of a much higher PO2 average which will minimize deco more safely.

People do have different thresholds for Oxygen toxicity convulsions. some have a very high tolerance and others not.

I guess in the military they run you at very high PO2's (2-3 range) just to find out what you can tolerate.

[CaptnJack]

Many of us on OC:
Bottom max = 1.4
Bottom average ~1.2
Deco max = 1.6
Deco average ~1.2
Personally, I have not formulated a "best mix", maximizing ppO2 on the bottom, in over 3 years.

Running an average of 1.4 is considered imprudent by some agencies and instructors (including mine). It offers you a few minutes less deco and there's rarely a race to exit the water (suit floods being a possible exception). This incident certainly has validated my gas choice practices if nothing else.

The reality is the only way to stay 100% "safe" is to stay out of the water. E.g. a FFM prevents you from donating gas smoothly, prevents you from orally inflating your wing without simulateously losing your vision, and they can accumulate CO2 far easier than a ordinary 2nd stage. Overall not a good tradeoff for most people's diving when ppO2s can be backed off by alternate gas selection with minimal deco (or NDL) penalties.

[Curt McNamee]

Many of us on OC:
Bottom max = 1.4
Bottom average ~1.2
Deco max = 1.6
Deco average ~1.2
Personally, I have not formulated a "best mix", maximizing ppO2 on the bottom, in over 3 years.

Running an average of 1.4 is considered imprudent by some agencies and instructors (including mine). It offers you a few minutes less deco and there's rarely a race to exit the water (suit floods being a possible exception). This incident certainly has validated my gas choice practices if nothing else.

The reality is the only way to stay 100% "safe" is to stay out of the water. E.g. a FFM prevents you from donating gas smoothly, prevents you from orally inflating your wing without simulateously losing your vision, and they can accumulate CO2 far easier than a ordinary 2nd stage. Overall not a good tradeoff for most people's diving when ppO2s can be backed off by alternate gas selection with minimal deco (or NDL) penalties.

Yes, there are tradeoffs with everything that we do and use. The question was " could a full face mask help in the case of a seisure " the answere is YES.

In the bigger picture you have to weigh out all the pros and cons in any situation you are in to come up with somewhat of a controlled/managed risk for yourself.

I have a full face mask with a pod that can be removed underwater for access to your mouth for another reg, air sharing etc without flooding the mask. All of the negatives that you pointed out are addressed in the design of this FF mask that I have.

[CaptnJack]

I am only familiar with the AGA FFMs and I don't like them. The aquarium makes us use them and with lots of ups and downs in shallow water I often end up with sore ears. I pretty much need a valsalva to clear in shallow water. Deeper I'm ok with other clearing techniques. I've also had the valves come loose and give me a whopper CO2 headache.

I have considered getting a FFM for IWR (in water recompression). However I have yet to travel so remotely that I felt it was worth the cost and the negatives (including travel weight). Even in Prince William Sound 35 miles from Whittier I didn't feel so remote from EMS that I'd go full tilt at IWR.

None of the plane recovery pics showed a FFM that I recall, when do you choose to use it?

In this case I'm not convinced that much of anyting could have saved Liz.

[airsix]

I am only familiar with the AGA FFMs and I don't like them. The aquarium makes us use them and with lots of ups and downs in shallow water I often end up with sore ears. I pretty much need a valsalva to clear in shallow water.

CaptnJack, if you can't valsalva in your AGA then it needs to have the nose block position adjusted. I dove an AGA for many years and I actually find it easier on my ears than any other mask because I can breath through my nose. My ears clear automatically and much more easily when I'm breathing through my nose. Anyway, back to your comment - Did the aquarium people show you how to do a valsalve in the mask? If not, it's simple enough. Using the pads of your fingers press against the hard plastic right above the regulator. This will push the nose-block against your nostrils and seal them and you can now perform a valsalva. If pushing the front of the mask in this way does not seal the nose-block against your nostrils remove the mask and adjust the nose block on the two metal posts such that it will contact your nose properly when you press against the front of the mask. I hope this helps.

Other benefits: no jaw fatigue, no dry mouth, no fogging, and no lost mask when you get finned in the head. I sold mine because I wasn't diving in 32F water any more and I have a lower SAC rate with a standard regulator.

-Ben

[airsix]

Liz, if you can't valsalva in your AGA...

Ah crap. You're not Liz. Oops. And no editing my post to fix it. <sigh>
Sorry Cap'n.

-Ben

[Joshua Smith]

Liz, if you can't valsalva in your AGA...

Ah crap. You're not Liz. Oops. And no editing my post to fix it. <sigh>
Sorry Cap'n.

-Ben

Fixed it for ya, there, Six!

[airsix]

Thanks Josh.

[RDW]

I have used the following FFM on OC and CC:

Mk-20/AGA: OC/SS they're fine. Bit of a jaw fatigue after a few hrs. ONe must take the time to make sure the nose pad is fitted right for
Not my fave for CC. Too much CO2 build up, does take some flushing, loses Diluent.
MK48: Not my favorite at all for CCR. I didn't like the poor visibility of the mask. Comms were good..kind of. If one can find the NATO Pod, cool. On OC/SS, it was ok for that, again the mask viz was not on my "I like it" list.
Drager: This one I liked for CCR. The attachment made for the DSV at ISC worked very well on the Meg. Good viz, good loop management. I liked the comm ports for the OTS comms units. Great also for OC but not real nice for SS. Downside: Pricey little bugger. This is not to take from the thread re the accident. That is a sad incident. What a shame.

[Sounder]

I have used the following FFM on OC and CC:

Mk-20/AGA: OC/SS they're fine. Bit of a jaw fatigue after a few hrs. ONe must take the time to make sure the nose pad is fitted right for
Not my fave for CC. Too much CO2 build up, does take some flushing, loses Diluent.
MK48: Not my favorite at all for CCR. I didn't like the poor visibility of the mask. Comms were good..kind of. If one can find the NATO Pod, cool. On OC/SS, it was ok for that, again the mask viz was not on my "I like it" list.
Drager: This one I liked for CCR. The attachment made for the DSV at ISC worked very well on the Meg. Good viz, good loop management. I liked the comm ports for the OTS comms units. Great also for OC but not real nice for SS. Downside: Pricey little bugger. This is not to take from the thread re the accident. That is a sad incident. What a shame.

Randy (or anyone else) - what is SS? Can I assume it is the same as CCR? (Pleading CCR ignornace ... again ... with the plan of learning more an picking up a CCR in the coming months - 1 year ... yes the time frame as decreased! )

[dsteding]

SS is surface supply, I am guessing.

[Sounder]

SS is surface supply, I am guessing.

Gracias!

[peo]

Using a FFM as a safe-guard for ox tox on a "normal" dive sounds like a solution waiting for a problem to pop up.

The real problem here is diving at a too high PPO2 to begin with. Back off to 1.2 or 1.0 as max PPO2. There's no point in diving a high PPO2, apart from when you're accelerating decompression, and then you have to be very picky about when you do it.

[peo]

Using a FFM as a safe-guard for ox tox on a "normal" dive sounds like a solution waiting for a problem to pop up.

The real problem here is diving at a too high PPO2 to begin with. Back off to 1.2 or 1.0 as max PPO2. There's no point in diving a high PPO2, apart from when you're accelerating decompression, and then you have to be very picky about when you do it.

A clarification: there are several types of dives where a FFM is warranted -- I'm talking about recreational dives or the bottom portion of decompression dives in "normal", non-polluted water, without SS and needs of communciation here -- just focusing on PPO2.

[CaptnJack]

The nose block is as high as it goes, its not shaped for my schnooz and doesn't seal worth a damn. I have to jam the mask up with my hand to clear. Since aquarium dives continually bounce between 0 and about 15ft for 1.5 hours it a pita real real fast.

I dove a normal reg+mask in there once and it was like a dream compared to the freakin' aga.

[Gill Envy]

In this case, the person seemed to have a history of not quite feeling right at depth. Because pressure changes in the sinuses and inner ear often lead to some degree of vertigo on a dive, it seems all to easy to shrug off signs of such a sensativity. Sounds like it could have been a hyper sensitivity to O2, but we can't be sure. Was it an Ox tox or some other kind of seizure? why did it last so long? These are questions that come to mind, but as usual some degree of speculation is all too tempting. I prefer to turn such scenarios into a hypothetical instance to de-pursonalize the tone and open it up to a broader discussion of "what if's" for sharpening ones understandings without stepping on so many toes.

So, lets say you determine you have a greater sensitivity to O2. a rebreather would allow you to manage it better than on OC. You could still do deep dives by allowing you to maintian a lower, constant po2 of say 1.2 and not subjecting you to the spike in po2 as the MOD is approached. It seems that o2 seizures due to a po2 of 1.3 are nearly unheard of but even less so at 1.2. Some folks choose to fly around 1.1 to 1.2 to give some extra buffer, allowing for more time to react to rising po2 with increase in depth and allowing added wiggle room for descrepencies in o2 cell accuracy. To give yourself that much leeway on OC you subject yourself to a lot more nitrogen and dramatically extended decompression, trading one risk for another. by deep diving on OC we seem all to tempted to push po2 limits to avoid the limits of nitrogen. As training standards and build quality of rebreathers has improved, using rebreathers have become more and more viable as an option to mitigate these risks.

my o2 cents,
g

Hey all-

Posting this here not to be inflammatory but because I often hear some cite a constant 1.4 PPO2 as one of the advantages of a rebreather-if someone was sensitive to hyperbaric oxygen like this victim apparently was then it would most likely manifest itself on extended exposures to a 1.4 PPO2, like one can get on a rebreather (I recognize that others set their setpoint at a lower level).

Detailed accident analysis is here.

A sobering read.

[John Rawlings]

.......I often hear some cite a constant 1.4 PPO2 as one of the advantages of a rebreather-if someone was sensitive to hyperbaric oxygen like this victim apparently was then it would most likely manifest itself on extended exposures to a 1.4 PPO2, like one can get on a rebreather (I recognize that others set their setpoint at a lower level)......

You "often" hear that? Hmmmmm.....please understand that I mean no offense by this at all, but I find that odd. I think that you may have zeroed in on a comment made by one person.....made sometime.....somewhere.....and that it is being repeated.

In fact, most CCR divers dive with a set point lower than 1.4. The training manuals now in use encourage divers to establish their set-point no higher than 1.3.

I'm a CCR diver, as are many of my dive buddies. In my role at ADM I also deal with literally hundreds of CCR divers each year.....some of them at expedition level. Having said that, I personally do not know a single one of them that dives their unit at a constant 1.4 PO2.

You need to remember that CCR divers are faced with the exact same O2 clock that Open-Circuit divers are, but CCR divers rack up OTUs at a far greater rate than do OCR divers simply because of the far greater time spent at higher PO2 levels. It is for this reason that in situations where you are making multiple dives on consecutive days it is even MORE important to scale down the PO2 levels.....that ol' oxygen clock builds up FAST if you don't pay attention to it!

Further, if your O2 sensors are nearing the end of their life they may not be reading high PO2 levels accurately, (which they tend to do near the end of their life, hence the recommendation to change them each year). In this situation you could be getting a reading that is in reality LOWER than the PO2 that you are actually breathing.....for example, your sensors might be reading 1.4 when in reality you are breathing 1.5 or even 1.6+. If you are flying your unit at a 1.2 PO2 (or even lower) this gives you bit of a buffer that will keep you from plunging up into an enormously high PO2 level without even being aware of it. If, however, you are flying your unit set at 1.4 you have virtually no buffer at all should your sensors be incorrectly reading low.....you could be swimming around happy as a clam at 1.6 or higher and not even be aware of it until the lights go out, (and probably not even then!). It is for this reason that O2 sensors are checked against both Air and Oxygen to ensure that they will read correctly at high levels.

Just as with Open-Circuit diving there are lunatics among CCR divers that will take higher risks to "push the envelope" a bit farther, avoid longer deco, or obtain some more bragging rights. That ain't me.....nor is it the divers that I dive with. As an older diver that has spent more time than I care to in a chamber, I fly my unit no higher than 1.2 and usually less.

It is my intention to ultimately quietly die of old age with my great-grandchildren at my bed-side.....not twitching on a deep wreck because I wasn't paying attention to my PO2 levels.

- John

[dsteding]

You "often" hear that? Hmmmmm.....please understand that I mean no offense by this at all, but I find that odd. I think that you may have zeroed in on a comment made by one person.....made sometime.....somewhere.....and that it is being repeated.

No offense taken. However, I went back through my email in box before I made that statement and had at least four people who've referenced the "maintain PPO2 at 1.4 and minimize decompression" as one of the reasons they are going CCR. So, just responding to that.

And, it isn't that far off from reality, if the setpoint is 1.3 for the whole dive. I'm wondering, how has this practice evolved? Are CCR divers setting PPO2 lower than they used to?