Article: Daily Limits for CNS Oxygen Toxicity

[Doppler]

You can view the page at http://www.scubaboard.com/forums/content.php?r=357-Daily-Limits-for-CNS-Oxygen-Toxicity

 

[TSandM]

It's a bunch of good points for staged decompression diving, but where on earth do recreational divers dive 50%?

 

[JamesK]

Excellent article.

 

[ianr33]

I've only ever done a handful of dives that have exceeded 80% CNS. I figure doing back gas breaks (12 on, 6 off or similar) I can breathe O2 for longer than I will ever need to. I certainly don't do two of those dives in a day.
Thoughts?

 

[fdarden]

Thank you. Great post

 

[Doppler]

It's a bunch of good points for staged decompression diving, but where on earth do recreational divers dive 50%?

When advanced nitrox first came out as a stand-alone, this type of scenario was used as an example of why instructors should teach ALL the NOAA limits. Agree that things turned out differently for OC divers; however, with the potential opening of recreational CCRs, this type of thing does become possible.

Excellent article.

Thank you. Appreciate the support.

I've only ever done a handful of dives that have exceeded 80% CNS. I figure doing back gas breaks (12 on, 6 off or similar) I can breathe O2 for longer than I will ever need to. I certainly don't do two of those dives in a day.
Thoughts?

The disturbing thing and CNS and tech diving is not the extreme dives but the borderline tech exposures that one is tempted to stack up day on day... for example, when we go cave diving for a week and try to pull 28 to 30 dives. That's what probably killed one of my friends. By the way, I have always thought "air breaks" where as much to reverse capillary bed shutdown than for CNS loading.

Thank you. Great post

Thank You. Hope it serves as food for thought.

 

[Kevrumbo]

No direct data or statistical study . . .but there are indirect anecdotes about extreme pO2 exposure as a contingency. . .

Full post from Joel Silverstein:

(i.g. PO2 of 2.4 for 10min -->100m on deep air)

High PO2 for bail out is just that -- it's for a bail out in an extreme condition. I've been at this a long time and I will tell you that I only trust about 4-5 people in the ocean with me. When I mean TRUST i mean that they will risk all to bring me back. (mostly cause if they don't they have to deal with my wife) ----

Nevertheless..... it takes approximatley 3-5 minutes for the brain to register the high PO2. We routinely run 2.8 atm PO2 in a hyperbaric chamber for up to 30 min at a time without issues too often. Keep in mind this is a dry environment, on mask, without exertion. So to run a high PO2 for an extremely short period for an ascent to a lower PO2 is not out of the question.

We removed the Exceptional Exposure Oxygen tables from the NOAA diving manual 4th editon because there was fear that if the general public saw them printed that they might take it as an endorsement to use them.

The NOAA exceptional exposure limits are set for extreme emergencies only and are not for routine use. IE: should be used for life saving only.

These are for a working dive meaning with lite exertion. Remember that there are a variety of factors that come into oxygen toxicity, and crossing the 1.6 atm 45min line does not guarantee convulsion, it also does not guarantee it won't.

NOAA OXYGEN EXCEPTIONAL EXPOSURE LIMITS PO2 Minutes 2.8 5 2.4 10 2.0 30 1.9 45 1.8 60 1.7 75 1.6 120 1.5 150 1.4 160 1.3 240

As you can see the exceptional times allow you a fairly large margin to use this method for an "escape." The table is NOT linear. Note that exceptional exposures are DANGEROUS and can only be done once in a day.

Having done more than a few dive fatality investigations. It bugs me hard when we find divers at 200+ dead with empty main tanks and full stage bottles. Someone put the notion in thier head that a deco gas used deeper than an MOD will kill them. Well i guess that point is moot. When one is out of gas for whatever reason you need to put something in the lungs to keep the water out. Breathing ANYTHING is better than nothing.

You may want to assume that your dive partner will not crap in his pants when you pass out and assume he can save you. You may want to assume that the regulator some friend of yours just rebuilt will work 100%, you may want to assume those things and many others on a dive and may NEVER need to go to your deco gas to bail out but this one little post .. this little tiny bit of information may save YOUR life one day.

Someday, and that day may never come ... you will be coming out of a dive and visibility went to ****, your $1200 flashlight took the crapper and your back up light flooded, and your dive partner is GONE... you look for him and the flow or current is so strong that your back gas is done ... you will say OH ****... think fast and remember .... Oh well Joel said I can use my deco gas, you will put your deep deco in your mouth, take a breath and head on up (dont dick around, just get to the MOD) then complete the deco and get on the boat, have a beer, smoke a cigarette and realise that today you had TWO choices.... Live or Not Live. Your choice with just this little information makes the difference.

Cheers,
Joel Silverstein, VP COO
Tech Diving Limited
Tech Diving Limited, a subsidiary of Scuba Training and Technology Inc.

 

[billgraham]

To me, the most interesting thing in the article is the suggestion to ignore the 90 minute half life. I hadn't heard that before, has anyone else? Certainly way more conservative.

 

[Teller]

Great article. Thanks for sharing. I also have never heard anyone suggest to ignore the 90 minute half times but I guess that is one way to add conservatism.

 

[Doppler]

To me, the most interesting thing in the article is the suggestion to ignore the 90 minute half life. I hadn't heard that before, has anyone else? Certainly way more conservative.

Great article. Thanks for sharing. I also have never heard anyone suggest to ignore the 90 minute half times but I guess that is one way to add conservatism.

Thanks!

OK, a little housekeeping. I suggest ignoring the 90-minute half-time because I have never found any credible scientific reference to it in relation to CNS toxicity. (There is reference in some studies on Pulmonary Toxicity which is irrelevant in this discussion, IMHO). Unfortunately, I only had one conversation with Bill Hamilton and wish I'd taken better notes; however, if memory served he used a rather choice expletive to describe the notion that CNS exposures decayed over time. "If that were the case," he said. "We would not have had to develop a 24-hour CNS table."

Do the maths and it becomes virtually impossible to approach the daily CNS limits if one subscribes to a half-time for CNS toxicity.

Also, worth thinking about, is that there are documented episodes of CNS "accidents" to divers whose oxygen exposure was well within NOAA depth and time limits (for example 1.3 bar for 95 minutes). This has lead to a general community speculation that some folks are more prone to CNS that others. THIS IS ABSOLUTELY PLAUSIBLE, however, it's my strong belief that the strongest factor in these cases is that the diver ignored or was unaware of the daily CNS limits. The dives on which incidents occur are invariably the last of a series of dives... all within single dive limits but taken as a group, beyond the limits of the recommended daily dose.


Therefore, I plan (and teach) dives based on daily limits without any reference to a half-time. CNS is one of the few things I fear... actually, one of MANY (LOL) but it seems reasonably easy to manage.