Thread split: CNS toxicity limits

[Seameadow]

Speaking to CNS vs older NOAA data and more practical info, this is a slide from Dr Neal Pollocks presentation at RF4.0 on the relative risk of different PO2s and time vs symptoms. The study was Arieli and Aviner (2020).

I can recommend waiting for his presentation to be put on the RF4 site hosted by GUE.

 

[Seameadow]

It has already been posted.

Hazards In Rebreather Diving.

Hazards In Rebreather Diving

gue.tv

 

[Capt Jim Wyatt]

Do you think it is possible to cite a reference showing the data is useful? Anecdotally, I have not found it to be remotely accurate in predicting tox.

I would think cases where pulmonary O2 toxicity has been diagnosed and treated would suffice. Anecdotally I have heard of divers getting lung/respiratory inflammation, difficulty breathing, burning sensation etc. after being exposed to higher PO2s for longer times. I have no hard data or actual cases to support this.

Just look at it with your eyes.

It is evident that 100% isn’t 100% of anything. If it was, you me and all our friends would be toast because we’ve all passed 100% many many times.

@PfcAJ -- We relied on that stuff while doing dive ops in the Navy - we have relied on it for 3+ decades in the sport diving world & now to claim it is bogus science leaves me flat-footed in terms of a response with any validity other than what my reply is to @grantctobin above.

 

[PfcAJ]

I would think cases where pulmonary O2 toxicity has been diagnosed and treated would suffice. Anecdotally I have heard of divers getting lung/respiratory inflammation, difficulty breathing, burning sensation etc. after being exposed to higher PO2s for longer times. I have no hard data or actual cases to support this.

@PfcAJ -- We relied on that stuff while doing dive ops in the Navy - we have relied on it for 3+ decades in the sport diving world & now to claim it is bogus science leaves me flat-footed in terms of a response with any validity other than what my reply is to @grantctobin above.

“This is how we have always done it” does not make it valid.

 

[Capt Jim Wyatt]

“This is how we have always done it” does not make it valid.

That was not my claim. Empirical evidence to the contrary does not seem to exist.

I have a couple of sources I will ask about it. One is a hyperbaric doc who did his fellowship in hyperbaric medicine at Duke and the other is my son who is a saturation diver/supervisor in the oil fields in the Gulf of Mexico.

 

[Superlyte27]

If we take literally thousands of dives where CNS was over 1000 with zero effects by cave divers exploring the deepest longest caves in the world, when does that become empirical data instead of anecdotal? I personally have thousands of hours as a commercial diver where CNS was completely disregarded with no ill effects. Imagine diving a 200’ tall water towers on air for 2 hours. What is your CNS after 2 hours at a 1.6PPO2. And this was a working dive sucking out silt at the bottom of a water tower, so definitely not at rest. And not just me, but a whole company doing dives like this every day for decades. When does it quit being anecdotal? I’m serious. That’s not rhetorical.

I couldn’t tell you how many 6-10 hour dives I have well over 1000% CNS and then multiply that by every explorer in cave diving. Without any ill effects from oxygen toxicity. When can we call that empirical. And I know the history of 1.6PPO2 vs when it was 2.0PPO2. That’s not this discussion.

Honestly, the math is bogus. If you think 100% is really 100%, you haven’t been paying attention.

I don’t agree with Pfcaj often. But he’s so obviously right this time, it can’t be helped. The math needs to change. 100% is NOT 100%.

 

[stuartv]

Can one of you link me to a reference that says that the NOAA CNS tables/guidelines say they (to GREATLY simplify it) are "100%"?

 

[kierentec]

100% very rarely means 100% in diving (except gas supply ). Look at deco or scrubber duration, those limits are often shattered, but there needs to be some sort of guideline to follow and build your own experience on.

While the CNS clock table isn’t a great measure, oxygen tolerance is just way too variable to have any better guide. “Oxygen and the diver” by Kenneth Donald is probably the best resource we have, but it’s pretty difficult to get your hands on these days. I used to have a PDF copy, but lost it in a hard drive crash. The analys of thousands of trials showed results that were highly inconclusive. So all we can really do is try to be conservative based on the tools and experience we have.

So for newer divers, the CNS clock table and air break procedures are pretty conservative guidelines that are easy to use. For divers wishing to push beyond, it takes a lot of experience and knowledge to know which limits and under which conditions they can be pushed. For example, running high PO2 under high CO2 loads while cold is extremely dangerous even for short durations, but at rest while warm (especially while in a habitat) many find they can reliable and safely go well beyond the CNS clock. But we have to give those newer divers a baseline to start with. So CE ratings for scrubber duration, conservative gradient factors for deco, and the CNS clock and OTUs are the best tools we have for that.

 

[kierentec]

I want to add, when I say “newer divers”, I mean the vast majority of cave, technical, and CCR divers. We’re talking thousands of hours underwater, not hundreds, as divers with the experience to start pushing established limits.

 

[Capt Jim Wyatt]

OK my son who is a sat diver supervisor told me: The chambers on deck stay around .45 ppo2 and diver usually locks out for 5 hours at a .8 in order to account for this. (Bolded text are my words).

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My friend who is a hyperbaric Dr and a Peer of Pollock & Simon told me this in answer to my question "Are NOAA PPO2 tables wrong? I have a friend who says they are wrong"; His answer:

They aren't "wrong"....just hyper-conservative and allow for back-to-back table 6 treatments after reaching "100%" on a series of dives. The table had a different goal, which it achieves.

I don't use them to calculate O2 for a big dive...because I have a different goal and never have access to a deck chamber (there is always going to be a delay to treatment). As for Pulm Tox existing....it most definitely does exist and I have seen it several times and had it at least twice on exposures that were significant.

We can measure it in subjects using pulmonary function tests and the results are repeatable. On those who press forward and continue an exposure, we can even tell you what the cellular pathology will be at autopsy.

There were millions of Americans alone that died of it during COVID (Alpha wave) when we couldn't dial the FiO2 on the vents down. It takes 3-4 days at 100% at 1 ATA to kill a person... unfortunately... To quote a famous scientist: "Science doesn't care what you believe..."

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So my thinking is it depends upon what level of "risk" you are willing to take. That also depends on a lot of varying physiological factors. My 65+ year old lungs may not tolerate high PO2s as well as my 20 year old student. OTOH mine may tolerate it better.