Air Breaks and CNS

[Doppler]

Steve, I've read your article before, and it always leaves me with the same question; If gas breaks don't affect the CNS o2 'clock', how can we explain people going WELL over the 'limit' time and time again by employing gas break strategies? I'd be long gone if 100% (heck, even 300%) equaled a toxic event.

If the model doesn't match reality, then the model is wrong

I do agree that a component of the gas break is to maintain deco efficiency, which is why I don't count my off-oxygen time as additional deco time.

The one thing I think we all can agree on is that the entire thing is poorly understood and deserves a lot of respect.

Certainly agree with you on that final sentence.


In answer to the first few: I really don't know, but logic causes me to ask the question: HOW DOES THAT WORK... because it is illogical. I understand the practice during a Table six (as mentioned in the article's preamble), but that would not seem to translate for recreational exposures.

And to provide a possible answer your 100 percent and over scenarios. The CNS clock is not a concept that Bill Hamilton embraced; so we are on our own with that one... no science just a wing and a prayer.

As I do more and more of this stuff, and learn how little we can be sure of, I am being to think that the NOAA 24-hour limits are a better gauge of what limits to follow than the single dive limits (which many of us have transgressed during long decos... time and time again). Perhaps it will remain unresearched and a mystery. Who knows?

However, the logic remains that CNS loading is time sensitive; there is no well-understood "decay" or half-time for CNS loading (at least Hamilton refuted the notion explaining that if this were the case, the 24-hour CNS limits would be unnecessary... and you and I know many people who have died on dives on which CNS loading was well within single exposure limits... but not so daily limits). An air break or a break on EAN50 or (my preferred) 50/50 heliox, deliver the same results... effective off-gassing without any further padding. I have also executed long decompressions during which no air-breaks were taken... I did not tox, but I was bubbling a level 3 (unusual for me under any conditions) an hour or so post dive. The day before, similar dive, same gases but with 50/50 heliox breaks, the reading was one.

So who knows?

I am not trying to argue with tenets held by anyone; but I am offering up an alternative explanation and continue to ask the questions: How and Why?

 

[PfcAJ]

I dont have to understand how it works to understand that it does work In my mind, getting on the lowest possible ppo2 during the break makes the most sense.

But yes, all the science out there is severely lacking. The limits are kind of made up, the data are all over the place, and its mostly a wing and a prayer!

 

[Doppler]

I dont have to understand how it works to understand that it does work

And I guess that's the real difference between us. If I don't understand what makes it work, I want to take the bloody thing apart and "find the man behind the curtain".

 

[Bobby]

First I want to be clear that I am in no way suggesting to anyone that they should copy or use how I do things. What I do has no math our science behind it and will most likely get you hurt or dead.

Now that we have that out of the way I agree with Steve and others that it is the 24 hour total exposure that is most important. I have run 4+ hours of decompression without an air break, though I don't recommend it. I have also found that lowering PO2 on the working part of the dive and having regular low PO2 breaks during the decompression helps considerably with off gassing.

I have also found that repetitive high CNS dives compound the issue. In the past I ran 24 hours between long exposures (for me 1+ or more hours of decompression) Now I have pushed that out to 48 hours between them. I find much less lung irritation and that I clean out my off gassing more efficiently. CCR is a good tool for gauging off gassing.

 

[kotik]

Judging by accounts I've heard of CNS (not pulmonary) events, my impression is that we understand the stuff about as well as we understand the weather. We know some of the mechanisms at work, but there are too many significant variables involved for us to make a very reliable forecast without sticking in a wide margin of error. The limits have gotten progressively more conservative over time because every so often the statistical nature of our practices continues to reach out and grab someone.

That being said, my own imagination would say there is probably some kind of 'stable' PO2 far as the CNS (again, not pulmonary) clock is concerned- so that a person at that PO2 is neither adding to the clock or subtracting. Go over this number, and they would eventually experience CNS toxicity. Dip under this number, and they would get farther from CNS toxicity-- that is, if they spent some time below the stable point, and then went above it, then it would take longer to tox than if they went directly from the stable point.

If this stable point really exists, then I'd doubt it would be .21, nature just doesn't seem to work that way. Since I'm imagining, my hunch is that air breaks at 20' probably wouldn't reset the clock, but I think they would set it back some. Who knows how much...

Just two extremely overvalued cents from someone very much a lay person.

Edit-- I was going somewhere with that, lost my train of thought...

To the OP: I found this paper by Eric Baker, who says:

There is some limited data in the academic literature which indicates that the practice of intermittent oxygen breathing (i.e. 20 minutes on O2, then 5 minute air break) can extend the limits of pulmonary oxygen tolerance by a factor of more than 2. Presumably, the CNS oxygen limits are extended by some factor as well. The problem is that, to my knowledge, a reliable method has not yet been developed to calculate the rate of recovery from oxygen exposure during these periods of intermittent oxygen breathing.

 

[Superlyte27]

Just to stoke the fire...

Air Breaks? what are they, and do people take them for the wrong reason? | Doppler's Tech Diving Blog

LOL, thank you.
People forget that CNS has a per dive clock and a per day clock. A 5 minute break doesn't do jack for someone with 2 hours at 20' on pure O2. They are so over their clock no matter how you slice it.

What I do know is this...
If I got extended times on pure O2 without a break, my lungs hurt.
If I take breaks, they don't hurt.
Being a commercial diver, I've seen some jacked up deco both in water and in chamber where the 1.6 for 45minutes and 2.8 for 60 minutes were completely dismissed without ill effects. One thing is for sure, we don't have it all figured out.

 

[Doppler]

First I want to be clear that I am in no way suggesting to anyone that they should copy or use how I do things. What I do has no math our science behind it and will most likely get you hurt or dead.

Now that we have that out of the way I agree with Steve and others that it is the 24 hour total exposure that is most important. I have run 4+ hours of decompression without an air break, though I don't recommend it. I have also found that lowering PO2 on the working part of the dive and having regular low PO2 breaks during the decompression helps considerably with off gassing.

I have also found that repetitive high CNS dives compound the issue. In the past I ran 24 hours between long exposures (for me 1+ or more hours of decompression) Now I have pushed that out to 48 hours between them. I find much less lung irritation and that I clean out my off gassing more efficiently. CCR is a good tool for gauging off gassing.

Bobby: thanks for your input, mate. Hope all is well. Getting very good feedback on your lights by the way. Gonna have to take one for a spin sometime.

 

[Bobby]

Steve,
Thanks, give me a ring when you're in the area & we will get you some to try out.

Sent from my DROID RAZR using Tapatalk 2

 

[kotik]

I had an idea. It seems like just last week there was a post by a college student looking for a diving related experiment to run, and those type of posts come in every so often. It seems like it would be possible to get some idea of how air breaks influence this so-called clock with an experiment using lab rats or mice.

I'm thinking of something like keeping them in a small chamber with pressurized O2 (flowing, to keep CO2 from building up), and observe them for seizures. A range of pressures could be tested, anywhere from high enough to seize very quickly, to just high enough to reliably cause a seizure before they die from pulmonary toxicity. Once the range is established, experiments could be repeated within the range but with air breaks of varying lengths inserted. Multiple rats would be needed I'm sure, so this could also give data on individual variation in susceptibility to CNS.

It wouldn't give us any better understanding of why CNS occurs, and rat physiology probably doesn't directly translate to ours so that we'll know exactly what an air break schedule should be once the report comes out. But it seems like it might teach us some rough idea about the decay rate, if there is one.

Thoughts or opinions? Does it sound useful? I'm thinking about posting this in a thread of its own, maybe it could shape up with all the details, maybe someone could run it someday.

 

[PfcAJ]

I've read a few studies that are procedurally similar to the one you envision, but they used implanted electrodes for EEG. Cool stuff.