CNS %

[PfcAJ]

The clock doesn’t mean anything definitive and it doesn’t give you any useful knowledge.

It’s possible to tox below 100% and (clearly) hitting 100% doesn’t mean you will tox. 1000% doesn’t mean you’ll tox either. There’s no useful information to be gained from it.

Dive a low po2 on th bottom (1.2 or less), limit your po2 to 1.6 when resting on deco, and take breaks to a low po2 gas during your oxygen stop.

 

[Trace Malinowski]

Beau, what AJ said pretty much sums it up. Here is a related study concerning special forces divers I thought you might find interesting since it's written in your language: Oxygen Toxicity and Special Operations Forces Diving: Hidden and Dangerous

 

[Beau640]

It’s a really interesting paper. They say we really don’t have definitive information on where the cut off will be for time to cause toxicity. however they do repeatedly state that time is a major factor and a big concern for risk of convulsion. It mentions we have poor human studies and some animal studies that cant necessarily be extrapolated but that’s about it. They propose a formula to try to calculate risk and in a way they are using their own version of a CNS Clock but it’s different than the one on our computers.

So as mentioned above by multiple people, the CNS clock we use on our computers really might not be accurate in telling us the risk of short term complications like convulsions. However I’m concerned we are not considering the long term effects oxygen can have with repeated high exposure. in the last five years, paper after paper has come out showing how Unneeded oxygen exposure in our patients will actually increase mortality. Oxygen has a lot of negative effects including CNS, pulmonary, and other organ system effects which are actually described by the paper trace linked above.

Just like high blood pressure will not kill you today but will kill you 20 years from now, these repeated exposures of high partial pressures of oxygen over long periods of time could be causing CNS and Pulmonary toxicity that you won’t know about until 20 years from now. Maybe the clock is a better indicator of acceptable exposure over the long term as having repeated dives with a clock over 100% could be putting you at unacceptable risk of complications down the road. Yeah we think it’s garbage right now because dive after dive right now we are not having any immediate issues. That doesn’t mean we won’t end up in the nursing home 20 years from now with early onset dementia and pulmonary fibrosis from these dives. It’s a fact that oxygen causes these things it’s just a matter of how much exposure over how much time will lead to it in a certain individual

 

[PfcAJ]

It’s a really interesting paper. They say we really don’t have definitive information on where the cut off will be for time to cause toxicity. however they do repeatedly state that time is a major factor and a big concern for risk of convulsion. It mentions we have poor human studies and some animal studies that cant necessarily be extrapolated but that’s about it. They propose a formula to try to calculate risk and in a way they are using their own version of a CNS Clock but it’s different than the one on our computers.

So as mentioned above by multiple people, the CNS clock we use on our computers really might not be accurate in telling us the risk of short term complications like convulsions. However I’m concerned we are not considering the long term effects oxygen can have with repeated high exposure. in the last five years, paper after paper has come out showing how Unneeded oxygen exposure in our patients will actually increase mortality. Oxygen has a lot of negative effects including CNS, pulmonary, and other organ system effects which are actually described by the paper trace linked above.

Just like high blood pressure will not kill you today but will kill you 20 years from now, these repeated exposures of high partial pressures of oxygen over long periods of time could be causing CNS and Pulmonary toxicity that you won’t know about until 20 years from now. Maybe the clock is a better indicator of acceptable exposure over the long term as having repeated dives with a clock over 100% could be putting you at unacceptable risk of complications down the road. Yeah we think it’s garbage right now because dive after dive right now we are not having any immediate issues. That doesn’t mean we won’t end up in the nursing home 20 years from now with early onset dementia and pulmonary fibrosis from these dives. It’s a fact that oxygen causes these things it’s just a matter of how much exposure over how much time will lead to it in a certain individual

I suppose that's *possible* for scuba related hyperbaric oxygen exposure to result in some sort of measurable long term issue at some point (there's no data that I'm aware of to support that), the CNS clock does not attempt to prevent that from happening. Its purpose is to prevent CNS toxicity symptoms, ConVENTID, for scuba divers. That's it and that's all.

OTUs DO try and quantify "whole body" oxygen toxicity and they are tracked in hospital settings. For nearly all divers, the durations and po2s are low enough that OTUs are a total non-issue.

 

[rjack321]

I think the highest CNS %age I’ve hit was right above 200% - not very high compared to some of my friends/aquaintences. On a few of the longer open circuit dives, I’ve have the experience where I have suddenly gotten incredibly irritated / irritable on deco. In those cases, I immediately switched to a low O2 mix for a decent period of time. I can’t know for sure, but I would not be surprised if I was starting to experience CNS toxicity in those instances and managed to hold it off. CNS toxicity is definite something about which I am concerned, but I definitely started pushing my exposure up very gradually over time. I think that is the best way to understand how ones body deals with all of the different types of stress coming from extended decompression diving - to gradually build up exposure.

Ruh oh, what about those of us that are irritable all the time?!?

I've hit 140-145% but my dives aren't that long. I get pretty snotty after 5 hours so much longer is going to be pretty gross, hurt my ears, and probably have some pulmonary damage as well. I don't really like it, but shrugs, its not really possible to keep CNS <80%.

 

[kensuf]

Ruh oh, what about those of us that are irritable all the time?!?

I've hit 140-145% but my dives aren't that long. I get pretty snotty after 5 hours so much longer is going to be pretty gross, hurt my ears, and probably have some pulmonary damage as well. I don't really like it, but shrugs, its not really possible to keep CNS <80%.

I typically do my 50' stop on OC to dry out my sinus cavities. It makes a huge difference to me.

 

[Dr Simon Mitchell]

Hi Everyone.

I agree in general terms with the posts (eg AJ and Tom) inferring that the CNS clock limits have little predictive value. They were a conservative guide produced by Chris Lambertson based on little more than synthesis of experiences he had. They are certainly not based on a database of dives of known outcome.

For many of the dives undertaken by modern technical divers it is a simple matter of necessity that these limits are exceeded, often by multiples of 100! As others have pointed out the rarity of seizures despite this has caused serious technical divers to largely ignore the limits. There are just a couple of caveats I would throw in. Part of the reason we seem to be able to tolerate high cumulative oxygen exposures is that they typically peak during resting shallow deco when the risk of CO2 retention is very low. We proved the latter in a field study at Bikini Atoll in 2015. CO2 retention markedly increases the risk of oxygen toxicity so you would not want to be swimming around deep breathing dense gas etc when your O2 exposure % is very high. The other point is that risk is very sensitive to the inspired PO2, and so I would counsel against breathing high-ish PO2s when your exposure has been very long. For example, I generally keep my inspired PO2 1.3 or less during long decompressions. I don't do 100% oxygen at 6m (inspired PO2 1.6) on dives like that.

There is a great article by Dick Vann and Bill Hamilton in the DAN Technical Diving Workshop Proceedings that give some excellent perspective on this subject.

Simon M

 

[Beau640]

Hi Everyone.

I agree in general terms with the posts (eg AJ and Tom) inferring that the CNS clock limits have little predictive value. They were a conservative guide produced by Chris Lambertson based on little more than synthesis of experiences he had. They are certainly not based on a database of dives of known outcome.

Simon M

Thank you for your reply. This was the exact information I was looking for. I’m still telling my friend they are crazy though.

 

[lv2dive]

I’ve gotten plent irritable on other occasions during long hangs, the key word I used to describe the situations I was referencing was “sudden”- as in, out of the blue, after being in a relatively normal mental place, to *suddenly* crawling out of my skin irritable / wanting to get out of the water right.this.minute.

Thanks for pointing that out- yes, general irritability is very different from what I was referencing.

Ruh oh, what about those of us that are irritable all the time?!?

I've hit 140-145% but my dives aren't that long. I get pretty snotty after 5 hours so much longer is going to be pretty gross, hurt my ears, and probably have some pulmonary damage as well. I don't really like it, but shrugs, its not really possible to keep CNS <80%.

[

 

[Trace Malinowski]

That's exactly how I felt when Kazbor's shut down.

So, too much oxygen and too little hot wings produces the same effect.