CNS Toxicity.

[Finless]

After reading the Inspiration thread on this forum and in particular Dominic's post on callibrating O2 sensors and varying atmospheric pressure at sea level I got to thinking about how much impact that would have on PPO2 calculations for OC divers.

I assume that if atmos pressure at sea level is 0.9 bar then at 10 mtrs the pressure is 1.9 bar and so on?

Obviously you don't suffer CNS toxicity immediately you exceed 1.6 bar ppO2 BUT that got me wondering if there were any figures available that give any comparisons on times/depths/workloads etc before people succumb to CNS toxicity.

I know someone who got to over 3 bar ppO2 on RB training due to fauly O2 cells.

On ascent from a 36 mtr dive I had CNS 99% flashing on my computer (set to 1.4 bar ppO2 with 5% conservancy) all the way up on ascent. Previously used it set to 1.6 bar ppO2 all the time so I was not worried BUT I was extra careful about being aware of any symptoms + had a FFM so no lost reg for me IF anything went wrong.

Just curious to know really - has any chamber work been done to try and quantify this problem? I assume most real life situations end in unpleasant results so there are limited conclusions to be drawn.

I know there would be tremendous differences between people and their susceptibility (even from dive to dive) but some info may come in useful one day if it hits the fan.

[Hyperbaric]

<font color='#0000FF'>The following has some formulas derived from chamber experiments on pulmonary and CNS O2 toxicity and recovery.

Arieli R, Yalov A, Goldenshluger A
Modeling pulmonary and CNS O(2) toxicity and estimation of parameters for humans.
J Appl Physiol (United States), Jan 2002, 92(1) p248-56

[Finless]

Hyperbaric,

I did a search on &quot;Modeling pulmonary and CNS O(2) toxicity and estimation of parameters for humans.&quot; but received a 9 page file. It is on the correct topic so I am printing it out as I type.

Tks.
Bryan

EDIT :-

I definitely down loaded the correct pages. I have not done integral and/or differential calculus since A levels at school 25 years ago and I was not very good then, so I'm not sure how much I will be able follow.

It makes me feel really stupid sitting here staring blankly at the report.

[Mark Chase]

OK Having just planned three dives with over 100% CNS I can tell you a couple of things.

First the dive plan and the dive are not the same thing. Dive plan gave 100+ cns actual dive cleared deco at 87% day one 82% day two. That was two dives of 65+m with 40min bottom time and 90-100min deco on PP of up to 1.6.

So any one saying they are doing 125% profiles off of tables might not actualy be doing 125% profiles.

My VR3 constantly monitors PP02 at all variable presures throughought the dive rather than just taking the planned deepest profile and doing it on that. Thats where I got the comparison.

Most CNS incidents are wrong as switch events where people switch to 80 -100 % at 30m rather than to travel gas. Unless you are into big profiles with high 02 off gassing it is not going to be an issue. I supose a 40% dive on nitrox to 30M fro 1.0 will show up a few problems but there are not many people planning that sort of profile.

How common is a CNS hit??

One member of my dive club toxed due to an incorect gas switch and bit the end of his toung off when he convulsed. His buddy sent him to the surface missing loads of stops but his PP02 was so high he wasn't bent??

He was back in the water two weeks later ??

Freind of a one of my Dive contacts was the chap who toxed sank and died earlier this year.

Thats two in the last twelve months so it dose happen. There are no set figures for who will and who wont surfice as to say that wen reaching limits of 100% CNS you should not be exerting your self in any way and should have some contingancy. IE a necklesed rig that straps the reg into your mouth, a full face mask, Clipping your self to a lift bag or suitable SMB and SUPPORT DIVERS

ATB

Mark Chase

[Finless]

Mark,

Tks for the reply.

Originally Posted by [b

Quote[/b] ]First the dive plan and the dive are not the same thing....My VR3 constantly monitors PP02 at all variable presures throughought the dive rather than just taking the planned deepest profile and doing it on that. Thats where I got the comparison.

My dive computer also works on the same principal and I knew before the dive I would probably get to this position. From previous dives working on ppO2 of 1.6 I knew I would be &quot;no worse off&quot; that previous dive profiles - just that the computer wouldn't like it being set to 1.4

Originally Posted by [b

Quote[/b] ]Most CNS incidents are wrong as switch events where people switch to 80 -100 % at 30m rather than to travel gas. Unless you are into big profiles with high 02 off gassing it is not going to be an issue. I supose a 40% dive on nitrox to 30M fro 1.0 will show up a few problems but there are not many people planning that sort of profile.

I only dive with two mixes and provided I start on the right mix I am not going to suffer this problem (one for the future though).

Originally Posted by [b

Quote[/b] ]How common is a CNS hit??

In a round about way that was what I was trying to find out.

Originally Posted by [b

Quote[/b] ]Freind of a one of my Dive contacts was the chap who toxed sank and died earlier this year.

Sorry to hear it.

Originally Posted by [b

Quote[/b] ]Thats two in the last twelve months so it dose happen. There are no set figures for who will and who wont surfice as to say that wen reaching limits of 100% CNS you should not be exerting your self in any way and should have some contingancy. IE a necklesed rig that straps the reg into your mouth, a full face mask, Clipping your self to a lift bag or suitable SMB and SUPPORT DIVERS

I dive with a FFM anyway.

I guess the question I wanted to ask, and to which there is probably no answer, is that based on current information is there a minimum threshold that we could ALL say we are safe (probably) up to this point BUT after that you are in the convulsion zone.

Rgds
Bryan

[Spike Jackson]

I think the best you can say is that CNS tox is highly unpredictable. The way I understand it is that there is not only massive variation in individual susceptibility but also within the same individual on different occasions. When you factor other things in like oxidants and free radicals derived from food and drink as well as drugs it becomes a scary concept.
Bearing in mind that for an unsupported diver with a normal mask (ie me) the consequences are likely to be lethal I like to use the tried and trusted CNS clock system advocated by virtually all training agencies. &nbsp;I know the WKPP are doing dives that fly in the face of these systems but the CNS clock technique is one I understand and can use and one that has data accessible to me.

[Mark Chase]

Totaly agree with Spike on this one. The agencys say do not pass 100% CNS and recommend air breaks after hitting 80% CNS as a precaution.

I would compare it to a solo dive. I wont do heavy wreck penitration on a solo dive as there is no one to pull me out of the wreckage or get help if I get seriously stuck.

I wont pass 100% CNS without adiquate support from trained safety / suport divers.

I wont pass 80% with out incorperating air breaks and preporation in the form of avoidance of CNS accelorators, like Orange, Coffie, Cigeretts, etc.

You can do some serious diving without passing 100% so why bother with the risk.

Mark Chase

[Finless]

Originally Posted by [b

Quote[/b] (Mark Chase @ Aug. 21 2003,15:11)]You can do some serious diving without passing 100% so why bother with the risk.

Mark Chase

Mark,

I totally agree and I NEVER plan to go to that high an O2 figure. The planned wreck was changed at the last moment and was a little deeper. I spent too long on the floor looking round for unbroken pots. Truth be told the CNS clock sneaked up on me - I was checking my guages and bottom time regularly but did not pay attn to the CNS sitution until the computer started bleeping warnings at me.

I was already &quot;bagged off&quot; from the top of the wreck and making sure I had not left anything behind as the CNS clock hit the 80% mark - but up to 99% by the surface (on a 80% deco mix). No opportunity for an air break as I did not have any.

At least, that is how I remember it. I have not downloaded the log file yet so it will be interesting to see what happened when and where.

I just made sure I was clipped to the reel during deco stops and ascent over last 10 mtrs when on the 80% and was smugly happy I had a FFM.

[Spike Jackson]

A full face mask will stop you spitting your reg out but couldn't it flood during a convulsion? Also a nightmare scenario could have you unconscious and floating downwards, ffm still in place! Not a wind-up, just an observation.

[Finless]

Originally Posted by [b

Quote[/b] (spike jackson @ Aug. 21 2003,18:30)]A full face mask will stop you spitting your reg out but couldn't it flood during a convulsion? Also a nightmare scenario could have you unconscious and floating downwards, ffm still in place! Not a wind-up, just an observation.

No, it don't think it will flood whilst there is gas in the tank - lift the mask skirt up and the reg would free flow and any water that gets in is vented through the exhaust valve. Also, the mask is held in place by 5 straps and the mask skirt is very forgiving with regards to to movement (mask or face).

The floating down bit is a worry. If I get into a position where an O2 hit is possible I clip my reel to me so I would dangle on the end of my DSMB (just hope I don't get towed along by a pssing boat).