CNS toxicity symptom occurrence...or not

[Dr. Lecter]

I can point out that my reaction to the sensation was driven by a combination of my training with regards to nitrox and potential CNS symptoms and the fact that I'd just spent an extended period of time decompressing on 100% O2 at or near 1.6 pO2 -- or that I've tweaked my preferred deco conservatism level away from those that leave me feeling anything but good even after the exercise of reboarding with gear, just like everyone else does -- until I'm blue in the face. Mindlessly dogmatic repetition of a screed like 'dive this number at this depth or else, because I said so, and anything that ever goes wrong after you violate that rule will be because of that' is best left in church.

 

[zaberman1]

This may be an incredibly unhelpful post but several years ago (i do not recall the dive profile or back & deco gas composition except that the max depth was 175') but I know I started 100% at 20ft and after about 2 min, began feeling extremely nauseous, not thinking clearly, and my vision began to tunnel. I had the sense to know something was seriously wrong and switched to my backgas. The feeling of relief was pretty instantaneous....not sure if it was CNS toxicity but I now only max out at 1.5 to be slightly more conservative...That said, I do recall that this was a particularly challenging dive and I exerted myself more than I should have (long story but it was to take control of someone else's potentially bad situation).

Did you notice a difference when you switched to your bg?

 

[Dr. Lecter]

This may be an incredibly unhelpful post but several years ago (i do not recall the dive profile or back & deco gas composition except that the max depth was 175') but I know I started 100% at 20ft and after about 2 min, began feeling extremely nauseous, not thinking clearly, and my vision began to tunnel. I had the sense to know something was seriously wrong and switched to my backgas. The feeling of relief was pretty instantaneous....not sure if it was CNS toxicity but I now only max out at 1.5 to be slightly more conservative...That said, I do recall that this was a particularly challenging dive and I exerted myself more than I should have (long story but it was to take control of someone else's potentially bad situation).

Did you notice a difference when you switched to your bg?

No and I expected that I would have if it was in fact a CNS related symptom; at the time, though, no other explanation occured to me -- I lack DD's profound depth of knowledge regarding the range of sensations that can be provoked by marine life stings, which I assume he psychically 'imprints' on each of his students, as they are apparently incapable of making such a rookie mistake -- so I remained wary. Given that the feeling persisted for some time after I finished the dive, and a relatively low 85% CNS, I assume it must have been a sting even though I did not see anything in the water around my face when it occured and there weren't any apparent sting marks on my face.

 

[DevonDiver]

The thread was placed in A&I/Near-Misses & Lessons Learned forum. The purpose of the forum is that " we all may learn from little mistakes that could ultimately be costly".

A purely academic discussion on CNS% calculation would be better placed somewhere like; 'T2T' (open and frank discussions among tech diving peers on more advanced topics) or 'Marine Science and Physiology' (all medical questions related to decompression or diving).

So.. lessons that "we all may learn from little mistakes that could ultimately be costly":

1) Obtain the correct training and experience to undertake dives of a more complex or hazardous nature. This would be a leading 'factor' in the 'incident' by any study... and remains a very relevant 'learning point'.

2) Remain within industry/agency-standard guidelines on OTUs, maxPO2 and (not or) CNS% when planning/conducting dives using nitrox mixtures for bottom or deco purposes. OP opted to exceed the maxPO2 for his bottom mix and this led him to nearly misdiagnose a simple marine-life irritation; and could be seen as the first rung on an incident chain that nearly led him to omit decompression.

3) Seek a robust understanding of the signs and symptoms of oxygen toxicity (and decompression illness), so that you don't risk misdiagnosing other factors, due to an overly-simplistic or generic interpretation of standard signs/symptom abbreviations. Whilst I think that point #2 had a direct relevance to the initial misdiagnosis by the near-victim, the need for more complete understanding proves beneficial - especially when a diver may face a situation where they believe they are faced with choosing/selecting a 'lesser of two evils' (risk OxTox or omit deco/bail-out), either of which has the capacity to seriously damage them.

Readers who are not technical qualified and experienced divers should accept that there are significant differences in approach (including mindset) when comparing technical with recreational-level diving. I mean nothing elitist or snobbish in that, it is simply a statement of fact. It is why Scubaboard and other forums typically isolate technical diving threads away from generic/recreational board discussions.

This isn't a recreational nitrox diving incident, whereby immediate access to the surface was an option for the OP. Fudging dive planning/computer algorithms to permit early access to the surface at the expense of ideal decompression isn't a prudent strategy, nor one taught to divers in any formal course, nor recommended by any credible technical diver that I know of.

You can fudge numbers, but you can't fudge what happens in your body when off-gassing. ANY decision to omit/shorten decompression below what was planned and selected as the ideal for that diver, on that dive, is a SERIOUS issue for technical divers. It may seem acceptable from a recreational diving-only perspective - but that merely illustrates the difference in mindset/perspective to which I have alluded.

My concern is only to bring valid learning and safety points from the OP's incident. This may damage the OP's ego, but it is the purpose of this particular forum. Inquiring about training levels isn't an insult - it is directly relevant to any analysis.

Mindlessly dogmatic repetition of a screed like 'dive this number at this depth or else, because I said so, and anything that ever goes wrong after you violate that rule will be because of that' is best left in church.

If you feel that issues like exceeding community-standard PO2 or omitting planned deco are not useful learning outcomes from your incident, then you have over-intellectualized what was, in reality, quite a serious incident. I class any risk of omitting planned, ideal decompression as a serious incident.

I see a fundamental flaw with happily skipping/reducing decompression 'because of concerns over OxTox', when the same diver has made the deliberate decision to exceed all community and industry accepted limits on PO2 max. Such a flaw, I would predict, stems from insufficient training and education. That's a valid learning point for this forum area.

If/when you seek proper training to conduct these dives, you will learn the difference between 'mindless dogma', common-sense and best practice. Diving safely using nitrox isn't some hard-core 'DIR-dogma'. There is a flaw in spending to much time on the forums seeking hypotheticals and opinions that merely enable you to justify unsafe diving... and not enough time actually learning from credible technical divers who will tell you the blunt truth.

Please don't get yourself hurt; it can happen to you. The internet is a safe place and it's easy to act confident on a keyboard, but in the water.. OxTox or DCI is not a 'virtual' risk.

 

[Mr. Black]

All this p*ssing contest aside a valid point has been made here:

This is the "Near misses" forum which is open to anyone of any level of training. Most posts in this sub-forum are in the context of recreational diving and it would be undesirable to give people the impression that planning a dive with a working PPO2 of 1.66 is acceptable practice.

Which agency teaches a working PPO2 of > 1.5 as being an acceptable plan for any dive ? I don't know of any.

Unless the mission is worth risking my life for I do not think there's many people out there who would support a planned working PPO2 of 1.66.

Letting this point stand unchallenged would risk some newbie reading this and thinking "Hmm so when I am a bit more experienced I can just dive 23% to 190 feet and probably be OK".

As in aviation, we have SOPs in place here to tweak the odds of catastrophe in our favor. Yes, there's a lot of conservatism built in. But no, we do not want to advocate breaking SOPs. Just like I would not want an airline pilot to skip some of his pre-takeoff checklists because he is a 10000 hour veteran and didn't have problems the last few times he done it.

Who trained the OP to plan his dive with such a high PPO2 ?

If this PPO2 was outside the scope of the training, why does the OP have a problem with someone pointing out that the first problem in the chain of events would have been the incorrect PPO2 ?

 

[Dr. Lecter]

If this PPO2 was outside the scope of the training, why does the OP have a problem with someone pointing out that the first problem in the chain of events would have been the incorrect PPO2 ?

Simply put, because given the same context (~17 minutes into 100% oxygen deco at 20' and 10' stops) I'd have reacted the same way to the sensation/potential symptom regardless of whether I'd used 1.4 or 1.7 as my max bottom pO2. I went above a pO2 that is now commonly - though not universally - regarded as acceptable for bottom gas that was the root cause of my reaction; all I can say is that I know that not to be the case.

I've found some value in the critique of my assessment of the sensation as a potential symptom -- though the exchange's utility is somewhat hampered by my limited ability to accurately convey the nature of the sensation, and by the lack of explanation of why it falls outside the twitching/tingling category of potential symptoms.

I have no problem with anyone pointing out that 1.66 is no longer a generally accepted pO2 for bottom mix, though I'm aware of at least one agency conducting technical training at a bottom pO2 above 1.7. Indeed, the trend seems to be towards 1.2 as the ideal maximum for more extreme exposures, as DDM and I think various GUE sources have stated. But the idea that 1.4 working/1.6 deco is "safe" is as incorrect as the idea that 1.7 is "unsafe"; if this was a case of absolutes, there'd never have been cases of divers within established limits toxing and nobody who dove air past 187' would ever have returned to tell the tale. Rather than safe/unsafe, the issue is one of what the diver deems acceptable -- hopefully based on as complete an understanding of the risk and mitigation possibilities as present understanding of O2tox allows.

 

[Quero]

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A ScubaBoard Staff Message...

The antagonistic engagement amongst some of the participants of this thread threatened to undermine a valuable discussion. Therefore, edits have been made to a significant number of posts to remove caustic remarks.

Please be reminded that the purpose of this forum is to learn from near misses. As such, it has been specifically designated as a no-flaming zone. Overly harsh critique is prohibited, and further display of such will draw sanctions to any member who violates this standard.

Furthermore, in the spirit of learning, it is expected that the near-miss-victim will approach the discussion with a willingness to accept constructive criticism, to learn from it, and to allow his/her experience to stand as a cautionary tale for other divers. Lack of humility and defensiveness are counter-productive to this goal.

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[Abdullah]

Doppler, with regards to CNS: the prior day's dive ran up 17% (172' max depth, but following a steep bottom contour and less than 10 minutes on O2), and concluded 24 hours and 6 minutes before the start of the dive when this happened. It was the dive in question that ran up 85% CNS.

Can you post your dive profile from the Shearwater Desktop?

 

[kwinter]

I guess what surprises me most is the absence of any discussion regarding the OP's choice of nitrox instead of trimix for his back gas. Independent of the discussions of pO2 is the issue of gas density. Not to mention the clarity of mind that would be necessary at 7 ATA if a problem developed. But work of breathing is not just an issue for rebreather divers. There is interplay among the issues of gas density, WOB, CO2 retention, and OxTox. While already flirting with extremely high pO2, I see no reason to push the envelope further with a high density, highly narcotic gas choice.

And what about the choice to switch from 100% at 20 feet to 23% at 10 feet when he suspected a problem? Why not the 50% he was carrying that would still be only 0.65 but not prolong his omitted deco that much? Or the choice of 100% in the first place instead of 80% or so after a dive with such a high bottom pO2?

I applaud the OP for being brave enough to post honestly and openly about this. My first impression was that this was a troll post. I still believe that to be likely. If not, then Darwin will have his way.

 

[Dr. Lecter]

Can you post your dive profile from the Shearwater Desktop?

First picture is from the earlier day's dive, second is the dive in question.