CNS toxicity symptom occurrence...or not

[Dr. Lecter]

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.

I don't see a discussion of your first paragraph leading anywhere other than the standard deep air back and forth, so I'll leave it be.

I'm not sure I understand the second point you raised. Aside from DDM's raising of "off phenomenon" here, I've never heard any suggestion that when CNS symptoms manifest one wants to do anything other than reduce pO2 as much as possible as quickly as possible. One could argue that getting out of the water sooner is better and thus the 50% would be a better choice while still achieving significant pO2 reduction; I opted for greater pO2 reduction.

 

[dorkito]

Dr Lecter, may I ask what was your objective for this dive? I was taught that you go deep only for a good reason. Seems to me this applies even more for a decompression dive. If we are looking for a chain, that may be the start.

 

[Dr. Lecter]

Dr Lecter, may I ask what was your objective for this dive? I was taught that you go deep only for a good reason. Seems to me this applies even more for a decompression dive. If we are looking for a chain, that may be the start.

The objective was the same as any other open water reef dive: to explore and enjoy the environment and fauna found on the ledge in question. I don't think hitting 200' was any kind of a personal depth record for anyone in the group, nor was it the point of the dive. A couple of the divers stayed around the top of the ledge, while I and a rebreather diver conducted a more multilevel profile along the ledge/into its pukas.

 

[AggieDiver]

Dr. Lecter, I am not a tech diver, but I think I see the point that several posters have made regarding your initial choice of gas being part of the issue here. I am not coming at it from the standpoint of saying that it was the wrong choice or right choice to pick that mix for the dive. Instead, I am thinking that YOU likely felt some level of concern with the ppO2 of your gas choices that led you to respond the way you did to what was probably a thimble jelly sting based on your description of what you saw in the water.

If you can stand a little story telling, I think I can maybe explain what I mean. Many years ago, my wife and I were renting a house down the street from a guy who was habitually drunk and who apparently liked to beat his wife. His wife called the police on him one late afternoon and with all the lights and commotion outside, my wife and one of the neighbors happened to be standing out on the sidewalk watching as the cops walked the guy out of the house in cuffs and stuck him in the squad car. As the cops drove away, he was staring out the window of the car at my wife and the neighbor with a scowl on his face. My first thought at that moment was "great...when he gets out and comes home, next time he gets drunk, he is going to have something to be upset and irrational about instead of his wife".

I didn't think much about that for the rest of the evening. Around midnight, I was sitting on the couch watching latenight tv and heard a tremendous crash in the kitchen, and the immediate image that popped into my head was that it was the drunk from down the street, out of jail and trying to come into the house through the kitchen window. I jumped up and ran to the bedroom to get a gun, while yelling at my wife to call 911. When I went back around the corner from the bedroom, I could see into the kitchen and saw that the source of the noise was a potted plant that our cat had been chewing on and had pulled off the windowsill and into the sink on top of some plates and glasses. I managed to slow my heart rate down and stopped my wife before she actually got 911 dialed. I am not sure who was breathing harder, my wife, the cat, or me...but we can all laugh about it now looking back.

The reason I recount that story here is this...the immediate leap of my mind to think that the sound came from the guy coming through the window was completely irrational for several dozen reasons. The window there was small and high off the ground, and would not be the window of choice for anybody trying to break into the back side of the house. There is no way that guy got out of jail that quick, and there is very little reason why he would have tried to come into the house that way if he had. I could go on and on...but the point is that because of a little pre-thinking about a future hazard, my mind when it panicked, skipped past several rational explanations for the sound I heard (cat being high on that list) and immediately settled on an irrational explanation instead. I reacted based on that irrational perception and went from there.

In your case, you mentioned having seen a number of jellyfish in the water during the dive. In most cases, if you felt a tingling/stinging sensation during a dive when you had been seeing jellyfish, you would immediately think "marine sting"...not Ox Tox. I believe it was only that you had selected a gas mix that put you beyond the widely recommended ppO2 for your dive that caused your mind to immediately assume Ox Tox when you felt the sting. As a result, you considered ommitting planned deco and deviated from your dive plan. I am not trying to say that those actions were right or wrong, or prudent or not. But I think even if you won't admit it to yourself, your subconscious mind knew that you were doing something dangerous by choosing to dive that gas mix, and in the moment of decision, it caused you to skip the obvious marine sting possibility and jump to the conclusion that you might be suffering from Ox Tox instead. If you had assumed marine sting underwater, but then considered the OxTox and deviated your plan accordingly, I could see it being a rational series of events. But to skip past the obvious explanation to one that was much less likely (albeit signficantly more dangerous) suggests to me that your initial choices on the dive altered the way you reacted to an unexpected event. It turned out ok for you in the end, but I think it is wrong to dismiss the role of the initial gas selection in affecting how you reacted later on down the road.

 

[uncfnp]

Aggiediver, I see his response a little differently. Using your scenario, what if you had been right. What if an unreasoning drunk had broke out your window? What if unsuccessful, he then looked for an alternate entry while you calmly rationalized that it was probably just the cat at play in the kitchen? After all, you had just seen him in there. How would you then feel if you had not taken the precautions that you did? Is it better to assume the worse case scenario and act accordingly until proven otherwise or just hope for the best?

 

[AggieDiver]

The point I was making was not that you should not react and blow off possibilities that could kill you. I am saying that if you have subconscious worries that predispose your mind to skip the "consideration of obvious possibilities" step of reacting to an unexpected event and go straight to reacting to the the worst possible scenario, you should listen more carefully to those worries before you are in the middle of a dive. In my case, had I rationally considered it, I would have poked my head around the corner first to see what caused the noise instead of racing into the bedroom screaming "call 911" and grabbing a gun. If it had been a drunk crawling through the window, I could have then reacted accordingly. I didn't have enough information to know how to react to the sound...the rational step would have been to gather more info before reacting by looking around the corner...I acted irrationally instead.

As divers, we should all strive to react to any incident with rational decision making. If your mind is skipping consideration of obvious possible causes, you are not thinking rationally in deciding how to react. Like I said, I am not saying his subsequent actions were irrational...they may have in fact been quite prudent for avoiding or mitigating the possibility of CNS toxicity. But the fact that he didn't even consider the obvious likely cause until he was out of the water suggests that something affected his thinking, and I think it was likely to be subconscious worry over the dive profile and gas mix selected for it. What if the sting had happened while he was still at 150+ feet in that frame of mind? If he had convinced himself he was suffering from CNS toxicity at 170 feet and thought seizures or convulsions might be imminent, would he have done something dangerous to try to mitigate that?

 

[uncfnp]

So your position is that if he had not been pushing the, in your opinion, PPO2 limit (measured by his max depth and not the more correct measure of time and partial pressure - as I understand he was within that limit) that the possibility of Oxtox would not have occurred to him? Only Dr Lecter can answer that. But I find it interesting that many divers will post threads of possible DCS symptoms when their dives are well within NDL limits. Why does this possibility occur to them when, at least theoretically, they should not be at risk?

 

[AggieDiver]

My position is that he was subconsciously aware that he was on the edge regarding his PPO2 due to his mix and profile. Because of that, when the incident occurred, his mind was preoccupied with that as a possibility. If he had been diving a more conservative gas/profile combination, he might have still considered the possibility of CNS toxicity, but might have also considered other possible causes with the subconscious knowledge that he was not "on the edge" with his mix/profile.

As for divers posting about possible DCS with "safe" profiles, undeserved DCS hits are well documented and widely known. CNS toxicity is also obviously not a set linear relationship with clear boundaries either, so I do believe it should have been something considered by Dr. Lecter in reacting to his incident. It just should not have been the only thing considered, and diving a more conservative mix would have given him some level of comfort in considering other possibilities. When you are diving a gas/profile that you know might be risky, it forces you to assume the worst when in doubt.

 

[uncfnp]

Looking back at his posts it seems that you may be at least partially correct. But it was the time spent at 100% O2 at 20 feet that brought oxtox to mind and not a subconscious awareness based on his gas choice at depth. Again, only he can answer that question.

 

[AggieDiver]

True, and good point. Again, I am not a tech diver...just nitrox certified, so I don't claim to know all of what goes into planning a tech profile. However, I know if I were planning to dive an enhanced air mix to those depths, among my foremost concerns would be the MOD of the mix I was using and calculating what my PPO2 might be. I suspect Dr. Lecter was aware of those things as well, which may have precipitated his concern about the cumulative affects of that plus the 100% O2 at 20'.