OP
Dr. Lecter
Contributor
The armchair psychology continues unabated, so I'll try this one more time. It was not the knowledge I'd spent a brief time at 1.6+ pO2 during the bottom phase that was going through my mind at the time -- it was that I'd just spent over 15 minutes on 100% O2, much of that at a 20' stop, after a relatively long/deep dive. While I knew I was not over my CNS single dive exposure, I also knew I was close to it (largely because of the shallow time on 100%) and that bad things can still occur at the margins.
So, why didn't I consider that what I was feeling was a sting and not a symptom? Foremost, I think it's because the sensation was quite unusual to me. I've been stung by jellys and hydroids before and while the intensity has vaired from 'eh' to 'AHHHH' they've always been more direct/localized/sting-like than the feeling I experienced here. Moreover, because there wasn't anything in the water with me that I saw during the shallow stops, much less in my face around the time of the sensation, that would have put the possibility in my mind. While I have a distinct recollection of seeing what you have likely correctly IDed as thimble jellys at a deeper stop, they were forgotten until I was back on the boat trying to figure out why the left side of my face still felt prickly/tingly. Whether my failure to consider the possibility of a jelly sting was an unreasonable jump or not, I can say that under similar circumstances I'd go to a lower pO2 again...it was just too weird a feeling without a confirmably benign explanation to not do so.
But what are the lessons learned for me? First, don't hesitate when faced with a possible symptom/indication of an oncoming problem: stop, think, act. I'm happy with the first part of my reaction here, going to a lower pO2 and ascending to the top of my stop without hesitation after concluding I was experiencing an unusual facial sensation after an extended period on 100% O2. Knowing what DDM has mentioned about potential effects of dropping the pO2 too much after a long O2 exposure, I'd go to my 50% if I had it to do over.
I'm still on the fence about my other decision, which was to stay in at the stop on a lower pO2 rather than bail out and surface at once because to my contingency profile I had finished deco quite some time ago. DD has expressed the one side of the issue rather well: bailing out to a limited deco profile has serious DCS risks associated with it. On the other hand, I knew I had cut a relatively conservative bailout profile for this dive (VPM +2 square 205' profile, versus VPM +3/GFS 75% multilevel computer generated profile) and I was only running the low surfacing GF that I was because I normally had a lot of work to do soon after surfacing (climbing a less than optimal ladder/transom with a lot of tanks in the open ocean)...I could have mitigated the exercise risk by ascending, establishing buoyancy, and just waiting a spell there on the surface.
The real lesson learned for me is that while I've always generated a bailout profile that is more aggressive than I would like but not so aggressive that I consider following it a serious DCS risk and thought generally about when I might need it, I've never really rigorously considered the go/no go decision for it. In this case, that caused me to hestitate to bail out and instead stay in the water hoping that a pO2 reduction would be sufficient despite training that divers have still toxed minutes after experiencing symptoms and reducing pO2 (now I think DDM's comments bring some light to bear on that subject). DD said earlier that part of the problem of this dive was that I almost scared myself out of the water and omitted deco time; I'm of the opinion that given the risks posed by oxtox, the sensation and context I had, and the fact that a reasonable contingency profile said I could go up right away, the main problem was my failure to rely on the bailout plan I'd constructed for, in part, this kind of problem.
The hypothetical of how I'd of reacted at 170' or 205' on air or 23% to the same sensation, equally unexplained, is a good one. Ascending at a prompt but not excessive rate to the first stop would be my first instinct, since that would be the only pO2 lowering technique available to me. I do not think I would have considered omitting any deco stops but I would been more likely to go to the contingency profile, not so much to get out of the water faster but to limit how much O2 exposure I'd have for the rest of the dive. The real difference would have been how much I was interested in communicating to the other divers in the water with me: if I'm going to have to spend the next 40 minutes worrying about an possible tox event rather than the next 4 minutes at 10' doing the same thing, I am going to want to be all but attached to someone who might possibly be able to help keep me from drowning myself. But at that point, I think I would simply accept that I have deco I have to do and if a hit is coming then it's coming.
True enough, but the thing is that while 23% is an annoyance for what is better done as an air dive, it simply doesn't move the CNS needle much more than 21% does. And my personal comfort level with bottom O2 exposure is pretty far from what occured on this dive. If we were talking about a 20 minute, 240' square profile air dive, then I'd be much more worried about 23%. Scratch that: 240' is my personal air MOD, and you would never catch me that deep on 23% absent a true emergency.
