CNS toxicity symptom occurrence...or not

[Dr. Lecter]

Context: morning dive on a ledge running from 170' to 205' on EAN23, with 50% and 100% for deco. First dive of that day, but had conducted one shorter dive the previous afternoon to 175' on air with 100% for deco. Exposure protection was a lavacore with no hood, gloves, and a pair of shorts.

Dive was uneventful, dropped to the ledge rapidly and spent most of my 25 minutes of bottom time around 190' with a couple excursions to 200-205' to snag a giant spiny lobster/nearly lose my Scout light/look for sunrise shells. No signs of anything amiss at the bottom despite the fact I would have preferred straight air for a dive of this depth.

Ascended with the rest of the group to our first stop around 100', and moved from there at a pace about one stop behind the rest of the group (had a new backup Petrel on 20/75 that seemed to want more time than anyone else's plan, and I chose to humor it). Very clear water until 50' or so, with one big eagle ray and a bunch of drifty little translucent creatures in the water. Switched to 50% at 70' and ascent continued uneventfully; no current or swells to speak of, held all stops within a foot or so without much effort. Did lazy circles around the group on the drift float to amuse/warm myself.

Pulled up to the 20' stop, dialed in the buoyancy carefully in preparation of having a deco ceiling and a MOD floor that were very close together, and switched to the 100%. Was on the 100% for around 17 minutes without much incident other than a random swell drifting me to 23' for a second, with most of the time spent between 20' and 15', when I suddenly felt a stabbing, prickly pins & needles pain across the left side of my face.

Oh , oncoming CNS hit! was my first thought, so I spat the O2 reg within a second or two of feeling the pain in my face and went to backgas, ascended to 10', and switched gases on the computers. Signaled the other guy left in the water towing the float that I had an issue with the O2, but was going to finish deco rather than heading straight up. I had 3 minutes of 100% deco remaining when I spat the O2 reg, and my computers wanted another 5-6 long minutes on EAN23 before clearing me to head up. I was a bit confused by the fact that the strange tingling/pins & needles pain remained, localized on the left side of my face, despite the fact that I was sitting more or less motionless at a pO2 of 0.3.

After surfacing in what appeared to be a canoe race going on around our dive flag, I headed over to the boat and hauled myself back aboard. Once I got myself settled in, I realized my face still felt the same. Then it clicked: the little floaty critters I'd seen an unusual number of on the dive had been trailing tentacles, and I'd surely taken what amounted to a mini-jellyfish to the face. Huge relief can apparently come in the form of having the crap stung out of your face That also made a lot more sense than having a CNS hit after a relatively limited pO2 exposure.

My only real concern is that I opted to wait and see what happened after I got off the O2, rather than blow off a couple minutes of deco when I run a very conservative profile anyway. Based on what I know about CNS hits, I figured that going off the O2 and lowering my pO2 more by ascending should have been enough. Perhaps the wiser course would have still been to blow off the deco and get floating on the surface in case a seizure hit.

 

[rsingler]

Assuming for discussion purposes that it wasn't the j-fish that stung you, those (peripheral nerve) symptoms would be more suggestive of a DCS nitrogen bubble near a facial nerve, than a cerebral precursor to a seizure. You did the right thing finishing out your deco obligation. And, you did the right thing decreasing your PO2, just in case.
Yes, you could make the case that "if I'd made the wrong call, I could have seized and drowned", but you didn't. A precursor to a seizure is either no symptoms at all, or something MUCH less localized.
Let's see if the Duke guys weigh in here.

Doc Singler

 

[openmindOW]

Oh , oncoming CNS hit! was my first thought,

That's scarey. I'm glad that you're okay.

 

[DevonDiver]

So... I'm interested.... what does the OP feel is the lesson learned? That you should have skipped deco? Really?? (is that what you were taught for suspected O2 Tox??)

For me... the lesson would be that technical divers should apply the most basic, entry-level, nitrox principles, especially in regards to MOD and max PPO2.

I don't think this would count as 'undeserved'.

Short-cuts, complacency and a "it'll never happen to me" attitude seem to underline the dive planning for these dives. Not good tendencies in a technical diver.

Glad you're okay though. I'd concur that it was much more likely a jellyfish/marine life issue. I suspect the only reason your knee-jerk reaction was to automatically fear O2Tox was because you knew your gas/depth selection was flawed (but dived it anyway). That 'nearly' led to an omitted deco situation. A bad chain of mistakes.

Let it be a lesson learned... time to go back to basics and smarten up.

 

[TSandM]

Interesting story. I don't think you were at huge risk of O2 toxicity, but I don't honestly know what I would do if I got symptoms I'd never had before, after 17 minutes on O2 in unsettled water.

We get enough jellyfish stings here in Puget Sound that we generally recognize them for what they are.

I think, if I had symptoms I was worried about, I'd get off the O2, signal my buddy for increased deco, and let them know I didn't feel right. Wetnotes might be handy here.

 

[Duke Dive Medicine]

Dr. Lecter,

Glad you're ok. It's easy to go, "Oh, duh, jellyfish" in retrospect but it sounds like you assumed the worst possible scenario and took action based on that assumption, which is rarely a bad thing.

That sort of sudden, dramatic stabbing pain that you described is not characteristic of the "tingling" associated with CNS O2 toxicity. The tingling is primarily a mild pins-and-needles feeling, and every time I've seen it it's been in the fingers. It sets in slowly, and our attending physicians (Dr. Moon among them) have described it as more peripheral than central in origin. The symptoms you described would also be an extremely unusual presentation for DCS.

The question of whether you should have omitted decompression is a good one and deserves careful consideration (which you admittedly don't always have time for in that situation). The problem with simply lowering the inspired pO2, even to normoxic levels, is that a diver can experience CNS O2 tox symptoms immediately after doing so. This is sometimes described as the "off phenomenon", and the mechanism isn't fully understood. We know that the reactive oxygen species (ROS) generated by hyperoxia effectively scavenge the circulating nitric oxide in the blood. Nitric oxide is a vasodilator, so when it reacts with ROS, the net effect in the body is one of vasoconstriction. Meanwhile, the relatively high inspired pO2 causes oxygen to be dissolved in the plasma in increased amounts. When the inspired pO2 is lowered, the vasoconstrictive effect goes away relatively quickly but the plasma still may hold an increased amount of oxygen. Vasodilation + dissolved oxygen = increased cerebral blood flow and increased O2 delivery to the brain, which can lead to CNS O2 toxicity.

So, if you start to experience CNS O2 toxicity at your deco stop, at what point is it safe to blow your deco and surface so you don't drown if you seize, or is it safer to chance it and just lower your pO2? There's no easy answer to that. It would be a seat-of-the-pants decision under less-than-ideal circumstances and would involve weighing the risk of seizure with the risk of DCS, and it's not always easy to quantify either. This would make for a good discussion in the tech diving or T2T forums.

As an aside, you may want to consider lowering the pO2 in your bottom mix - 1.66 is no longer considered a safe pO2 for back gas, especially at 200-foot depths where gas density and the resultant hypercapnia become significant factors. Dr. Vann is even considering lowering his recommendation from 1.4 to 1.2.

Best regards,
DDM

 

[Dr. Lecter]

I suspect the only reason your knee-jerk reaction was to automatically fear O2Tox was because you knew your gas/depth selection was flawed (but dived it anyway).

I was really looking forward to your commentary on this one, DD

With regards to this specific bit, it’s an incorrect assumption on your part. This occurred long after I left the bottom and the CNS % difference for EAN23 vs air is negligible. While localized, the sensation was nowhere near intense enough that I would have associated it with a jelly sting -- something I've had happen several times before and always knew what had happened right away -- except on reflection. I’m still trying to ID the jelly-like things I remember seeing during a stop, which had a body around the size of a thumbnail or so and only two obvious tentacles.

A significant part of my response came from this being a rare instance where I was using 100% even though I don’t like how it leaves me sitting at 1.6 for the full length of the 20' stop without benefit of the CNS depression provided by a high pN2. I almost always use 80% as my final deco mix, because I like getting on it earlier and getting nearly the same efficiency without the maxed out pO2. I don't have any regrets about the gas change part of my response to the sensation experienced (except per the "off phenomenon" issue DDM raised below) and would have done that part the same even if I'd run 20/35 on the bottom and 80% as my final deco gas. It was an unexpected prickly sensation in the face without warning or apparent explanation while on an extended period at 1.6pO2, and unless I actually saw the critter that caused it, I'd treat it as a possible symptom if the same thing happened to me again tomorrow.

I'm still on the fence about whether the lesson learned is 'do what should work and nothing more drastic', which is what I did, or 'take a different calculated risk and omit a couple minutes of deco that’s not nearly called for by my contingency profile because bent is better than drowned'. Normally I run VPM/GFS +3/75, and in this case was following the slightly more conservative GF 20/75 profile. The last few minutes of the 10’ stop in that profile is because I find I feel a lot better carrying a lot of gear up a tiny ladder with sub-optimal hand holds with that extra few shallow minutes…according to the VPM +1 contingency table I’d cut for this dive, I was well past ‘acceptably safe’ time to get out of the water if events dictated the need to do so.

That sort of sudden, dramatic stabbing pain that you described is not characteristic of the "tingling" associated with CNS O2 toxicity. The tingling is primarily a mild pins-and-needles feeling, and every time I've seen it it's been in the fingers. It sets in slowly, and our attending physicians (Dr. Moon among them) have described it as more peripheral than central in origin. The symptoms you described would also be an extremely unusual presentation for DCS.

The question of whether you should have omitted decompression is a good one and deserves careful consideration (which you admittedly don't always have time for in that situation). The problem with simply lowering the inspired pO2, even to normoxic levels, is that a diver can experience CNS O2 tox symptoms immediately after doing so. This is sometimes described as the "off phenomenon", and the mechanism isn't fully understood. We know that the reactive oxygen species (ROS) generated by hyperoxia effectively scavenge the circulating nitric oxide in the blood. Nitric oxide is a vasodilator, so when it reacts with ROS, the net effect in the body is one of vasoconstriction. Meanwhile, the relatively high inspired pO2 causes oxygen to be dissolved in the plasma in increased amounts. When the inspired pO2 is lowered, the vasoconstrictive effect goes away relatively quickly but the plasma still may hold an increased amount of oxygen. Vasodilation + dissolved oxygen = increased cerebral blood flow and increased O2 delivery to the brain, which can lead to CNS O2 toxicity.

So, if you start to experience CNS O2 toxicity at your deco stop, at what point is it safe to blow your deco and surface so you don't drown if you seize, or is it safer to chance it and just lower your pO2? There's no easy answer to that. It would be a seat-of-the-pants decision under less-than-ideal circumstances and would involve weighing the risk of seizure with the risk of DCS, and it's not always easy to quantify either. This would make for a good discussion in the tech diving or T2T forums.

DDM,

Thank you for the substantive and useful response. I probably should have been more clear that while it wasn’t like putting a chilled extremity into hot water ‘pins & needles’, it was still very unlike prior marine stings I’ve felt before. As I said to DD, if I felt more or less the same thing again under such circumstances, unless I saw the critter that could have caused it, I think I would still at least lower pO2.

But maybe not as dramatically as I did in this case. The “off phenomenon” you describe is interesting, because it sounds like lowering pO2 may be a balancing act: too little, and one isn’t addressing the potential problem; too much too fast could trigger a seizure in and of itself. Maybe going to a leaner deco gas, rather than straight to backgas, would be a more prudent response.

When and by how much to change a deco schedule on the fly is always tricky. Looking back, I think it would have been more prudent for me to switch to my contingency profile rather than sit out the last few minutes of my ‘ideal’ deco profile, since according to it I could have surfaced at once. This is made somewhat easier by having done similar dives on VPM +1 and had no issues other than feeling extremely tired; a similar situation occurring at a deeper stop where even the contingency profile still calls for significant deco time would be much less debatable.

 

[DevonDiver]

With regards to this specific bit, it’s an incorrect assumption on your part. This occurred long after I left the bottom...

...and yet that was the first thing that entered your mind, despite the symptoms being extremely atypical for O2Tox?

the CNS % difference for EAN23 vs air is negligible.

I guess 1.4 versus 1.6 versus 1.8 versus 2.0 is all "small" numbers. Negligible huh?

You were aware of the MOD and bottom PPO2 for 23% when planning your 205ft dive??? Chose to ignore it?

What were you taught?... in the qualifications you've gained thus far to undertake dives of this nature?

While localized, the sensation was nowhere near intense enough that I would have associated it with a jelly sting

Do you seriously think all jellyfish stings are of the same toxicity/intensity?

 

[Dr. Lecter]

...and yet that was the first thing that entered your mind, despite the symptoms being extremely atypical for O2Tox?

Why don't you tell me more about how otherwise unexplained tingling sensations in the face and/or extremities is "extremely atypical for O2tox"?

I guess 1.4 versus 1.6 versus 1.8 versus 2.0 is all "small" numbers. Negligible huh?

I was referring to the impact that choice of bottom gas had on overall CNS% exposure (both in terms of per dive and 24 hour exposure), which factors into the potential to later tox while at 1.6 during deco.

You were aware of the MOD and bottom PPO2 for 23% when planning your 205ft dive???

Quite aware of the max 1.66 pO2 at the bottom, yes.

Chose to ignore it?

Not at all. Chose to address it differently than you would have -- by limiting O2 exposure during the previous day's dives, limiting time spent above 1.4 on the bottom, total bottom time, and exertion level during the bottom portion of the dive -- which is quite different from ignoring the pO2.

 

[Duke Dive Medicine]

DDM,

Thank you for the substantive and useful response. I probably should have been more clear that while it wasn’t like putting a chilled extremity into hot water ‘pins & needles’, it was still very unlike prior marine stings I’ve felt before. As I said to DD, if I felt more or less the same thing again under such circumstances, unless I saw the critter that could have caused it, I think I would still at least lower pO2.

But maybe not as dramatically as I did in this case. The “off phenomenon” you describe is interesting, because it sounds like lowering pO2 may be a balancing act: too little, and one isn’t addressing the potential problem; too much too fast could trigger a seizure in and of itself. Maybe going to a leaner deco gas, rather than straight to backgas, would be a more prudent response.

When and by how much to change a deco schedule on the fly is always tricky. Looking back, I think it would have been more prudent for me to switch to my contingency profile rather than sit out the last few minutes of my ‘ideal’ deco profile, since according to it I could have surfaced at once. This is made somewhat easier by having done similar dives on VPM +1 and had no issues other than feeling extremely tired; a similar situation occurring at a deeper stop where even the contingency profile still calls for significant deco time would be much less debatable.

I wondered about your bailout tables and I'm glad you brought that up. Re how much to lower the pO2: with the caveat that this has not been studied extensively with immersed individuals and this shouldn't be taken as gospel, the vasoconstrictive effect begins at an inspired pO2 of about 0.6 ATA. There's no hard-and-fast with this, though, and I would not stake my life on it.