Today I think i survived a CNS hit at depth

[DevonDiver]

O2 Tox Symptoms (CNS Toxicity/Paul Bert Effect)

CON - convulsions, short tense muscle contractions (tonic); followed by rapid spasms/convulsive jerking (clonic)
V - visual disturbances, especially tunnel vision or blurred vision
E - ear ringing, such as tinnitus
N - nausea, vertigo and dysphoria ("just didn't feel right")
T - tingling, muscle twitching and cramping
I - irritability, personality changes and anxiety
D- dizziness, confusion and decreased consiousness

Illustration of symptom onset, from study:

Oxygen Poisoning at 90 ft (27 m) in the Dry in 36 Subjects in Order of Performance –
by K W Donald (1947). px"Oxygen Poisoning in Man: Part I". British Medical Journal

Exposure (mins.)	Num. of Subjects	Symptoms
96	1	Prolonged dazzle; severe spasmodic vomiting
60–69	3	Severe lip-twitching; Euphoria; Nausea and vertigo; arm twitch
50–554	4	Severe lip-twitching; Dazzle; Blubbering of lips; fell asleep; Dazed
31–354	4	Nausea, vertigo, lip-twitching; Convulsed
21–30	6	Convulsed; Drowsiness; Severe lip-twitching; epigastric aura; twitch L arm; amnesia
16–20	8	Convulsed; Vertigo and severe lip twitching; epigastric aura; spasmodic respiration;
11–15	4	Inspiratory predominance; lip-twitching and syncope; Nausea and confusion
6–10	6	Dazed and lip-twitching; paraesthesiae; vertigo; "Diaphragmatic spasm"; Severe nausea

The PPO2 in the described by the OP was low (1.28ppO2), which would generally/simplistically discourage agreement that his symptoms were, indeed, an O2 toxicity related event. However, the symptoms described do fit the model for O2 toxicity. The OP also describes maintaining a regular frequency of nitrox diving - which leads to concerns that their NOAA CNS Daily Limits may have been approached, or exceeded, over multiple dives/days - which is believed to raise susceptibility to oxygen toxicity onset.

Article on CNS Daily Limits by Doppler: http://www.scubaboard.com/forums/content/357-daily-limits-cns-oxygen-toxicity.html

The issue of multiple, over single, exposure also needs to be accounted for - especially as the OP described repetitive, deep nitrox dives over a multi-day period. I'm not suggesting that the OP might have exceeded these limits (without knowing precise schedules for past diving), but it can be seen that there is more to consider than simply the specific PPO2 and exposure time for the single dive where symptoms may have been symptomatic...

Normal Operations
Oxygen Partial Pressure (ata) 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6	Maximum Duration for a Single Exposure (min.) 45 120 150 180 210 240 300 360 450 570 720	Maximum Total Duration for any 24-Hour Day (min.) 150 180 180 210 240 270 300 360 450 570 720

Also see: NOAA CNA Percentage Oxygen Exposure Table.

Other factors which effect susceptibility (referred to as "oxygen tolerance") are:

- There is a large individual variation in susceptibility and time of onset to symptoms.
- Compared to dry exposures, immersion decreases oxygen tolerance a great deal, decreasing exposure times up to a factor of 4 or 5.
- Exercise decreases oxygen tolerance a lot, compared to rest - believed issues with CO2 retention and tox susceptibility.
- Diving in very cold (<49°F / 9°C) or very warm (>88°F / 31°C) water seems to decrease oxygen tolerance.

Read: DAN discusses the dangers of oxygen toxicity when using nitrox as a breathing gas, By Dr. E.D. Thalmann

In sum, the issues of individual oxygen tolerance, exposure time and %24hr exposure are all factors that could point towards this being an O2 Toxicity event, even though the maximum PPO2 for the symptomatic dive was safely below the maximum threshold.

It would be a rare occurrence - but O2 toxicity (and DCS) is not an exact science, nor is safety/immunity guaranteed simply by following the standard guidelines and remaining within 'advised' thresholds. As such, it is absolutely wrong to assume that this cannot be O2 toxicity, simply due to PPO2/Time exposure on a single dive, without recognition of many other, potentially contributing, factors and an understanding of the variances involved.

 

[bbarnett51]

I'm very interested in hearing what DAN says.

 

[PfcAJ]

snip

In that table, you'll see that it states that some folks use a 90min halftime for the CNS% value on the surface. A fun game to play is to try and learn exactly how that number was derived.

 

[lamont]

...the temperature dropped markedly (maybe 4-5C)...

colder temperature

and vis reduced to around 1-2m.

dark and low viz.

We continued down to our max depth at 36m.

END more than sufficient to produce narcosis.

This was at around the 7min mark. I then completed the narcosis test (fine),

Those are largely useless, you were narc'd. The best test of narcosis at depth is to memorize random words and then try to recall on the surface. Doing tasks at depth that are trained on the surface (e.g. the classic math problem) is testing the kind of recall that is /not/ impaired by narcosis.

checked our NDL's (mine said 8) and we turned around and started to swim back in the direction from which we had come. At this stage we were swimming into the current

swimming into a current, so exertion level was up.

in very low visibility. I had dropped to the back of the 3 person team. At around 1min elapsed time, I started to feel.. wrong. We were around 31m at this stage I believe. I struggle to describe it, but I just felt really really weird. I have experienced vertigo before, this was not vertigo. My vision started to go weird too. It was like when you look through a thermocline, it's just a bit warped, and it started to narrow.

Symptom of a CO2 hit.

I instinctively knew something was wrong, kicked one or two kicks to get to the DM, and grabbed him. I signaled a problem, pointed to my head, and thumbed the dive. He signaled OK and took off to grab the Instructor, who was only 2-3m away but was already disappearing into the haze. I reached for my inflator, and tried to ascend a couple of meters. I don't think I was successful (certainly I see nothing on the computer to indicate that I was). I was remarkably calm, my SAC rate was fine, but I was feeling like I was going to pass out.

I suspect you were not all that calm and your SAC wasn't that hot, video probably would have showed something different. This is exactly the kind of recall which is impaired by narcosis.

Then I started to shake. I could feel my arms and legs shaking. I managed to grab the reg and hold it in my mouth, but could not work the inflator button. My hands simply wouldn't do as commanded. Around now the DM re-arrived with the Instructor, not 100% before seeing me understanding the gravity of the situation (I believe). I let go of the inflator, grabbed the DM's arm, and held on for dear life. I consciously then ceded control to them (not that they didn't take over anyway), and became a passenger.

That sounds like you've made the transition to panic, not seizures.

The ascent from 31m to 20m is a bit of a blur. I didn't pass out, but I certainly was "out of it", and don't recall much of it. The shakes subsided on the way up, and by the time we got to around 20m, I felt almost normal. We spent a few minutes swimming around with them checking on me constantly, then ascended up. I think I came up with 120bar left.

So at this stage, I need to go have a drink, and re-asses my diving plans. I certainly would never ever want to do a deep dive again with anyone who is not Rescue trained.

And I need to go buy my DM and Instructor a large number of drinks.

"when you hear hoofbeats, think horses, not zebras..."

a tox on 1.3 ppO2 is a zebra, the obvious horse in this race is a CO2 hit and panic.

 

[Griffo]

So to answer a few questions...

Spoke to DAN. They were not as dismissive of it being an O2 hit as some people in this thread. What they said aligns pretty much with what Lynne said. They said the po2 limit could vary from person to person and dive to dive etc. and the shaking is a pretty firm sign.

Yes I have been deeper before, 40m is my max but on air. I,ve never gone that deep on Nitrox before, the deepest Nitrox dive was the previous days at 28m on 32%.

I strongly believe that this was not a panic event. I was calm enough to wait for the DM and Instructor to return from the soup - I had lost complete sight of them. And the shaking and inability to command my hands properly. I was thinking it through the whole time. I was most probably narked, but not panicy. And cold, low vis doesn't worry me, I dive in it all the time.

Oh, and it's too late to have the tank analysed. However I have analysed almost all my tanks with my co tester, and they have never been above 0.1. This day I happened to leave the tester in my hotel room by accident. Maybe that's the lesson, but I've seen the shop change the filter stack at least once since I've been here, they seem very on top of maintenance. The shop is owned by westerners, one of whom is a tech diver.

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[Quero]

What treatment or follow-up measures did the DAN team suggest?

 

[Griffo]

What treatment or follow-up measures did the DAN team suggest?

None. As I was symptom free, they were happy with an 18 hour SI.

On another note, I measured my SAC for the dive. It was actually lower than normal at 13l per min averaged across the dive.


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[Quero]

And did they make recommendations in regard to your future dive planning? What are your own thoughts in this concern?

 

[japan-diver]

Although I would tend to lean toward a combination of a CO2 and Narcosis problem there still is a possibility of a O2 hit. Did you take any kind of medication that day? There are some meds that will tend to lower O2 tolerance.

 

[Griffo]

And did they make recommendations in regard to your future dive planning? What are your own thoughts in this concern?

They didn't really say much. I plan to dive air or keep my pO2 below 1.2 until I get home and speak to some hyperbaric specialists.


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