Oxygen Toxicity vs Narcosis

[Rick Murchison]

I'm still curious. Several people have stated that as you head for the surface, tox symptoms would improve. I understand that there's a threshold at which this would be true, but until this threshold is reached, wouldn't you still be accumulating exposure? It seems a basic integration problem...

As Charlie has mentioned, the mechanism for CNS toxicity is not well understood, but, like narcosis, symptoms diminish quickly as PO₂ falls below 1.4. In the case of narcosis, what's odd is that the narcotic affects of nitrogen seem to be almost entirely directly related to PN₂ and have practically nothing to do with total absorbed nitrogen - that is, if you dive deep enough to get narced, you'll get narced right away and it won't get measurably worse over time if you don't go any deeper, and it will subside almost immediately when you ascend. At least with oxygen, there's a reason it doesn't accumulate like nitrogen, as oxygen is both metabolized in all the tissues, and scavenged by hemoglobin from the blood.
Therefore...
In the cited case, where the nausea persisted all the way to the surface and for awhile afterwards, I'd rule out both CNS oxtox and narcosis, and point the finger at food, drink, a bug or CO2 as the most likely culprits - and a possibility of CO/bad gas... but not oxtox or narcosis.
Rick

 

[peterbkk]

An interesting and somewhat related sidebar to this discussion:

Read Steve Jones new book Coral (http://www.amazon.co.uk/Coral-Pessimist-Paradise-Steve-Jones/dp/0316729388). He blames all our oxygen related problems (e.g. cancer) on the corals, who, millions of years ago, filled our atmosphere with oxygen. Without them, we would have had to learn to breathe CO2. All these oxtox problems would not have existed (but neither would the human brain)...

Regards
Peter

 

[grazie42]

It is my understanding that the risk of oxtox is a function of time and po2 but that there is always some risk (however small) of a hit. If you die from a "1 in a million occurence" doesen´t really, dead is dead. An individuals total risk of oxtox is dependent on a whole host of other issues as well as po2/time so I wouldn´t rule out the possiblility of it being oxtox, though I agree that it seems very unlikely...

I don´t think it was narcosis because while the risk of narcosis is subject to the same variances as oxtox it too tends to go away as you ascend...

It could be a food/drink/physical condition thing as it persisted, or CO2...

The "take home" of this incident should as most everyone has pointed out, be a change in diving practices, whether it was oxtox, narcosis or anything else really is beside the point...

 

[miketsp]

..snip..
We were using a 32% mixture
..snip..

Just curious, did you actually check this yourself?

I've been offered tanks that were 10% over the requested mix.

 

[Rick Murchison]

It is my understanding that the risk of oxtox is a function of time and po2 but that there is always some risk (however small) of a hit.

Well, there's always some risk of a seizure in folks who have a seizure risk anyway... and there have been a few cases where that seizure susceptability was discovered first by the early onset of oxtox symptoms at elevated PO₂'s, but for the rest of us, within the NOAA exposure time limits, I don't believe there's ever been a seizure at 1.4 or below, nor has there been any seizure at 1.6 or below where there has been no exposure to more than 1.6 just before the seizure. That's pretty good odds...
What I see happening is that everyone jumps on the "early oxtox" bandwagon whenever a diver is diving Nitrox and has an episode that could be explained by oxtox (if it were oxtox), and from these, anecdotal evidence of early hits emerge - but that ain't science, and I personally think oxtox is getting far more credit than it is due.
For example, last summer one of our divers had a seizure on the surface after a dive, and the "general consensus" quickly began pointing the finger at oxtox, even though there wasn't the remotest chance that it could be. But nothing showed up on any of the post dive workups, and the oxtox believers weren't disuaded until the same fellow had a surprise heart attack three months later... you see, he was young and a runner and a "healthy" dieter and in excellent health according to all tests, so it just had to be oxtox. Except that it wasn't.
Rick

 

[TheRedHead]

I'll venture sea sickness as the cause.

 

[miketsp]

..snip..
within the NOAA exposure time limits, I don't believe there's ever been a seizure at 1.4 or below, nor has there been any seizure at 1.6 or below where there has been no exposure to more than 1.6 just before the seizure. That's pretty good odds...
..snip..

There were some discussions on the aggravating effect of medicines or CO2 buildup.
http://www.scubaboard.com/showthread.php?t=119941

 

[rjack321]

Well, there's always some risk of a seizure in folks who have a seizure risk anyway... and there have been a few cases where that seizure susceptability was discovered first by the early onset of oxtox symptoms at elevated PO₂'s, but for the rest of us, within the NOAA exposure time limits, I don't believe there's ever been a seizure at 1.4 or below, nor has there been any seizure at 1.6 or below where there has been no exposure to more than 1.6 just before the seizure. That's pretty good odds..
Rick

Seizure at 1.4 ppO2 3 weeks ago. http://www.divers4life.com/TheCrack.html
Never say never.

 

[Rick Inman]

Seizure at 1.4 ppO2 3 weeks ago. http://www.divers4life.com/TheCrack.html
Never say never.

I personally am skeptical that this was really an OxTox induced seizure.

Official cause of death was ruled an air embolism, however with the caveat that it was the only condition evident that could be ruled as a cause. The medical examiner stated there was no way he could determine the actual cause of death, and we would likely never know for sure.

 

[limeyx]

Well, there's always some risk of a seizure in folks who have a seizure risk anyway... and there have been a few cases where that seizure susceptability was discovered first by the early onset of oxtox symptoms at elevated PO₂'s, but for the rest of us, within the NOAA exposure time limits, I don't believe there's ever been a seizure at 1.4 or below, nor has there been any seizure at 1.6 or below where there has been no exposure to more than 1.6 just before the seizure. That's pretty good odds...
...
Rick

I agree (mostly). I think within the "standard" 1.4/1.6 limits, it is more related to time, rather than a high probability of some of the high-variability that has been observed over 1.6 (where things got so random that I think NOAA was forced to conclude there was essentially no rhyme or reason)

There *is* a (suspected) CNS hit at 1.4 (before switching to deco gas) ongoing on another board regarding an incident in a cave "The Crack." The current assumption I guess is CNS hit, but I dont know how that can be 100% verified.

Once you get out of the range of using nitrox to extend bottom time, it seems (to me) prudent to keep PPO2 to 1.2/1.6 instead of 1.4/1.6, but I will dive 32% to 100 feet on recreational dives, which kind of violates this.