Oxygen Toxicity vs Narcosis

[Abbo]

Sounds like time to pass round the chill pills.
Soon after I first started with nitrox I did a 'braille dive' in low viz in ?Plymouth Harbour back in the UK. It can't have been much more than 100 feet, but I was starting to black out so I thumbed the dive. The dive shop owner de-briefed me and laughed a big belly laugh when I said I thought it was ox tox. "I was a Navy diver during the war: we had to dive all over this harbour on pure oxygen, and when we started to tox out we just ascended a little!" 100FSW on oxygen is a PO2 of 4!! Don't go trying that at home! The problem, incidentally, was a tight neck seal on my new dry suit [cringes]!

Some things sensitize you to ox tox: CO2 is one; coffee is another. I've heard of rare cases of ox tox at way less than 1.4B. Chances are, for most people, that 1.4 gives a massive margin for error. Sensitivity varies person to person and day to day: you just don't know till it bites you in the butt. If your PO2 is above 1.6 and you're not in deco, you need to bring it down, but it's not an emergency. Large doses of coffee, of course, increase DCI risks as well as ox tox, which is something I too would do well to remember before big dives.

The fact that the nausea went on for so long after surfacing makes it highly unlikely that the problem was ox tox.

Taking calculated risks is fine: being unaware of the risks you are taking is decidedly unwise. We've all done bone-headed things underwater: the important things is to admit to them, analyse the event and develop strategies to avoid making an *** of yourself like that again. Underwater, I like to play 'guess my depth and time' - EVERY time I look at my computer, every dive. Buoyancy should tell you how much your depth has changed since you last checked; the game ensures you always know depth and time: God knows, computers can fail, and some make a habit of it. And if you're not where you thought you were it's a big sign you have another problem.

 

[Rick Murchison]

...The fact that the nausea went on for so long after surfacing makes it highly unlikely that the problem was ox tox....

Bingo!
Wasn't narcosis either.
Something else... likely food, but could be allergy or a 'bug' (or CO₂) - but neither oxtox nor narcosis survive an ascent.
Rick

 

[brutus_scuba]

So first of all. Let me start off with a disclaimer that I have never dove above a 1.4 ppO2 and I stick to the limit.

I find it very hard to believe that a 1.51 was enough for a tox hit. I have seen people diving ~1.8. The navy used to require people dive to 2.0, and the chamber rides will see even higher pp02 levels before they see any signs of tox hits.

But now to play devils advocate. Many people in here are stating that the period was too short for him to take a hit. Correct me if I'm wrong, but there are two types of Oxygen Tox Hits you can suffer from. The first is CNS Oxygen Toxicity which is more common and caused by breathing high partial pressures. CNS Oxygen toxicity has nothing to do with exposure time and is more likely the cause of the incident. A CNS hit can happen in the matter of one breath. On the other hand high pp02 exposure (even within the diving standards less than 1.4) over a period of time can cause a Pulmonary O2 Hit.

If the diver was suffering a CNS Oxygen tox hit then it doesn't matter if he was at 123 for a minute or an hour. The time only matters with Pulmonary hits.

 

[Snappahead]

Or a residual hangover from the night before?.....

 

[ianr33]

If the diver was suffering a CNS Oxygen tox hit then it doesn't matter if he was at 123 for a minute or an hour. The time only matters with Pulmonary hits.

This is totally and utterly WRONG.

The allowed time decreases with increasing PO2 but it is most definitely time dependant.

 

[victor]

Ooooooops

 

[brutus_scuba]

This is totally and utterly WRONG.

The allowed time decreases with increasing PO2 but it is most definitely time dependant.

I stand corrected I always thought that tracking the time was to prevent a Pulmonary Ox Tox Hit.

 

[Charlie99]

I stand corrected I always thought that tracking the time was to prevent a Pulmonary Ox Tox Hit.

Time is an important factor for both CNS and pulmonary.

A crude way to look at things is that pulmonary effects are the reduction in breathing capacity due to damage to the lungs over the period of many hours. The recovery time is that required for the lungs to heal --- many hours to a few days, kind of as if the lungs had been burnt a bit.

The exact mechanisms of CNS toxicity are unclear, but generally are considered to be some sort of modification or interference with the nervous system chemistry. It takes place over a few minutes to few hours. The recovery time is a half life on the order of 90 minutes.

 

[biz_nate]

What Charlie said. Just to add a couple more data points for reference:
1) From what I understand, most divers no longer track exposure for pulmonary tox. Given the ppO2s to which most divers (who track this, at least) are exposed, CNS becomes a problem before pulmonary does.
2) Since (I don't think) anyone's thrown out this number, I will: the maximum allowable exposure time to a 1.6 ATA ppO2 is 45 min (according to NOAA). This is why most people are citing that ~1.5 ATA ppO2 for ~1 min is no problem. Of course it could be, but the odds look pretty slim.

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...

 

[Charlie99]

QUOTE=biz_nate]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...[/QUOTE]If you look at the allowable CNS exposure times, it would seem that once you have gotten below 0.5ata ppO2 or so, then you should no longer be acculating exposure. OTOH, if you have started having CNS problems, then your system is messed up and you are prone to more seizures or other ill effects. There seem to be some delayed effects.

In the reports of oxtox that I've seen, the victim had additional seizures, even when moved to a low FO2 backgas and to a shallower depth. The bottom line is, that if someone has oxtox symptoms such as seizures, there is a pretty high likelyhood that it will happen again in the next few minutes, even if you reduce the exposure dramatically.

===========================================

Here's a detailed report of an oxtox incident during a GUE Tech1 class in Croata. Seizures started after 19 minutes at 2.3ata ppO2. 36 meters / 120' on a gas with 50% O2. He was put onto 30/30 (1.38ppO2 at depth) and after two minutes of convulsions and then a 1 minute ascent to 17 meters/57'/0.8aata ppO2 he went into convulsions again. No further convulsion as he was taken to the surface after that convulsion stopped. No further convulsions on the surface even when on 100% O2 (Yes, as contrary as it may seem, it's best to put an oxtox victim on 100% O2).