EAN32 diver below you at 40-odd meters?

[Makhno]

OK, so:

You're diving EAN32 on a vertical wall with great viz. Everyone else is also diving EAN32. You see a diver (also diving EAN32 -- it's clearly marked on his cylinder) dropping deeper and deeper, faster and faster ... until you're at about 32 m (that's 105 fsw; near your MOD) and he's at least 10 m below you (138 fsw) and going deeper quickly.

What do you do?

I chose to exceed my MOD and swim down fast, grab him and tap him on the shoulder, and point to the depth on his gauge. He adjusted his buoyancy and then we both ascended quickly to above our MOD; no problem. But by the time I got down there and tapped him on the shoulder, we were at about 45 msw (148 fsw) or deeper. That's a PP02 of > 1.6; a CNS 02-tox risk.

Divers learn in our rescue courses not to put ourselves at too much risk when trying to rescue others -- 2 deaths are worse than 1; 2 lost or injured divers divide the rescue resources in half; etc. etc. But I still think I made the right choice. Countless divers have exceeded PP02s of >1.6 on air and EAN32 for short periods of time and had no problems, whereas a diver who's narced and keeps plummeting down a wall to 55 m or 65 m or deeper on nitrox is at a great degree of risk.

I'm receptive to those who can explain persuasively why I made the wrong choice, as well as those who can persuasively back me up. I think there's more to it than just PP02 because divers on air and EANx seem to tolerate greater PP02s than divers on trimix (in theory because the narcotic effect of N2 counteracts the neuroexcitatory effect of 02 toxicity, but helium doesn't). Any empirical data on what % of divers tend to tox at what depth, on what gas mix?



P.S. As I learned later, he's a scuba instructor qualified to teach nitrox courses. :depressed:

 

[Rainer]

You made a perfectly fine choice (duh, everything worked out).

Was this guy just swimming around by himself? Did you see him later on the surface and ask why he was that deep on that gas?

 

[Makhno]

You made a perfectly fine choice (duh, everything worked out).

Yeah, but I have known a lot of folks who by the same logic will tell you that they can drive a car just fine when drunk and on a lot of drugs because last weekend they drove home completely wasted and everything worked out. No disrespect intended, but that logic doesn't entirely convince me.

Was this guy just swimming around by himself? Did you see him later on the surface and ask why he was that deep on that gas?

We spoke different languages so we couldn't exactly discuss the matter in detail. He was diving with a group of buddies who were far shallower than he was. He thanked me later in simple English but the language barrier prevented any further discussion. Basically, though, he just got narced and lost track of the depth in the good viz and forgot to watch his depth gauge.

 

[vjanelle]

Oxtox is something that takes time + high PPO2. I'm not sure how i would've reacted personally either.

 

[Rainer]

Yeah, but I have known a lot of folks who by the same logic will tell you that they can drive a car just fine when drunk and on a lot of drugs because last weekend they drove home completely wasted and everything worked out. No disrespect intended, but that logic doesn't entirely convince me.

The point is pretty simple. Humans can tolerate high PPO2s for brief periods. You were at considerably less risk than the other diver if he was just going to stay there.

That said, you didn't know the guy. I wouldn't fault you either for not chasing after him. Clearly there is SOME depth at which chasing strangers around isn't going to be worth my effort / the risk. I hardly see a brief, calm swim down to 1.7, though, as threatening. I've worked much harder at 1.6 (decoing in currents) several times.

 

[Slamfire]

You did right. Like vjanelle said it is a time/dose relationship. When I took my PADI nitrox course they did not teach us about the time dose relationship. I only learned about it when I took the TDI advanced nitrox course. Before learning about it my impression was that you should not go beyond 1.4 ppO2 for any lenght of time because you are at an imminent risk of toxing. Then you learn about the NOAA oxygen exposure table and that according to it, you are allowed 45 mins of 1.6 exposure. And then you notice that the PADI %CNS table is based in this NOAA table. So in short they took the NOAA table and gave you more conservative rules that should compensate for out of shape, overweight 50 year old divers.

Several of my buddies that have not taken advanced nitrox believed as I did that going to 1.6 was a big no-no.

 

[Scott L]

That said, you didn't know the guy. I wouldn't fault you either for not chasing after him. Clearly there is SOME depth at which chasing strangers around isn't going to be worth my effort / the risk. I hardly see a brief, calm swim down to 1.7, though, as threatening. I've worked much harder at 1.6 (decoing in currents) several times.

The majority of the very experienced divers I know have been nitrox certified for 8+ years and still operate by what was taught them regarding PPO which was 1.6. They chuckle when I indicate the industry has reduced their recommendation to 1.4. No one has fatality toxed here in years that I am aware of...

 

[Rainer]

Um, you had a diver tox and die at 1.4 in FL last year... (there have been other deaths at 1.4). I'm not advocating long exposures to high PPO2s (I plan my deeper dives at an average of 1.2), but short exposures to higher PPO2s *when attempting a rescue* are certainly worth considering for some. That said, for normal diving, I just can't see why you'd want to push limits all for gaining what, a few minutes less deco?

It's also worth remembering that the NOAA guide isn't exactly a table worth taking too seriously. Across *and* within subject variation in toxing is extremely high.

The majority of the very experienced divers I know have been nitrox certified for 8+ years and still operate by what was taught them regarding PPO which was 1.6. They chuckle when I indicate the industry has reduced their recommendation to 1.4. No one has fatality toxed here in years that I am aware of...

 

[Slamfire]

...That said, for normal diving, I just can't see why you'd want to push limits all for gaining what, a few minutes less deco?

It's also worth remembering that the NOAA guide isn't exactly a table worth taking too seriously.

Why? I've heard/read statements like yours before that basically say, "DCS risk is overestimated and risk of oxtox is underestimated, that's why I prefer to keep it at 1.2". I've always been curious to find out on what is such statement based upon. There must have been a systematic methodology to develop the NOAA table. I believe if it is to be effectively discredited, there should be an equally systematic methodology to prove it wrong. Has there been any statistical hypothesis testing?

Across *and* within subject variation in toxing is extremely high.

I would love to read the research paper where this determination was published and reported. Do you know where can I find it?

 

[TSandM]

As Rainer said, individual variation in susceptibility to oxygen toxicity varies widely, and what is more frightening, varies from day to day in the same subject. (I know where I have the table from the papers on this, and if I have time, I'll find it.) Once you leave the "generally recognized as safe" area, you are really in no-man's land as far as knowing what the risk is that you are taking. That said, I'd probably have chased the fellow a ways, myself, simply because if I hadn't and he had toxed, I'd have a hard time living with myself.