> While I certainly realize that there's a fair amount of
> research that indicates O2 may very well be 
> narcotic

Very little research, I think.  I know of only one published paper that 
looks into it.  The rest of the knowledge base is anecdote.

>, my current thinking is that the whole issue is 
> basically irrelevant anyway.  Here's why:

For the most part, I think you are correct.
 
> [1] O2 is theoretically about as narcotic as N2, so as

Actually, *theoretically* O2 is about twice as potent as N2 for narcosis 
(if the theory you're looking at is the correlation between lipid 
solubility and narcotic potency).  The one empirical study suggested that 
it was roughly equipotent for narcosis as N2 (the rationale being that 
because O2 is metabolized, its concentration in the tissues is 
considerably less than the inspired concentration).

My collective anecdote tells me that O2 is less potent for narcosis when 
the PO2 is less than about 1.4 or so, and more potent when the PO2 is 
above 1.7 or so.

>     you dial down your PN2 in a Nitrox, the narcotic potential of O2
>     increases accordingly, so for Nitrox/Air it's a totally
>     moot point.

Not exactly moot - but close to it.  Some folks like to give themselves 
"narcosis credit" when breathing enriched air (i.e., would experience 
less narcosis with EAN at a given depth than with air). However, assuming 
O2 as equipotent means they would expect equal narcosis as they would 
with air.

> [2] Narcosis is not usually considered a problem by most
>     divers in the Nitrox (well, EANx) depth ranges.  So again, 
>     it's a meaningless concern for Nitrox.

In this case, I think you are right. But more importantly, narcosis is 
such a sploogey factor anyway, that the variance in narcosis from other 
factors is likely MUCH greater than the variance from the O2 component of 
narcosis, especially if the EAN divers are good boys and girls and keep 
the PO2 at 1.4 or less.

> [3] In the Helium-addition depth ranges, 
>     PO2 is fundamentally limited by toxicity concerns, and thus 
>     any O2-enduced narcosis is constrained by the fact that your MOD
>     limits the FO2.

This is probably the main reason why it's not an issue of practical 
concern to most divers (which doesn't make it a meaningless topic - just 
one of little practical concern).

> [4] In the trimix/heliox range, the selected PO2 is
>     likely to be the same, or less, than that selected
>     for the Nitrox range, so the O2 narcotic potential
>     is a relative constant, and amounts to only a small
>     fraction of your total END. 

Again, I agree - as long as the PO2 doesn't get much higher than about 
1.6 or so.

> For example, let's say we're doing a trimix dive and wish
> to dial in our END and PO2, and fill the gap with Helium,
> and we want a max END of 130ft and a max PO2 of 1.4.
> 
> At 130fsw on air, your PO2 is about 1.0 and your PN2 is about 3.9, 
> for a total "narcotic partial pressure" of 4.9 (assuming O2 is narcotic).  
> Sticking with a max PO2 of 1.4, and a max PN2 of 3.9, the difference 
> *at any depth* in narcotic potential for an END of 130fsw, if you assume 
> O2 is narcotic, is .4 P(total narcotic gas), which is about 10 feet of
> Equivalent Air Depth.  And for 10ft of EAD, I'll bet most divers
> will opt for the deco advantage of the additional .4 PO2.
> 
> So basically, like I said, I think the whole O2 narcosis issue is 
> irrelevant to actual diving, and best left for academia.  Sad part
> is, it's easier to put the "O2 is narcotic" switch into the software
> than to include this long-winded explanation as to why I didn't.

Now that we've established that there is no functional difference between 
including and not including O2 in the narcosis computation, I think it's 
safe to say that this doe not need to be an option in the software 
calculations - the calculations should include the O2.  Why?

1. Slightly more conservative.

2. Easier math. e.g. for EAN there is no math, it's the same depth. For 
trimix, it's simply (PN2 + PO2 - 1) * 33 -- no conversions to equivalents 
of air factoring in 0.79.

Actually, in my opinion, it's *all* academic and of little practical 
value. Trying to predict the level of mental impairment based only on 
"equivalent air depth" ignores all the other factors that contribute to 
narcosis.

Aloha,
Rich

Richard Pyle
Ichthyology, Bishop Museum                deepreef@bi*.bi*.ha*.or*
1525 Bernice St.                          PH: (808) 848-4115
Honolulu, HI 96817-0916                   FAX: (808) 841-8968
       "The views are those of the sender and not of Bishop Museum"

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