Posted by Kendall Raine on June 15, 2001 at 12:07:14:
In Reply to: Re: STEVE- Meyer-Overton hypothosis posted by Steve on June 14, 2001 at 23:01:17:
Steve;
I reread Chapter 7 of Physiology and Medicine of Diving, 4th Edition, last night. I don't have time for lengthy quotes today, but will articulate my conclusions from the reading. These are my conclusions alone and anyone who doesn't like them can read the chapter themselves. When I've quoted directly, I've used quotation marks. :-) I'm not saying, either, that this is the last word on anything. The 4th edition was published in 1993 and lots more research has been done since then. Here goes:
1) Oxygen may act to "potentiate" the narcotic properties of nitrogen. Raising the partial pressure of oxygen while holding the nitrogen partial pressure constant can increase narcosis. Hence, END EANx 32 can be greater than END EANx 25 for a particular depth. This may be related to oxygen acting in concert with CO2, but the chapter doesn't say. "... raised partial pressure of oxygen alone can indeed produce narcotic effects and anaesthesia." The claim that Nitrox lowers narcosis versus air for any given depth is not supported.
2) Strong evidence exists to support Meyer-Overton. While it is a 100 year old theory, recent research supports the construct that the lipid solubility of an inert gas is the strongest, but not only, correlate to that inert gas's narcotic potential. This is not true of helium for other reasons.
3) Pure Oxygen by itself does not produce narcotic effect at depths shallower than 33 fsw (2.0 ata). This is obvious and not disputed. The chapter concludes that because of this, oxygen alone can be ruled out at the cause of narcosis. Whether oxygen exerts independent narcotic effect in the presence of inert gasses, or simply potentiates the narcotic property of inert gasses, is not discussed.
So, where are we? There is nothing in Chapter 7 which supports the idea that oxygen, based upon its physical characteristics alone, is more narcotic than nitrogen, or any other inert gas for that matter. The chapter title is, afterall "Inert Gas Narcosis." There is evidence that oxygen, perhaps with CO2 or other synergistic mechanism, makes nitrogen more narcotic and that the claims of ANDI/IANTD and others that Nitrox lowers narcosis are not correct.
As for implications in my diving, I'll continue to run my PO2's low, use helium shallow, assume no amelioration of narcosis (e.g. assume O2 is narcotic) from higher FO2's in my target END planning, avoid air when possible, say my prayers and brush my teeth every night. Thank you for listening.