CO2 and nitrogen are almost certainly additive when it comes to narcosis. Narcosis can be manifested by uneasiness or anxiety anyway, and if you add CO2, even WITHOUT any narcotic contribution from the gas, you heighten the anxiety. The body has a very strong reaction to elevated CO2 in most circumstances, because breathing is something your deep brainstem thinks is pretty important, and elevated CO2 means you aren't doing enough of it . . .
I think it's unfortunate that, in any discussion of narcosis, people focus on finding some clear abnormal feeling to identify that they are narced. Yes, severe narcosis can be manifested by euphoria, hallucinations, reduced responsiveness, unexplained anxiety and the like. But some of the scariest stories of narcosis, to me, are the ones where the diver is not aware of anything awry until he simply fails to perform up to usual standard.
My Fundies instructor told us a story that really made an impact on me. He was in Croatia, and his girlfriend wanted to dive. The only dive he could find near where he was, was a dive on a wreck at 100 feet. The only gas available was air. He decided to go along with her and do it. He was in a set of doubles. They descended on the wreck and reached the 100 foot level, where for the first time in his diving career, he had a big leak from behind him. He reached back and closed his valve, and the leak didn't stop, so he isolated and thumbed the dive. They did their first stop coming up, and he decided to check the valve again. Turned out it was open. Either he didn't close it, or he reopened it and didn't remember doing so -- but either way, this man who taught this procedure on a regular basis had messed it up without being one whit aware that he was doing so.
I have written here about a couple of episodes of just utter stupidity on my part, which have the common denominator that they have all occurred at or close to 100 feet. In none of them did I feel drunk or fuzzy or otherwise identifiably abnormal -- I just didn't process information the way I normally do, and that led to what could have been major mistakes.
As I said at the beginning of the thread, the difficulties I've seen with people remembering sequences of tasks and procedures, even in shallow water, has made me wonder from the beginning whether there is more cognitive impairment in shallow depths than we credit. Whether that's because of nitrogen, or because more people are CO2 retainers underwater than realize it (comfortable CO2 retention has been documented in divers before) I don't know. But I think you're missing the boat, if you are only diagnosing narcosis when someone feels as though he is drunk. That's just my opinion, based on my experiences of the last six years.
