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On Thu, 23 Feb 1995, Richard Servadei wrote: > Could you please describe the possible relationship between N2 narcosis and > HPNS . O.K. Well...HPNS seems to be a function of ambient pressure (not neccessarily helium...although I think there is some evidence that the actual helium molecules might partly cause the effect...) Basically, very high ambient pressures results in an excitatory (sp?) effect on the central nervous system. Elevated inspired partial pressures of N2 have a depressive effect on the CNS (narcosis). Theory goes: the depressive effect of narcosis counteracts the excitatory effect of HPNS. This is why trimix seems to result in reduced HPNS symptoms in commercial divers (Bennett did the work on this). Compression (descent) rate also has an effect on HPNS: slower descents yield reduced HPNS (nobody has a clue why, as far as I am aware). So, saturation divers typically take 2 days to go from the surface to 1000fsw. Tech divers, of course, decend at a much greater rate, so HPNS starts to become a problem beyond about 600 feet (according to the few tech divers who have made dives to such depths) and the symptoms may be potentially very severe. Sheck Exley and Jim Bowden were doing a series of very deep dives in Mexico during their attempts at 1000fsw. I had a long conversation with Sheck & Jim at Tek94, and one of the things we talked about was their ways of dealing with HPNS. Their approach was to use relatively nitrogen-rich trimixes for their bottom gas. They were aiming at an equivalent narcosis depth of about 300 feet on air (to get as much nitrogen as possible in the mix). Naturally, they did a bunch of deep-air work-up dives to build up their "short-term" tolerance to narcosis, so they could manage under an EAD of 300fsw. Sheck told me he noticed that his HPNS symptoms seemed worst when his "short-term" narcosis tolerence levels were high, and that when he tried the ultra-deep trimix dives without prior deep-air work-up dives (i.e., low "short-term" narcosis tolerance), the HPNS was not as bad. If, indeed, this is the case (they had a very small sample-size of dives), then that means there may be a true physiological adaptation to narcosis (at least for the "short-term" component), because if acquired narcosis tolerance was simply a "learned ability to deal with an impaired brain", then the extent of this tolerance should not have any effect on HPNS (because the physiological depressive effect of the nitrogen would be the same regardless of the extent of tolerance). I spoke with Jim Bowden a bit about this at Tek95 and asked him his thoughts. He said he definitly does feel signs of HPNS on his very deep dives, but he has not notcied the effect that Sheck did (i.e., decreased HPNS with decreased "short-term" narcosis tolerance), so he continues to build-up his "short-term" tolerance with deep air dives prior to an ultra-deep trimix push. I plan to be reaching potential HPNS depths this summer, so if I notice any of these effects, I'll be sure to post my experiences... Hope this sufficiently (and not overly) answers your question. Aloha, Rich deepreef@bi*.bi*.ha*.or*
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