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Doug, > 0 to 150meters 3500meters 7500meters >Ambient Pressure 1.0 Bar .68 Bar .38 Bar >Pressure of Inspired Oxygen .196 Bar .129 Bar .066 Bar >Pressure of Inspired Nitrogen .74 Bar .487 Bar .250 Bar >1 Bar = 1.01325 ATA >The fraction of O2 can be calculated from the data provided. > 0 to 150meters 3500meters 7500meters >Fraction of Oxygen .196 .189 .173 Ok, I'll accept this fact, however, the difference between .189 and .196 is for all practical purses non existant, it just won't make that big a difference. Personal physiological differences will greatly overshadow this. The 7500 m data, while interesting is largely irrelevant for this discussions since no one is diving at 25000' (other than the mile high club - and that's only muff). Joan later said... >If this is all true, then I feel that I need clarification in the use >of EANx mixtures for high altitude diving, because EANx mixtures have >a higher partial pressure of O2 relative to air. Would the use of >nitrox exacerbate (do to higher ppO2) the hypoxia phenomenon? If so, >because of the insidious nature of hypoxia, this could prove to be >fatal. I think that people need to *think* a little here. Let's compare two dives from an O2 aspect... Let's assume .75ATA pressure (remember, 3500m was .68bar). #1) - Air to 33' is now approximately .55 PPO2 which came from (.20 * (2 for the 66' +.75 from ambient)) #2) - EAN 40 to 20' is now approximately .55 PPO2 which came from (.40 * (.625 for the 20' dive + .75 from ambient) From an Oxygen perspective these dives are identical (not considering nitrogen or other inert gas loading issues for not). >Or has this altitude/hypoxia relationship been an assumption and >simply has been repeatedly stated without any real basis in fact. >That is a little far fetched, but it occurs to me that it could be >possible. I have not seen ANY real physiological studies cited in any literature. Unless someone can point me to such I think it's time for someone to get out the kneepads :^) >If this is all true, then I feel that I need clarification in the >use of EANx mixtures for high altitude diving, because >EANx mixtures have a higher partial pressure of O2 >relative to air. Would the use of nitrox exacerbate (do to >higher ppO2) the hypoxia phenomenon? If so, because >of the insidious nature of hypoxia, this could prove to be >fatal. One would normally conclude that *IF* we assume a potential for hypoxia then continuing breathing EAN for some period AFTER the dive could only help the "transition" (whatever the hell that means) to normal post dive activity. YES, you're right, I DON'T KNOW, but you know what - I doubt ANYONE DOES!!! As for the NOAA diving manual I think that 10.12.6 is USELESS, it doesn't define what altitude they're talking about and does NOT define the ranges of PPO2 of concern. I wouldn't bother buying a diving manual from these guys if it's all written like this. At least the NAVY manual gives specific PPO2 for the onset of various symptoms. >Cooling affects blood perfusion and slows down the transport of oxygen >to tissues. This could also contribute to the onset of hypoxia on >ascent to low ppO2 at the surface. What effect does breathing EANx at >depth, which means breathing higher than normal ppO2, and then >surfacing into very low ppO2s have on this process? >What also caught my attention is that in the very first paragraph it >appears that the brain is incredibly more sensitive and therefore more >susceptible to being damaged by the onset of hypoxia before other >tissues confirm the event. Circulation to the heart, brain, kidneys, and liver are generally NOT affected by moderate amounts of cooling (reducing blood flow to these organs are the "last ditch" efforts and won't be done unless you're near death) so I would believe that the effects of hypoxia really are due to the low PPO2 (remember, PPO2 in the lungs is less than the inspired air, but arterial O2 is basically in equilibrium with this value). >There has to be some one, some where, who completely understands and >has this information. Where are you? This is where you're wrong - tables and procedures have all been set up quite frequently by observation and experiment with what works and what doesn't. There are MANY things about the human body that we don't understand completely and since this is one of the less well studied areas I'd be willing to be that no one understands it completely - hell - they don't even understand DCS fully and that HAS been studied!!! -Carl-
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