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Gentlemen, My original question and concerns still stand. I brought this up because of several things. One, there are divers who dive at altitude using nitrox mixtures -- or at least are claiming that they do -- and two, I would like to have as much understanding as possible of this mixed gas technology (nitrox) and its possible applications related to diving at high altitudes. I have several other references that I would like to quote reguarding hypoxia when diving at high altitudes and with air as the breathing gas. My point is that air is 21% oxygen, and there are real -- or apparently real considerations -- when diving at altitude with the possible onset of hypoxia. 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. 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. REFERENCES: October 1991 -- NOAA Diving Manual: 10.12.6 "A diver surfacing from an altitude dive is moving from a breathing gas in which the oxygen partial pressure is relatively high to an atmosphere in which it is low. As a result, the diver may experience symptoms of hypoxia and breathing difficulty for a period after the dive". This is very similar to what Lippmann writes in his books titled Deeper Into Diving and The Essentials of Deeper Sport Diving. The reference to the "breathing gas" is air. Okay, some people think that NOAA is not to be taken seriously. U.S. Navy Diving Manual, Volume 1 (Air Diving) Revision 3, 15 February 1993. 3-5.1.1 Symptoms of Hypoxia. "Severe hypoxia will stop the normal function of any tissue cell in the body and will eventually kill it, but the cells of brain tissue are by far the most susceptible to its effects. Unconsciousness and death can occur from brain hypoxia before the effects on other tissues become very prominent. If hypoxia develops gradually, symptoms of interference with brain functions will appear. Symptoms of hypoxia include: o Lack of concetration o Lack of muscle control o Inability to perform delicate or skill-requiring tasks o Drowsiness o Weakness o Loss of consciousness The partial pressure of oxygen determines whether the amount of oxygen in a breathing medium is adequate. For example, air contains about 21 percent oxygen and thus provides an oxygen partial pressure of about 0.21 ata at the surface. This is ample, but a drop to 0.14 ata will cause the onset of hypoxic symptoms on the surface. If the ppO2 goes as low as 0.11 ata at the surface, most individuals will become hypoxic to the point of being nearly helpless. Consciousness is usually lost at about 0.10 ata, and at much below this level, permanent brain damage and death will probably result". And so on .... My comments: Okay. Here again, this is written with respect to the breathing air, not EANx. The reference to a ppO2 of 0.21 ata at the surface has to be at sea level -- assuming -- that this is an open water dive. If the ppO2 decreases to 0.14 ata at the surface, this would be equivalent to an calculated altitude pressure ratio of 0.6666 which puts the diver very close to an altitude of 11,000 feet above mean sea level. Other reading and research that I have done, appears to indicate that decreases in ppO2 that are slightly below 0.16 ata could begin the onset of hypoxic symptoms; 0.16 ata calculates out to a little over 7,000 feet altitude. I think that another factor is water temperature. Most lakes are very cold at altitude compared to elevations at or near sea level in the torrid and temperate zones. 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. There has to be some one, some where, who completely understands and has this information. Where are you? Joan
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