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>Posted on 25 Sep 1996 at 15:36:46 by Peter Heseltine >I think that Phillip is right. You are not going to get a handle on >nitrogen toxicity until you start thinking about the dose in relationship >to body size. >PS. The above will be reformatted in docspeak and transmittted to the >hyperbaric list. Report to follow to those that reply to this. Like last time this doesn't make any sense to me. The vapor pressure of N2 in the blood (a measure of the *concentration* of N2 in the blood) is equal to ppN2 of the gas in the lungs so the drug "dosage" is the same *irrespective* of the breathing rate and/or body mass. >Date: Sat, 21 Sep 1996 11:35:42 -0500 >From: Phillip Finch <pfinch@so*.ne*> >To: "Peter N.R. Heseltine" <heseltin@hs*.us*.ed*> >Subject: Deco theory questions > >1) Does the volume of a given gas consumed during a dive affect one's >saturation state? > >Seems to me that the answer to that one should be "yes," though every dive >table and dive computer I have seen assumes otherwise. We all know the >benefits of EANx at the right depth. By breathing a .36 mixture I am >consuming approximately 80 per cent of the absolute amount of nitrogen that >I would be consuming if I were breathing normox. And that difference >significantly affects deco and saturation status. But would I not enjoy some >of the same benefits if I reduced my air consumption from, say 2.0 cu.ft/min >to 1.6 cu.ft/min? You consume O2 but not N2 and/or other gases used in the mixture. I.e., the concentration of N2 in your lungs is for all practical purposes constant over one breath irrespective of how fast or slow you are breathing (assuming the depth is fixed). The *concentration* of N2 in the lungs equals the concentration of N2 in your blood (after it leaves the lungs) and it is the N2 gradients between your blood and your various tissues (theoretical tissues that is) that determine if you are "on" or "off" gassing. I.e., if the amount of N2 removed or added to your lungs during one breath is negligible, your rate of breathing is irrelevant because the ppN2 in the lungs has no dependence upon your respiratory rate. By controlling the ppN2 (EANxx) in the breathing gas you control the N2 gradients between the blood and the theoretical tissue groups and therefore the rate at which your tissues take up N2. With less N2 in the gas mix, the less N2 you have to get rid of during the decompression. > >2) Partial pressure within a tissue, relative to ambient pressure, is a >crucial determinant in Haldanean theory. But doesn't the actual volume of a >given compartment--blood, for example--determine partial pressure within >that compartment, for a given absolute quantity of dissolved gas? It would if the the concept of an "absolute quantity" made sense. Gas exchange is propelled by differences in concentrations so I my opinion the total amount of drug is meaningless and can only be found by first knowing the concentration(s) and then multiplying by the volume(s). >And doesn't the volume of a given tissue compartment vary significantly >between individuals, depending on height, weight, and body type? >Shouldn't a comprehensive deco model attempt to account for these >differences? It would be desirable if you could prove that some of these parameters made a measurable difference. >Certain combinations of these variables can place an individual well >outside the norm which all deco models seem to assume. My breathing >is considerably more efficient than it was two years ago--at the >surface. On the other hand these variables might be irrelevant. John
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