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At 06:05 PM 28/2/99 -0500, Rich Lesperance wrote: >BillyW, > >Thanks for responding to that, you _did_ point out something I wasn't clear >about, but I have to respectfully disagree with you on another point. > >1) Regarding CNS O2 toxicity > >>>you got that ass-backwards<< >>> 2) In the water, if you run 1.6 for the whole dive, you >>>>are more likely to take a CNS hit before you'll sustain pulmonary >>>>damage.<< > >You are quite correct in that CNS 02 toxicity is _also_ time dependant. I >was extremely remiss in not noting that there is a CNS 02 'clock', and at a >pO2 of 1.6, the safe limit (NOAA) is considered to be 45 minutes. Anything >longer than that and the risk of CNS toxicity is unacceptably high. As you >even decrease the pO2 to even 1.5 or 1.4, your max exposure increases to 120 >and 150 min, respectively. Rich, as long as five years ago, it was common knowledge in the technical diving community that the NOAA standards were too liberal for our kinds of dives. Although NOAA considers 45 minutes at 1.6 to be 'safe', for the technical diver, 1.6 is only acceptable during resting deco, and for no longer than 20 - 25 minutes without breaking to a lower 02 gas. I remain uncomfortable with your declarations of 'safe limits' without mention of these important provisos. >>>in the chamber, where the likelihood of a CNS hit is lower, the >>>>administration of a USN table 6 treatment, which includes a total >>>>of 70 minutes at pp02 2.5 and a total of 2 hours at pp02 1.6 >>>>generally does NOT cause pulmonary oxygen toxicity. That's over >>>>three hours at 2.5 to 1.6 WITHOUT 'burning' up your lungs.<< > >Actually, it's 60 minutes (not counting ascent or descent time) at a pO2 of >2.8, and 2 hours at a pO2 of 1.9. I'm not counting the air breaks. You're right. That's my poor estimate from a graph, not a table. >Yes, the >likelihood of a CNS hit is lower, but more to the point, the consequences >are less severe - if a patient starts seizing, you just remove the mask and >have him go on air. More to the point, the advice that you were handing out referred to IN WATER DECOMPRESSION where there is no chance of 'just removing the mask'. You were responding to someone who asked about decoing on 50% at 22 metres. The point is you advised this person that " At a pO2 of 1.6, you are relatively safe from CNS O2 toxicity, but you are 'burning' up your lungs from pulmonary O2 toxicity." I pointed out that a table six, with three hours of higher than 1.6 p02s, generally does not cause pulmonary oxygen toxicity. >I would disagree with you on the statement that it generally does not cause >pulmonary O2 toxicity. OK. Go ahead. Disagree with me and the combined Navies of the English speaking world (an some of the other ones). >A standard TT6 with no extensions will cause the >patient to accrue 642 UPTDs. This is above and beyond any UPTDs he may have >incurred through his dive (hopefully he kept his pO2 respectable). The USN >lists the max UPTDs per 24 hour period as 1425, but Rutkowski (Recompression >Life Support Chamber Manual, 1990) sources Lambertson and Wright as >suggesting a limit of 615, unless serious DCI is present. >I can only speak for USN chamber operations practices, and what Rutkowski >teaches, but they generally agree that two TT6s may be run in a 24 hour >period, but again, this approaching heroic measures. Again, the limiting >factor is the PULMONARY toxicity. Two Table 6s? *TWO* Table 6s? That's a bit of a stretch Rich! > 642 UPTDs correlates to a 2-3% decrease in Vital Capacity (basically lung >elasticity, which is used to guage the effects of pulmonary O2 tox), whereas >1425 is a 10% decrease. That's _alot_. Short term, mild doses, it is self- >correcting within a day or so. It _can_ lead to fibrosis of the lungs, and >permanent disability, though, with longer exposures. Perhaps this is what >you meant to say, instead of it doesn't cause it at all. I didn't write "it doesn't cause it at all" I wrote "generally does not cause pulmonary oxygen toxicity." With single Table 6 treatments, oxygen toxicity is the exception not the rule. Furthermore, the degree of oxygen toxicity equivalent to a 2 percent decrease in vital capacity is very difficult to measure, completely reversible and ASYMPTOMATIC. >>>I think, if you are going to use these lists to think things >>>>thru, you should clearly state that your ideas are just >>>>speculation on your part. Otherwise, dangerous misconceptions >>>>get started and spread around.<< > >>> snip<< > >>>>Rich, if the consensus of smart people that you refer to ever >>>>did exist, it would not argue against using 50 and 100%. And >>>>it would not use the argument that "you are exposing your lungs >>>>to high pressures of O2 with less of a benefit, since you still >>>>have 50% N2 in the mix."<< > >My apologies if you feel I was spreading disinformation. >When I mentioned >"concensus of smart people", I was trying to say, in a humorous manner, that >while _I_ would not make a pronouncement on it, several other people have. >If you comb the archives, I think you will find a thread on this same >subject, that I read not too long ago. In it, people were labelled "strokes" >and "morons" for using 50/50 mixes to deco. I prefer to keep my discussions >professional, but I stand ready to be corrected if indeed, I am misinformed >about something. Rich. Do you do any diving requiring multiple decompression mixes? Have you had the oportunity to do mix dives with, say, air and 80% as deco gasses and compared the deco (and the trips to the chamber) with the same dives done on mix, 50% and 02? Or even 50% alone? I mean, do you do this stuff or just play around with the ideas? The reason I ask is because there are people on this list who actually do the deco and try the concepts that are discussed here. And I'd hate for someone to get bent because they adapted their deco to accommodate one or more of your misconceptions. > >>>50% is a useful decompression gas. Waiting till you are at 6 >>>metres before applying a high pp02 (1.6) as you would have to >>>with 100% as your only deco gas, does not make good deco sense. >>>You should take advantage of the oxygen window right where your >>>decompression goes from being depth dependent to time dependent. >>>50% does it nicely.<< >/ >OK, so you _are_ advocating using _both_ 50% and 100%? That seems to be at >odds with the other thread I mentioned, but like I said, I was attempting to >show that information as not being my own, but garnered from another >discussion. Are you using air or trimix? Perhaps my confusion stems from the >fact that the other thread was (I believe) in reference to deco after >trimix, where the diver may already have shifted to a gas richer in 02 >content for his ascent. Or maybe it was felt that the logistics of requiring >an extra cylinder wasn't worth it. I will attempt to find that in the >archives again. Let me know if you want me to send the message numbers. Rich, there's enough confusion here without invoking another thread. If I am ocean diving in the 50 - 70 metre range and can, for whatever reason, only use one deco gas, it will be the 50 starting at 22 metres because by the time one gets onto the 02 at 6 metres, the success or otherwise of the decompression has already been determined. I'd rather get onto the 50 at 22 metres. O2 you can breath on the boat after. On deeper dives, I think you need the 50 AND the 02. > >>>Rich, there are some very good texts on decompression theory >>and practice which will help clear up a lot of your confusion..... >>Also check the archives here. George Irvine has consistently provided a >>stream of very useful information based on the WKPP's successful regimen for >>decompressing from deep >>exposures, long and short - the stuff that really works.<< > >Already read them, in addition to some of Lambertson's stuff, and Bove. >As >for the stuff Irvine has posted on the deco profiles of the WKPP, I have not >found that, but I _will_ go back and look, because it sounds like exactly >the type of info I'm interested in (for intellectual purposes only, I have >not done a deco dive in awhile). Just as well. rgrds billyw -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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