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Richard Pyle wrote: > > I *know* I am going to regret sending this. But it happens to be a > slow > day, and after skimming through the recent archives, I see some > threads > of discussion that starve for rational thought. Let's look at a few > of > them briefly: > > 1. Death of Hennie Pretorius > > I knew nothing of this incident until I glanced through the archives > today. First, Kudos to Dennis Harding for maintianing a rational tone > despite irrational attacks. Getting to the point: The bulk of this > discussion seems to focus on nit-picky little details that may or may > not > warrant heated (or otherwise) discussion, but do not in any way (even > from the most convoluted perspective) have any bearing on the cause of > his death. From what I have read, I have absolutely no idea what > caused > the observed symptoms. However, it is clear we can eliminate a few of > the > going hypotheses: > > Did he die because the instructor was from one agency but not another? > Did he die because the instructor was a smoker? > Did he die because he was 30 lbs overweight? > Did he die because he went to 105msw instead of 90msw? > Did he die because he breathed 32% O2 during deco? > Did he die because he breathed 80/20? > > I challange anyone on this list to present a rational explanation of > how > any of these factors would have cause the observed symptoms (i.e., > sudden > and dramatic neurological symptoms upon surfacing, after relatively > long > deco with no apparent indications of a problem). > > The only possible considerations I can see are that he had a history > of > neurological barotrauma, and that the chamber treatment may have been > suboptimal. > > As to the former, although I believe that a valid case can be made > that > history of neurological barotrauma should be considered a > contraindication to > deep diving; if we all followed that advice, many of the more > outspoken > individuals on this list (myself included) would have an assortment of > rebreathers, scooters, lights, double 104's, and other scuba gear to > sell. > > As to the latter, regardless of the wisdom of the chamber treatment > administered, that still doesn't explain the sudden onset of severe > neurological symptoms in the absence of any gross decompression > violations. > > Why is this important? When I was a kid, my dad would always hit me > with > the "I hope you learned somthing from this bad experience" line. I > understood where he was coming from and all, but the problem was, > because > I was such a brilliant little kid, most of my bad experiences were of > the > "sh*t happens" type, rather than "gosh, that was stupid of me - I > won't > do that again" type. As much as I would want to learn something from > it, > there really wasn't anything of practical value to learn, because the > circumstances leading up to the bad experience were either > ultra-particular to that specific event, or were cryptic and > unidentifiable. Don't get me wrong - whenever something bad happens > to > anyone, we should all try to learn as much from it as possible, so > that > the collective body of all these bad experiences will ultimately > reveal > trends to teach us how to reduce our chances of getting hurt. > > However, in this particular case, until we can get a better handle on > what actually caused or contributed to or otherwise led to his death, > we > don't really have much to learn other than "sh*t happens"; which we > already know. > > 2. 80/20 > > I've personally never breathed 80/20 on an open-circuit dive. However, > the arguments I've seen on *both* sides of this debate range from > "stretching it" to "truly rediculous". Guys: it's a gas. It happens > to > contain 80% O2, 20% N2. It is neither the cure to bad buoyancy > control, > nor a notarized death certificate. Neither side of this argument will > ever "win", because there is not enough substance to argue about. The > only point that makes sense to me is the one raised by Joel that you > can > get more total gas molecules in a given cylinder without a booster, > which > might convey logistical advantages for a 30-foot stop. (Joel doesn't > get > credit, though, because the tone of his message makes it sound like he > intended this as an example against 80/20, when it looks to me more > like > an example in favor of it). > > 3. Fudge Factors in deco calculations. > > Since before Abyss ever existed - when it was just a topic of > discussion on > the CompuServe scuba forum - I have been arguing with Chris about the > "meaninglessness" of all these super-hyper complex fudge factors, and > how > there was no real basis for the math behind any of them. I invented > the term > "titanium doorstop" in reference to the following analogy: why go to > the > trouble and expense to use high-performance machining technology to > manufacture a wedge of titanium to within 0.0001" tolerance -- so it > can be > used as a doorstop? Especially when a random chunk of wood will do the > job > just as well (if not better). The point here is that decompression > is, > always has been, and almost certainly always will be a very "fuzzy", > imprecise practice at its core; so why try to micro-manage the numbers > with > all these gee-whiz features that mean nothing? > > Chris' response has generally been twofold: 1) These fudge factors > only > affect the deco slightly; and 2) in most cases, they lead to longer > decompression times (which presumably means lower probability of DCI > symptoms). After giving it more thought, I finally stopped arguing > with > Chris for these reasons, and for the simple reason that even though > the > fudge factors don't necessarily reflect physiological reality, neither > do compartment-based deco models in general....so what's the diff? > > Also, I trust everyone realizes that the practice of deep stops - > which > seem to be gaining wider and wider acceptance these days - is a > similar > "unfounded" fudge factor. There are theoretical reasons why it should > help, but my particular method for doing it is an off-the-cuff SWAG. > There are also theoretical (and even empirical) reasons why smoking, > obesity, cold-water conditions, and age all may affect one's > disposition > to experiencing DCI symptoms; and Chris has developed his own > off-the-cuff SWAG methods for modifying the deco accordingly. In fact, > if I'm not mistaken, the option of including deep stops is yet another > one of the "unfounded" fudge factor options included in Abyss. If > these > factors don't give you a warm & fuzzy feeling, then don't use them. > If > they do, they probably won't help - but they even more probably won't > hurt. > > 4. Constant PO2 deco > > This one is always a riot to me. There are two issues: "jacking up > PO2 > on deco", and "constant-PO2 deco". The former is something that people > who dive with fully-closed rebreathers seldom do. The latter is > something that people without fully-closed rebreathers are not capable > of > doing (at least not within any practical logistical limits). So, the > "have-nots" tend to lump the two together to justify their choice of > diving equipment, because they can make a valid case about the former, > but have absolutely no experience whatsoever with the latter. > > Here's the score, guys: > > I have done a lot of deep decompression diving with air only (all the > way to > the surface). I have done a lot of deep decompression diving with > open-circuit mixed-gas and nitrox & O2 on deco. I have done a lot of > deep > decompression diving with constant-PO2 mixed gas. > > The air diving/deco thing really sucks. I never knew how much it > sucked > until I started using 100% O2 on deco after deep air dives. I > *really* > started to understand how much it sucked when I started breathing > helium > deep with nitrox & O2 on deco. However, I only actually grasped the > full > magnitute of how much air diving/deco sucked when I started diving > with > a constant PO2 rebreather. > > The weenie approach to this topic is to bust out some deco model and > compare deco profiles for OC trimix/nitrox/O2 and CC constant PO2 > diving > and say "hey look - there really isn't much difference!" Folks, the > difference is not in the numbers that a deco model spits out. The > difference is in how you feel after a dive. The CC-divers out there > already know what I'm talking about, but the rest will just have to > either take my word for it, or ignore me. If you started diving deep > air > and then switched to trimix & elevated PO2 on deco, you probably > noticed > the dramatic reduction of post-dive fatigue and other associated > symptoms. Well, when you switch to constant PO2, you'll notice the > same > level of improvement. > > As for the argument that constant PO2 will cause you to convulse from > O2 > toxicity, you'll have to show me the numbers of all those divers > convulsing at 1.4 or 1.3, which is where I keep the PO2 for the entire > dive. > > 5. CNS% > > I have tried and tried and tried and tried to discover the source of > this > concept, and so far it seems to fall into the "high PN2 causes RBC > rigidity" category. You don't see me publicly bashing the concept, > however, because people who choose to believe in it will tend to lower > their operational PO2 values and introduce low-PO2 breaks during long > exposures (both of which I believe to be wise practices). So, like the > Deompression Program Fudge Factors topic, while I don't see any merit > in > it, I see more good than harm - so I leave it alone. > > That's all I'm going to whine about today. I haven't even had the > courage to > read any of the messages with "Helium" in the subject line, for fear > of what > I might find. > > The general themes I'm seeing throughout all these threads are > "titanium > doorstopism" and "precision causation delusion". I already described > the > first. The second is in reference to the all-too-frequent knee-jerk > conclusion (delusion) that Accident "X" was a direct result of Cause > "Z" (or > the variation: Practice "Z" will lead to Accident "X") In almost every > such > conclusion (delusion) I've seen, the scenario seems to be that the > person > forming the conclusion (delusion) probably read somewhere or was told > by > someone that circumstance "Z" might in some way indirectly affect > outcome "X" > in a few situations. Unfortunately, without the basic understanding of > the > factors involved (rather than rote memorization of stuff written in > books and > spoken by "experts"), the delusion becomes conclusion. A case of "a > little knowledge is a dangerous thing". > > Kind of like the "HFS" dive instructor who once told my 5'7", 110-lb > rock-climbing wife that she was more likely to get bent than he was, > because > women, on average, have more body fat. > > 'Nuff said. > > Now...don't make me come back here again! :-) > > Aloha, > Rich > > Richard Pyle > Ichthyology, Bishop Museum > deepreef@bi*.bi*.ha*.or* > 1525 Bernice St. PH: (808) 848-4115 > Honolulu, HI 96817-0916 FAX: (808) 841-8968 > "The views are those of the sender and not of Bishop Museum" > > -- > Send mail for the `techdiver' mailing list to > `techdiver@aquanaut.com'. > Send subscribe/unsubscribe requests to > `techdiver-request@aquanaut.com'. Hi Rich Thanks for the best post on this subject so far. I was present at the accident investigation meeting. There is no clear answer to the death of Hennie Pretorius. The autopsy did not show any sign of a patent foramen ovale, which was our first choice of primary cause. Death was caused by cerebral oedema, which was compounded by the oxygen therapy. In other words, this was a situation where you are caught between conflicting interests. You need the oxygen to keep everything alive, and to maximise the inert gas gradients, but it contributes to the oedema. He was given anti-inflamatory drugs (I'm not sure now which), but to no avail. In the end, the only clear message that came out of this very unfortunate incident, was that Hennie had a history of idiopathic DCS, which should have precluded him from continuing any form of technical diving. However, I don't know about the USA, but here in SA we are not bound (like aviators) to stop diving on medical advice; the final decision lies with the diver. Hennie accepted the risk and paid for it. Regards Johnny -- Johnny van der Walt Chairman - CMAS-ISA 244 Anderson Street Brooklyn Pretoria South Africa 0181 +27 12 362-2035 (tel home) +27 12 529-8025 (tel work) +27 12 529-8305 (fax work) -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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