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I apologize if this message gets delivered twice (or four times for people who subscribe to both lists)....from my end it looks as though my first attempt failed. I see that Peter Bennett talks about the importance of ascent rates in his editorial in the latest "Alert Diver". Many of us have known for a long time that ascent rates are much more critical than most literature would have us believe. A number of us have also observed apparent reduction in DCI incidence (or "proto-DCI" incidence) after deep dives when we include additional short-duration "deep" stops on decompression (i.e., hang out at 120fsw for a minute even if our ceiling is 60fsw). On a related issue, I've seen some talk lately of the idea that actual ascent rates can be faster in deeper depths due to the reduced volumetric pressure differential for a given vertical distance ascended (i.e., ascending from 99fsw to 33fsw results in a halving of ambient pressure/doubling of volume; which is the same effect as ascending from 33fsw to the surface -- only half the vertical distance). The upshot being that a diver should be able to ascend at twice the rate from 99fsw to 33fsw that he/she could from 33fsw to the surface. By this logic, a conservative rate is one halving of ambient pressure over the span of 1 minute. I've been giving a lot of thought to these concepts and would like to submit some ideas to these lists for feedback. Recommendations for ascent patterns (i.e., decompression) only specify ascent rates for continuous ascents, and time at stops for decompression stops. What I have never seen is a limit for maximum pressure differential traversed without any stop at all. Envision a diver, having spent sufficient time at 231 fsw (8 ATA) such that his/her decompression computer/tables requires an initial stop at, say, 40fsw. (i.e., a bounce dive typical of what many professional divers and tech-diver-types might do). Even if the diver ascends at a proportionally slow rate (one halving of ambient pressure per minute or less), he/she still traverses the distance from the bottom to the first decompression stop in a straight ascent lasting as little as 2 minutes for nearly two halvings of ambient pressure. Now...many contemporary scholars subscribe to the "micronuclei" concept of bubble formation (or some derivative thereof). Is it possible that micronuclei formation/growth could be dictated largely by "fast tissues" (i.e. high-perfusion tissues: blood, muscle, etc)? If so, then here's a hypothesis: What if the 'halving-of-ambient-pressure-per-minute' rate is only conservative over relatively short distances (e.g., a single halving). What if this rate is not slow enough to allow the fastest tissues, which are at saturation at the initiation of an ascent, to "catch-up" to the total drop in ambient pressure when that 'total drop' exceeds a certain amount? For example, the diver described above traverses at total pressure drop of nearly 6 ATM during his/her initial ascent to the first decompression stop, which at the depths mentioned would be nearly two halvings of ambient pressure in a single shot. Would this diver have a reduced probability of DCI if he/she stopped at 100fsw for a minute or two to allow the fastest tissues to "catch-up"? Looking only at decompression math, that diver should have a higher probability of DCI due to the additional ongasing that occurs during this 100fsw stop (assuming he/she did not add that time to their total bottom time). If this is true, then there are two options for the diver: slow the ascent rate down even further, or introduce additional stops at each pressure halving (or whatever quantity ambient pressure drop) to allow the fast tissues to "catch-up". I suspect it is overtly clear that I am not a hyperbaric specialist. However, I believe I have some empirical evidence supporting my hypothesis. First, there is this unquantified observation by experienced divers that maintaining slow ascent rates even during deep-water ascents seems to reduce the probability of DCI. Second, there is the similarly unquantified observation by many deep divers that throwing in additional deep stops reduces incidence of DCI (or "proto-DCI"). Perhaps most compelling of all is the undisputed effectiveness of "safety stops" to reduce incidence of DCI after otherwise-no-decompression exposures. The implications to decompression computations should be obvious. First, it might be best if computers and/or tables maintained a 33ft/min ascent rate even during deep ascents, because deep ascents are the ones likely to include the greatest pressure differential in a "single shot". Alternatively (or additionally), perhaps there should be a limit to the total ambient pressure drop traversed in a "single shot" without a one- or two-minute stop to allow fast tissues to "catch-up" (subsequent decompression stops could be calculated to take the additional gas loading during such "deep stops" into account). The profiles most affected by such modifications would be deep bounce dives (greatest total pressure drop without required decompression stops). In escence, these deep stops would be like "safety stops" before reaching the decompression ceiling. Just some food for thought. I would appreciate feedback from anyone... Aloha, Richard Pyle deepreef@bi*.bi*.ha*.or*
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