The only thing I have on the subject is a bunch of notes collected over the years. I'd like to say that there are those that do and those that talk with regards to subjects like this. I tend to listen to those that do. Science doesn't always have the answers -- even Dr. Deco will concede that.
Enjoy!
Mike
-------------
I just found an article in Dive Training (Feb) that reports on a workshop with many key diving experts and, among others, AAUS and DAN, examining reverse dive profiles. I'm quoting the results verbatim:
Findings and Conclusion of the Smithsonian Institution Reverse Dive Profiles Workshop, Washington, DC, October 29-30, 1999
Findings:
- Historically, neither the U.S. Navy nor the commercial sector has prohibited reverse dive profiles.
- Reverse dive profiles are being performed in recreational, scientific, commercial and military diving.
- The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates an increased risk of DCS.
- No convincing evidence that reverse dive profiles within the no decompression limits lead to a measurable increase in the risk of DCS was presented.
Conclusion:
We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dive profiles less than 40 meters sea water/130fsw and depth differentials less than 12 meters sea water/40fsw.
--Dive Training Magazine, February 2000 issue, page 46
------------------------------
Here's another Tranmer-ism from the other night. Simon mentioned that at the recent reverse profile workshop, the starting point was "no one has a satisfactory reason for why we shouldn't perform reverse profile dives".
I brought up the issue of dislodging bubbles trapped in the lung capillary beds, to which Simon replied that reverse profiling refers to performing a second dive of greater depth after a correct surface interval, where bubbles in the lungs are no longer an issue. My concern was more related to zig-zag profiles on the same dive. Fair enough.
The question is, once I have surfaced, how long does it take the 'lung' bubbles to diffuse to a non-issue? Obviously, 'it depends', but has there been any research into this?
What triggered this was reading through the BSAC incident reports, I noticed at least a couple of incidents where a diver had made a shallow first dive (an aborted first attempt) followed by the correct deeper dive after a matter of minutes, and had been hit, even though the dives appeared well within required deco.
Ian
Reverse dive profiles are pretty interesting but the evidence seems largely anectodal. Erik or Simon, did you attend this workshop? There are an array of reasons that this may be an issue. I doubt we can completely discount bubbles passing through the lungs but bubble passing is likely to occur even on a shallow second dive. Of course, the inital dive may activate microbubbles that stabilize and seed bubbles for the next deeper dive where the gas load is higher. There are several interesting theories that play with the idea. As for the bubbles in the lungs, Art et al do you have anything interesting to say here? We did find that lung filtration rates appeared be very high in our doppler studies. Coming out of the water with a 3-4 score we found that on some of us this could drop wihin 30 min to 0.
JJ
>
This is some info from Erik's reverse profile workshop.
JJ
John Lewis and Glen Egstrom made presentations in which the historical roots of the prohibition against reverse dive profiles was investigated. It turns out that it probably had less to do with safety issues and more to do with "optimizing" bottom time over a series of dives. This comes from gas loading considerations which dictate
that more useable bottom time will be achieved by doing a deep dive first, followed by a shallow dive, rather than the other way around.
What is quite remarkable about the issue of reverse profiles is that nobody seems to have a clear answer as to why this prohibition is in place and where it came from. Apparently the rule against reverse profiles started for reasons of bottom time efficiency and then, over time, got distorted into a myth about diving safety.
This is not to say, however, that there have been no incidences of DCS with reverse profiles. I analyzed one such profile on this List before the Workshop and Till Mutzbauer analyzed two such cases at theWorkshop in his presentation. A common feature of these incidents is that while one diver gets hit pretty hard, his/her dive buddies go unaffected when doing the same exact profile.
In his presentation, Bill Hamilton provided an overview of the reverse dive profiles situation and pointed out that repetitive diving, especially deep repetitive diving, is a core issue. Reverse profiles are commonly undertaken in recreational, commercial, and scientific diving without incident.
A large portion of the Workshop was devoted to review of actual dive data in search of a "smoking gun" that would support the premise that reverse dive profiles are inherently dangerous.
Presenters in this effort were:
Paul Weathersby and Wayne Gerth - review of Navy/NMRI diving data
Richard Vann and Petar Denoble - review of DAN diving data
Karl Huggins - review of Catalina Hyperbaric Chamber diving data
Jon Hardy - review of sport diving practices and outcomes
Peter Mueller - review of European diving data
Terry Overland - review of commercial diving data
Drew Richardson and Karl Shreeves - review of PADI approach/data
Bruce Wienke - review of NAUI approach/data
Duke Scott - review of YMCA approach/data
Edward Maney - review of AAUS approaches/data
It was fairly obvious to all there that the diving data presented DID NOT point to any particular problems with reverse dive profiles in recreational, commercial, military, or scientific diving. It was also apparent that divers are doing reverse profiles all the time without incident. When ocassional incidences of DCS did occur with reverse profiles, it was at a statistically lower rate than for forward profiles. It appears that decompression algorithms and dive computers are adequately handling the issue of reverse dive profiles in the field.
There is no basis in diving experience to draw the conclusion that reverse profiles are inherently more dangerous than forward profiles. Accordingly, this statement (in different wording of course) is included in the Findings of the Workshop. There was essentially unanimous agreement about this point among the participants.
The discussion got a little heated when it came time to come up with a Conclusion or Recommendations. Several of the folks present who work with bubble models had serious reservations about a "complete retraction" of warnings against doing reverse dive profiles. I made comments to this effect as did Bruce Wienke, David Yount, Valerie Flook, Michael Gernhardt, and a few others. In other words, the bubble models show that you can really get into trouble on an improperly planned/executed reverse dive profile. Many were concerned that divers, especially inexperienced sport divers, would get the wrong message about reverse profiles and think that it was okay to do them without any special consideration. An argument can be made that the present lack of data that reverse profiles are dangerous could be, in part, due to the arbitrary prohibition against them that has been in place for many years.
A couple of key concessions were obtained by the bubble modelers. First, David Yount pointed out that practical diving experience showed that there had not been many problems with reverse profiles, but bubble models showed that there could be. So, some wording was changed to make it clear that it was only in the diving experience that there were few problems, not that there's a lack of evidence of any kind that reverse profiles are/can be dangerous.
Bruce Wienke prevailed in his argument that the pressure differential, or "delta-P," of most of the safely executed reverse profileswas 40 fsw or less between the repetitive dives. Accordingly, it was agreed by the body of participants that reverse dive profiles are most likely okay (i.e. "reasonably safe" ) as long as the depth difference between repetitive dives is 40 fsw or less. A point of final agreement was that the sport diving limit of 40 msw/130 fsw should apply to any relaxation of current prohibitions on reverse profile diving.
So, the bottom line from the Workshop is that the diving data shows that reverse profiles have not been inherently dangerous and that current prohibitions against it can be relaxed within the scope of no-decompression diving to the sport diving limit of 40 msw/130 fsw and "delta-P's" of 40 fsw or less between repetitivedives.
~ Erik Maiken
----------------
There is nothing inherently dangerous about a reverse dive profile providing that the decompression algorithm adequately takes into account excess dissolved gas (gas loading), unresolved free gas (gas bubbles), and changes in the underlying size distribution of bubble formation nuclei (gas nuclei). Only the first of these effects is accounted for in conventional dissolved gas algorithms. All three can be addressed with the context of the Varying Permeability Model."
http://www.phys.hawaii.edu/~dey/rdpw/rdpw.pdf
