Always Do Your Deep Dive First?

For many years, recreational scuba divers have embraced a maxim that it is unacceptable to perform reverse profile dives, i.e., to make a repetitive dive to a depth deeper than a previous dive or to make the deeper part of a single multilevel dive after a shallower part. While it is widely recognized among recreational divers that reverse dive profiles are forbidden, these profiles are often done anyway.

It’s not clear where the rule against reverse dive profiles came from or how significant it really is. Except in rare instances, neither the military nor commercial diving communities prohibit reverse dive profiles or even recognize them as unique. Most sport diving computer programs do not prohibit reverse dive profiles and even allow the calculation of such profiles without apology. The primary consequence is that there may be less no-stop bottom time available on a repetitive dive. So the long-standing prohibition against reverse dive profiles is being questioned by the sport diving community.

To get more bottom time, most sport divers have observed the prohibition of reverse order and perform dives only in “forward” order. However, for scientific divers the “rule” can impose a significant operational limitation. To reexamine the rationale for this rule, a highlevel international workshop was organized by Michael Lang, head of the Smithsonian Institution’s Scientific Diving Program, and by Charles Lehner, Diving Physiology Laboratory, University of Wisconsin, Madison. Divers Alert Network (DAN), the Diving Equipment & Marketing Association (DEMA), and Dive Training magazine were also sponsors. The October workshop’s objective was to examine whether reverse dive profiles cause increased risk and whether there is a justifiable reason to prohibit such profiles.

Workshop Sessions

In the first session, a discussion of the literature revealed that the prohibition against reverse profiles probably related less to safety issues than to “optimizing” bottom time over a series of dives. This comes from gas-loading considerations that allow more usable bottom time by making the deep dive first.

The next two sessions concentrated on physics, physiology, and modeling. Among the modeling approaches, bubble formation and growth models were prevalent. Although there was diversity among the bubble models, they tended to arrive at similar conclusions. For example, most call for lower allowable supersaturation gradients on the initial stops (deep stops) and shorter nodecompression limits than conventional dissolved gas models. The bubble models included David Yount’s varying permeability model (VPM), also known as the “tiny bubble” model; Bruce Wienke’s reducedgradient bubble model (RGBM); the Duke University bubblevolume model; the DCIEM bubble evolution model based on Doppler scores; a gas-dynamics model by Valenie Flook based on Van Liew’s concepts; and Michael Gernhardt’s tissue bubble-dynamics model.

Hugh Van Liew argued that they need experimental validation to confirm the existence and role of micronuclei for bubble formation, including whether such gas nuclei can be “crushed” to the point of elimination or inactivation. Another presentation showed that, although the reverse dive profile may have a higher predicted incidence of decompression sickness (DCS), the differences were trivial for pairs of no-stop dives, and decompression using the U.S. Navy tables would be adequate. However, for dives involving decompression stops or for more than two dives in a row, these tables might not provide a reliable decompression. All of this pointed toward an urgent requirement for more information and, to this end, Alf Brubakk suggested an animal model that might at least show which profiles result in the most bubbles.

Another session included a discussion by several divecomputer manufacturers. Many older computers on the market use conventional dissolved-gas (Haldanian) algorithms that take into account only gas loading and supersaturation limits (M-values) and do not specifically consider the order in which dives are conducted. In these cases, the user manuals accompanying the computers may recommend against reverse dive profiles. Some of the latest dive computers incorporate algorithms based to varying degrees on bubble models. These computers have specific warning features or penalties for dive patterns associated with increased risk (bounce, yo-yo, repetitive dives with excessive pressure differentials, etc.).

Many horror stories have been associated with reverse profiles, the classic one being the instructor making a short, deep dive to release the anchor chain after a day of diving and getting severe DCS. Such situations are hard to interpret because the number of subjects is very small and buddy divers doing the same profile may be unaffected. Other data show that studies of 100 dives may be insufficient for statistical analysis, but one comment put this issue into perspective: “We are better off having that 100 dives than no observations at all.” Many participants reviewed data from the U.S. Navy, commercial diving records, decompression chambers, DAN records, and various recreational dive sources.

An argument can be made that the present lack of data proving whether reverse profiles are dangerous could be due, in part, to the arbitrary prohibition against such profiles for many years — in other words, not many of these dives have been done.

Although there were some problems with reverse dive profiles in isolated examples, the conclusion drawn from the analysis of actual diving data was that reverse profiles have not shown a higher risk for DCS than forward profiles. However, this holds most confidently when the differential pressure for the reverse profile is not too great — one cannot get big differentials without having significant depth. It appears that decompression tables, algorithms, and dive computers adequately handle the issue of reverse dive profiles.

Another observation is that this subject may be a matter of repetitive diving and, in general, this is handled differently across the many decompression algorithms.

The discussion turned to the participants to arrive at findings and conclusions, and the discussion got heated. Several people who work with bubble models had serious reservations about a “complete retraction” of warnings against reverse dive profiles since the bubble models suggest that you might get into trouble on an improperly planned or 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.

The bubble modelers obtained a couple of key concessions. Practical diving experience showed few problems with reverse profiles, but bubble models showed there could be. Thus, they adjusted some wording to make it clear that it was only in the diving experience that there had been few problems, not that there’s a lack of evidence that reverse profiles are or could have a higher DCS risk. The sentiment prevailed also that there should be a pressure differential limit, noting that most of the safely executed reverse profiles were in 40 fsw or less between the repetitive dives. Another point of agreement was that the sport diving limit of 130 fsw should apply to any relaxation of current prohibitions on reverse profile diving,

Findings and Conclusion

Neither the U.S. Navy nor the commercial sector has prohibited reverse dive profiles, and they are performed in recreational, scientific, commercial, and military diving. Since the prohibition of reverse dive profiles cannot be traced to any definite diving experience that shows an increased risk of DCS and no convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS, the workshop participants found no reason to prohibit reverse dive profiles for no-decompression dives less than 130 fsw and depth differentials less than 40 fsw.

This is a summary of an article that appeared in Pressure, the newsletter of the Undersea & Hyperbaric Medical Society, Inc. (Volume 29, Number 2 2000 March/April). It was written by R. W. Hamilton and Erik Baker. Undercurrent takes all responsibility for editorial changes.

To order the full workshop proceedings, contact Underwater Hyperbaric Medical Society, 10531 Metropolitan Avenue, Kensington, MD 20895, call 301-942-2980, fax 301-942-7804, or visit their website at www.UHMS.org.

— Ben Davison