Those Reverse Diving Profiles

In a 1999 a workshop organized by The Smithsonian Institution and sponsored by DAN, DEMA (Dive Training Magazine) and several other organizations, the participants challenged the traditional view that both multilevel and repetitive dives should be undertaken from deep to shallow. The standard “forward dive profile” entails making the deepest dive or the deepest part of the dive first of repetitive dives, so the dive or dives become shallower. However, the participants promoted the concept of a “reverse dive profile,” diving from shallow to deep, either in multi-level diving or repetitive dives. While sport divers rarely, if ever, have a reason to conduct a reverse profile — which is not to say they don’t do them – scientific divers often find them useful.

While there was no consensus at the workshop, a compromise led to the reverse profiles being approved, though with specific limitations: a depth limit of 40 meters sea water (msw), a differential between dive depths of no more than 12 msw, and no decompression dives.

The belief that forward and reverse profiles are analogous and require comparable decompression is based mainly on the assumption that, given the same depths and durations, both produce the same load of inert gas dissolved in the tissues — despite the order of the exposures. This concept is inherent in many decompression computer algorithms, especially those that deal with dissolved inert gas loads, as opposed to induced-bubble models. However, there appears to be no experimental confirmation that reverse and forward profiles can safely have the same decompression requirements.

Research Called For

Three Australian researchers, S. McInnes, C. Edmonds, and M. Bennett, designed a study to test the hypothesis that there is no difference for decompression sickness risk between the forward and reverse profiles, as they apply to multi-level and/or repetitive dives. They selected two groups of actual guinea pigs and subjected them to dives within the recommended workshop limits, confirming the profiles with an Aladin Pro dive computer in the chamber.

For multi-level dives, the initial forward profiles were 36m for 30min, 24m for 30min and 12m for 30min. Ascents and descents were at nine meters/min. The reverse profile began at 12m, then dropped to 24m and 36m.

For repetitive dives, the initial forward profile was 30 msw (meters of seawater) for 30 minutes, 20m for 30 minutes and 10msw for 30 min, with surface intervals of 15 minutes. The series was reversed for reverse profile dives. In a second set of forward dive profiles, depth and time were increased, with the subsequent series just depth profile was reversed.

Results

During the standard forward dive profile, no animal showed signs of DCS. However, in the reverse multilevel profile, six animals showed DCS symptoms, a statistically significant difference. All six were immediately placed on oxygen and recompressed. Two were dead before treatment could be instituted. At 42 minutes into the treatment table all six were dead.

The experimenters ran two forward dive profile experiments, and no animal showed signs of DCS. However, in the first reverse repetitive dive profile, one animal died. In the second, this time at greater depths and longer, six animals suffered severe DCS and three died. The difference between the two groups was statistically significant.

Discussion

The researchers chose the dive profiles to comply with the workshop recommendations, but they shortened the time for guinea pigs so that their exposure would approach the no-decompression limits. As there is a direct relationship between DCS susceptibility of a species and its body mass, guinea pigs have a much lower DCS susceptibility than humans. So, “human” depth limitations should carry much lower risks of DCS for guinea pigs.

Despite the modifications in the profiles, six guinea pigs in the reverse multilevel profile died rapidly with severe DCS, unresponsive to either surface oxygen or to oxygen recompression. The catastrophic results show a substantial difference in the physiological processes involved in inert gas handling between forward and reverse profiles in multi-level dives.

That none of the pigs in the forward profile were affected during the repetitive dive, indicated that the dives approximated a no-decompression sequence. Nevertheless, the mirror image reverse profile produced one death, and extended profiles resulted in another catastrophic increase in reverse profile casualties.

The researchers were clearly disappointed in the deaths of the animals, noting that the planned oxygen was inadequate treatment. “We had not anticipated the enormous difference that was demonstrated by the no-decompression exposure of the FDP and the same exposure in reverse.”

Conclusion

Because there are so many potential combinations of repetitive dives, no experimental model can predict the overall risk of DCS from reverse profiles. But, the incidence and severity of DCS in the experiments showed a substantial difference in the physiological processes involved in inert gas handling between forward and reverse profiles in both multi-level and repetitive dives.

Reverse profiles applied to multi-level and repetitive diving are not the mirror image of forward profiles and do not carry equal decompression obligations.

“We advise against advocating reverse profiles, until the limitations of this format are determined more factually and the decompression requirements are redefined.”

McInnes , C. Edmonds, M. Bennett, Department of Diving and Hyperbaric Medicine, Prince of Wales Hospital, Sydney, Consultant Diving Physician, University of NSW, Sydney, Australia. Reverse Dive Profiles, The making of a myth; dive profiles. The making of a myth: South Pacific Underwater Medical Society Journal, 2005; 35(3):139-143.