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.”
