Many of us, reading our Open Water Diver manuals (including PADI and CMAS) have been instructed and warned about the danger of diving s.c. reverse profiles.
Briefly explained, reverse profiles occur when, within a single dive, the latter portion of the dive is deeper than the earlier portion or, in the case of repetitive dives, the repeat dive is deeper than the earlier one.
Although the rule against reverse dive profiles has been (and apparently still is) widely accepted and taught by major recreational diving agencies, it is not clear where did this rule came from and which was the scientific motivation behind it. Sure thing, historically this limitation could be explained by the use of dive plan tables and electronic dive planner that were only in the condition of calculating bottom time and nitrogen loading only based on the maximum depth and time figures.
With this in mind, in the year 1999 the Smithsonian Institution and AAUS (American Academy of Underwater Sciences) called a Workshop with the purpose of investigating, among others, if there is any physiological basis (read “DCS incidence”) for the rule against reverse profiles ( Reverse Dive Profiles Workshop, Washington DC, October 29 and 30, 1999).
The results of this study, that considered modern bubble decompression models, were:
Within this research, the possible increase of DCS cases possibly connected to reverse profiles was explained with the fact that very often reverse profiles are associated to repetitive dives on multiple days activity and connected nitrogen exposure.
The conclusion of the study was that if diving within no decompression limits and less than 40 meters, and within a maximum depth difference of 12 meters, the current evidence does not show any demonstrable increase in risk of DCS.
With this in mind, and with the generic commercial spread of dive computers with algorithms capable of adequately calculate bottom time and residual nitrogen loading for reverse profiles, today the only reason for avoiding reverse profiles in recreational diving seems to be purely based on the necessity of maximizing the dive duration.
In fact, even if apparently not relevant for DCS, reverse profiles still significantly reduce the available bottom time, and this is for sure a valid reason for avoiding them when possible: for sure, when we are on holiday, we want to maximize our fun and to stay below water as long as we can!
One last consideration: the position of the Smithsonian Institution and AAUS is not globally shared and there have been more recent studies showing different results, even though based on animal experiments (The relative safety of forward and reverse diving profiles. S. McInnes, C. Edmonds, M. Bennett, UHM 2005: VOL 32, ISSUE #6).
Nonetheless, the results of the 1999 Workshop are still widely accepted, among others, also by Divers Alert Network (DAN).
I share this article based on my best knowledge and research on public documentation as well as literature in my possession. If any of you fellow divers out there, especially tech divers, do have any further information or authoritative source, I would be happy to further investigate and discuss with you!
Many times, in my professional activity (mostly in the role of dive guide), I witness the inexplicable behavior of divers who, after a perfect safety stop at 5 meters performed in very good buoyancy, rapidly swim to the surface or even worse just inflate the BCD and pop up uncontrolled.
These divers are probably convinced that after the safety stop there is no more risk and it’s possible to return rapidly to the surface and thus to the boat… Well this could not be more wrong!
If we consider the Workman critical ratio 1,58:1, we can easily understand that a rapid ascent from 5 - 6 meters is at the very limit of this value! Moreover, the most modern VPN and RGBM decompression models imply the constant presence of bubbles in the body tissues, and in a previous article we learned that a full washout (off-gasing) of bubbles relevant for DCS only occurs in the 12 - 24 hours following the dive. So, given the presence of bubbles in the tissues while we still are underwater, the sudden pressure decrease caused by a rapid ascent from 5 meters could trigger the production and growth of bubbles potentially relevant for DCS.
Furthermore, a rapid ascent from 5 meters is not only relevant in terms of DCS, but also with reference to pulmonary injuries and barotrauma, such as reverse squeeze.
In particular we shall always keep in mind that, if we surface holding the breath with full lungs, a depth change of only 1,2 meters is sufficient to cause lungs over-expansion!
Finally, considering, the massive pressure change in this delicate phase, it is recommendable to choose an ascent speed even slower than the 9 m/s recommended by many training agencies. Considering an ascent rate of 6 m/s, we should take at least one minute to surface from 5 meters!
No need to say that in this phase, perfect buoyancy is the most important skill to be mastered, so keep training and, if necessary, join dedicated buoyancy courses.
In one of the first articles on my blog we discussed about smoking and its consequences on scuba diving ( https://scubadivinggoodpractices.b...
