REVERSE PROFILES IN RECREATIONAL DIVING

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:

• Historically neither the U.S. Navy nor the commercial sector have 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 was presented that reverse dive profiles within the no-decompression limits 
lead to a measurable increase in the risk of DCS.

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!