Monday, February 27, 2006

Debate: The relative safety of forward and reverse diving profiles.

Scuba instructors and dive masters all over the world are in a quandary about the proper method to teach and do diving profiles - with conflicting recommendations by the Smithsonian Reverse Diving profile Workshop in 1999 and a recent study reported in the UHMS Journal, supporting forward diving profiles already found to be safe over many years.

We have provided the abstract of the new article by McInnes, Edmonds and Bennett reported in the Undersea and Hyperbaric Medical Journal, Nov/Dec 2005. In addition we have obtained material reflected in the references to the article that gives most of the salient features of the workshop. Finally, we have provided you with a critique of the workshop and article so that you may personally take part in the debate. This is a pragmatic debate, as it affects every dive operator in so far as the safety of the diver is concerned. I specifically ask that you read the comments of Dr. Simon Mitchell linked to the bottom of this article.

1 Department of Diving and Hyperbaric Medicine,Prince of Wales Hospital,Sydney,2 Consultant Diving Physician,3 University of NSW,Sydney,Australia
McInnes S,Edmonds C,Bennett M.The relative safety of forward and reverse diving profiles.Undersea Hyperb Med 2005;32(6):421427.

A recent workshop found that with nodecompression dives,“reversed dive profiles ”(RDP) did not increase the risk of decompression sickness (DCS).Thus in multilevel dives, the deeper part of a dive may be performed later in the dive,and repetitive dives may progress from shallow to deep.This contradicts the conventionally recommended forward dive profile (FDP) when the deeper dive, or deeper part of the dive,is performed first.The RDP Workshop recommendations were made despite the absence of adequate data. We performed two groups of experiments to test this hypothesis. We exposed two matched groups of 11 guinea pigs each to forward and reverse multilevel diving profiles to determine any substantial difference between FDPs and RDPs.There was no evidence of DCS in any of the FDP animals,while six (55%) of the RDP animals exhibited symptoms of severe DCS and died. This difference was statistically significant (P =0.01). We then compressed two groups each of 11 guinea pigs to repetitive dives to determine any substantial difference in the risk of DCS when two equivalent sets of three dives were conducted from the deepest to most shallow on the one hand (FDP),and from the shallowest to the deepest on the other (RDP). Over two such series of dives (the second extended in time and depth to increase DCS risk), there was a significantly higher incidence of severe DCS in those animals in the RDP group. Seven of 21 exposures (33%) in the RDP group resulted in severe DCS versus none in the FDP group (P =0.01). Our findings suggest that multilevel and repetitive dives performed in the established FDP manner are less hazardous than those performed in the reverse profile mode, at least for the exposures we chose. We believe the recommendations of the workshop should be reexamined.

1.Lang MA ,Lehner CE.Proceedings of theReverse Dive Profile Workshop 2930 October1999.Undersea and Hyperbaric Medical Society,

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 no-decompression 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 reduced-gradient bubble model (RGBM); the Duke University bubble-volume 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 dive-computer 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 Conclusions

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.


* 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 and increased risk of DCS.
* No convincing evidence was presented that reverse dive profiles within the nodecompression limits lead to a measurable increase in the risk of DCS.

Conclusions: We find no reason for the diving communities to prohibit reverse dive profiles for nodecompression dives less than 40 msw (130 fsw) and depth differentials less than 12 msw (40 fsw).

2.Albano G.Principles and Observations on the
Physiology of the Scuba Diver .ONR Dept of Navy.

I was unable to find this reference.

3.Vik A,Jenssen BM,Eftedal O,Brubakk AO. Relationship between venous bubbles and haemodynamic responses after decompression in pigs.Undersea Hyperb Med 1993;20(3):233248.

We present a new pig model for studying relationships between venous gas bubbles and physiologic effects during and after decompression. Sixteen pigs were anesthetized to allow spontaneous breathing. Eight of them underwent a 30min exposure to 5 bar (500 kPa) followed by a rapid decompression to 1 bar (2 bar/min); the remaining eight served as controls. The pigs were monitored for intravascular bubbles using a transesophageal echocardiographic transducer, and bubble count in the twodimensional ultrasound image of the pulmonary artery was used as a measure of the number of venous gas bubbles. Effects on physiologic variables of the pulmonary and the systemic circulations were either measured or estimated. We detected venous bubbles in all pigs after decompression, but the interindividual variation was large. The time course of changes in the mean pulmonary artery pressure, in the pulmonary vascular resistance, in the arterial oxygen tension, and in the pulmonary shunt fraction followed the time course of the bubble count. In contrast, such a relationship to the number of venous gas bubbles was not found for the immediate increase in mean arterial pressure and for the changes in the other variables of the systemic circulation. We conclude that the number of venous gas bubbles, as evaluated by the bubble count in the ultrasound image of the pulmonary artery, is clearly related to changes in the variables of the pulmonary circulation in this pig model.


4.Boycott AE,Damant GCC,Haldane JS.The prevention of compressed air illness .J Hyg

5.Berghage,TE,David TD,and Dyson CV.Species differences in decompression.Undersea Biomed
Res 1979;6:113.

In an effort to bring together the diverse laboratoryanimal decompression studies, a literature review and statistical evaluation were undertaken. Although 22 different species that had been used in decompression studies were identified, systematic data were available for only 7 of these species: man, goat, dog, guineapig, rat, hamster, and mouse. Mathematical functions using physiological data on these seven species were developed to estimate 1) saturation time (the time for the body to equilibrate after an increase in hydrostatic pressure), and 2) nodecompression saturationexposure limits (the maximum saturationexposure pressure from which an abrupt return to 1 ATA can be tolerated). Data from man, rat, and mouse were used to develop physiological relationships for two additional decompression variables: change in pressurereduction limits associated with increased exposure pressure and time to onset of decompression symptoms. Finally, data on rats for two other decompression variables, gas elimination time and optimum decompression stop time, are discussed in the hope that this will stimulate additional animal laboratory research in other mammalians. The general functional relationships developed in this paper provide a preliminary and rough means for extrapolating among species the decompression results obtained during animal laboratory experiments.


6.Flynn,ET,and Lambertsen CJ.Calibration of inert gas exchange in the mouse.In:Proceedings of the
Fourth Symposium on Underwater Physiology , edited by Lambertsen CJ..New York:Academic, 1971,p.179191.

7.Hills B.Decompression Sickness,Volume 1 .1977.John Wiley and Sons.p.141.

8.Bert,P.La Pression Barométrique .Paris:Masson, 1878.Appendix I.[English translation by M.A.Hitchcock and F.A.Hitchcock ].Columbus, OH:College Book,1943.

9.Lillo RS Parker EC Evaluation of oxygen and pressure in the treatment of DCS in guinea pigs.
Undersea Hyperb Med .1998;25(1):5157.

These experiments examined whether increasing the partial pressure of oxygen (PO2), hydrostatic pressure, or both were responsible for the improvement in effectiveness of recompression treatment previously observed in guinea pigs with increasing depths of air. Unanesthetized male guinea pigs (600700 g) were subjected to 8.6 atm abs (871 kPa) air dives for 60 min and then decompressed at 1.82 atm (184 kPa)/min to the surface. Subsequently, animals usually displayed hypotension, cardiac arrhythmia, and tachypnea, indicative of a fatal bout (> 95% death rate) of decompression sickness (DCS). Animals that developed DCS were treated by recompressing to depths ranging from 2.5 to 11.6 atm abs (2531175 kPa), with 14, 28, 42, or 100% O2/balance N2. This design produced PO2's at treatment depth ranging from 0.4 to 3.6 atm abs (41365 kPa). Upon recompression, recovery of blood pressure, heart rate, and breathing rate generally occurred. The area under the breathing rate vs. time curve was used to examine the effectiveness of treatment over a period of 60 min. A dramatic improvement in recovery over time was observed with increasing recompression depth for all gas mixtures. Analysis indicated that the positive response to depth was related to increasing hydrostatic pressure, increasing PO2 had no statistically significant beneficial effect.


10.St Leger Dowse M,Gunby A,Moncad R,Fife C, Bryson P.A prospective Field study of reverse dive profiles in UK female recreational divers.South Pacific Underwater Medical Society Journal .2004; 34(4):183188.

Reverse dive profiles– Dr Phil Bryson quoted from the UKSDMC

Current understanding of decompression physiology suggests that diving the deepest dive first is safer. In the UK the principal recreational dive training organisations recommend deepest dive first but anecdotally divers do not follow this recommendation. In 2000 the Smithsonian Reverse Dives Profiles workshop concluded “We find no reason for the diving communities to prohibit reverse dive profiles for nodecompression dives less than 40msw and depth differentials less than 12msw. The primary question posed is whether reverse dive profiles (RDP) incur a higher risk of DCS than nonRDP dive profiles. In a review of the literature there was no convincing evidence to indicate that a repetitive dive must be shallower than the dive that precedes it. The exception was in a direct ascent from deep repetitive dives that have been shown to produce a high incidence of DCI. Scientific and diving medicine litera ture is not always consistent with current grass roots thinking and recommendations have grown from anecdotal data.

We observed the every day diving habits of female recreational divers and compared our data with the recommendations of the Smithsonian workshop. The divers were asked to keep diving diaries for three consecutive years but did not know the aims of the project and therefore made no changes to diving habits as a result. Volunteers were asked to record basic dive information including maximum depth, total dive time, and if a decompression stop was added. Symptoms and complications of diving were recorded as described in page 11. We categorised the multiple dive day data as follows: ·The 2nd dive is deeper than the first and the depth differential was more than 12m ·The 2nd dive is deeper than the 1st and is more than 40m ·The 2nd dive is deeper than the 1st, deeper than 40m, and the differential is more than 12m. We did not restrict the definition to nodecompression dives since there was insufficient granularity in the data to do this. We also looked at a further three categories substituting 30m for the 40m described above. 570 women divers returned diving records for up to three consecutive years. The basic characteristics are described in page 11. 532 (93%) women recorded 16,706 multiple dive days (36,487 dives). 479 (84%) women recorded 4,972 days (9,944 dives) with second dives of the day with reverse dive profiles. 36% of women logged 576 days with the 2nd dive deeper than 30m. A breakdown of the frequency of untreated selfassessed symptoms is shown in tables 1 and 2. Using the stricted interpretation only 29 (5%) of women recorded 41 RDP days (94 dives) outside of the workshop recommendations, which is only 0.2% of all multiple dive days. The relationship between RDP’s and maximum depth ever dived is shown in figure 1 and figure 2 shows the relationship with the total number of dives. Both correlate significantly (p=0.0013 and 0.0012 respectively). This study supports anecdotal observations that the practice of RDP’s in one form or another is taking place widely. Around 30% of multiple dive days within the study involved some combination of RDP although only between 0.2% and 1.5% of multiple dive days fall outside the recommenations of the Smithsonian Reverse Dive Profiles workshop. Symptom rates are higher (but not statistically) when analysing data outwith the recommendations.


11.Edmonds C,McInnes S,Bennett M.Reverse dive profiles.The making of a myth:South Pacific Underwater Medical Society Journal ,2005; 35(3):139143.

Background to the article (see above)
In 1999 the Smithsonian Institute held a workshop on RDPs which produced the following statement that many of you have heard of: "we find no reason for the diving communities to prohibit RDPs within the the no-decompression limits for dives less than 40 metres and with depth differentials less than 12 metres".

The authors in the above article have nicely critiqued the workshop findings and also presented some new data refuting the recommendations supporting RDP.

Some key points include:

1) All at the workshop agreed that there was an absence of hard data to make recommendations either way but then went on to make recommendations.

2) At the workshop, there was a disparity of opinion between those who favoured inert gas tissue models (felt that RDPs and forward dive profiles (FDPs) should be equal); and those who favoured bubble models (eg VPM and RGBM) who felt that RDPs would be less favourable for safe decompression. It was Bruce Wienke (RGBM) who suggeted the 40m and 12m caveats to the final recommendation. He didn't extend the concept to 3 or more dives or mutilevel dives.

3) There was some evidence of increased adverse outcomes with RDPs presented at the workshop (USN and Catalina Island) which was not brought out in the findings.

4) "The belief that FDPs were introduced only to obtain more bottom time is a myth that seems to have been developed at the workshop". The authors of this paper support the claim that the initial concept of avoiding RDPs (in the 1960's) was a safety issue.

5) The 40m depth and 12m differential is not clearly enough explained in the workshop findings, and that it can be manipulated in a number of ways to make deompression less effective. This was a recommendation put forward at the last minute without support and will most likely be forgotten.

6) Finally the authors present some new animal work they have performed which nicely shows a significantly worse outcome in their model using RDP.

There is an excellent discussion of this debate by Dr. Simon Mitchell at
He takes issue with Dr. Edmonds in the following quote: "However, the argument about reversing two profiles without adjusting either of them is to some extent an anachronistic throw-back to the days when all dives were planned with tables. It seems an academic exercise in the modern world of diving where virtually all dives are controlled by computers that derive adjusted no decompression algorithms in real time based on recent previous exposure. Almost by definition, computer users cannot perform true reverse profile dives. Another problem is that the reverse dive profile ban had reinvented itself over time as a total ban on any dive deeper than another during any single day of diving, regardless of any measures the diver was prepared to take to make the second deeper dive safe. Hence we were subjected to ridiculous practices like divers being banned from further diving just because a second dive was slightly deeper than the first, or worse, divers finding some deep hole to bounce down into on their first dive just to have "the number on their computer" which kept their depth options open for subsequent dives.

You can sum all this up by saying that divers want to do "reverse depth dives" with appropriate adjustment to the subsequent deeper dives to make them safe, NOT reverse profile dives in the strict sense."

It would be my personal (ESC) recommendation that "if it ain't broke - don't fix it". Forward diving profiles have a good track record. In addition, no decompression diving using computers would seem to be controlling, as stated by Simon Mitchell.

Let me know what you think!

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