Reverse Dive Profiles Revisited

[BillP]

The Nov/Dec issue of Undersea and Hyperbaric Medicine has an interesting study that takes another look at "forward" vs. "reverse" dive profiles.

The relative safety of forward and reverse diving profiles.

S. McINNES, C. EDMONDS, M. BENNETT, Department of Diving and Hyperbaric Medicine, Prince of' Wales Hospital. Sydney, Consultant Diving Physician, 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):421-427. A recent workshop found that with no-decompression dives, "reversed dive profiles" (RDP) did not increase the risk of decompression sickness (DCS). Thus in multi-level 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 II guinea pigs each to forward and reverse multi-level 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 II 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=O.OI). Our findings suggest that multi-level 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 re-examined.

(To see a summary and discussion of the Smithsonian workshop they’re talking about go to: http://www.scubaboard.com/showthread.php?t=362&highlight=smithsonian )

In the study, the researchers looked at two types of FDP’s and RDP’s. In the first, they looked at multi-level dives where the “divers” started the deepest part of their dive first and finished with the shallowest (or vice versa for the RDP) and in the second they looked at repetitive dives where the diver either did the deepest dive first (FDP) or shallowest dive first (RDP). The dives were reportedly within the no decompression limits for guinea pigs but were intentionally aggressive dive profiles. The researchers stated, “The dive profile was chosen to comply with the depth recommendations of the workshop on forward and reverse profiles, for humans but modified for this animal species, so that the 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 considerably lower DCS susceptibility than humans. The application of “human” depth limitations, therefore, should carry much lower risks of DCS when applied to our experimental animals. Despite this, we have found significant risk of serious DCS when applying RDP schedule to these animals. ”


Multi-level dive profiles and results

For the first part of the study the profile was 36msw (118fsw) for 30 min, 24msw (78fsw) for 30 min, and 12msw (39fsw) for 30 min with descent/ascent rates at 9 m/min (30 ft/min) for the FDP group, and of course the reverse for the RDP group. If they didn’t see any difference between the two groups with the first dives, the researchers planned to do progressively more aggressive dives, but they didn’t end up having to.

With this dive profile, none of the FDP animals experienced DCS whereas six of the eleven RDP animals developed DCS. The 6 animals who developed DCS were all treated for their problem (O2 and recompression treatment table) and all died.

Repetitive dive profiles and results

The researchers did two series of dives with this group.

In Series 1 with the FDP Dive 1 was 30m (98ft) for 30 min, Dive2 was 20m (65ft) for 30 min, and Dive 3 was 10m (33ft) for 30 min with 15min surface intervals between dives. The RDP group obviously did the reverse.

In Series 2 with the FDP Dive 1 was 36m (118ft) for 40 min, Dive 2 was 24m (78ft) for 40 min, and Dive 3 was 12m (39ft) for 40 min again with 15 min surface intervals between dives. Again, the RDP group did the reverse.

In Series 1 one of the RDP animals developed DCS and died, but none of the FDP animals developed DCS.

In Series 2 again no FDP animal developed DCS, but six of the RDP animals developed severe DCS and three died.

They ended their study by saying, “We advise against advocating reverse profiles, until the limitations of this format are determined more factually and the decompression requirements are re-defined.”

Food for thought. Hope you find it helpful.


BillP

 

[Walter]

Bill,

That's interesting, but the reverse profile dives in the study violated a basic principle of diving reverse profiles. You don't get as much total bottom time with reverse profile diving. Bottom time is sacrificed by diving reverse profiles. The concept that equal bottom time on reverse profiles is dangerous was never in dispute. This study would have meant something if they are followed the guinea pig tables for reverse profiles. As it is, it merely confirms something on which everyone was already in agreement. A similar study in which basic principles are not violated would be useful.

 

[BillP]

You bring up a good point, Walter, and one that I noticed too. You would, of course, modify your dive plan for a reverse profile. That just seems to be common sense. But I'm not so sure that the Smithsonian workshop (or people that read about it and interpret the findings for themselves) would agree with you that a modification is necessary. The Aussie study above seemed (to me) to be intentionally designed to call into question the conclusions of the Oct. 1999 Smithsonian workshop on reverse dive profiles (that RDP's are equivalent to FDP's within certain limits) rather than to study how reverse dive profiles can be modified to make them as safe as forward dive profiles. They were following the recommendations of that workshop to see what happens.

That Smithsonian workshop pointed out, "In professional practice, neither the military nor commercial diving communities prohibit or even recognize reverse dive profiles as unique, except in rare circumstances. These facts have recently called into question the long-standing prohibition against reverse profiles in the recreational diving community." The conculsion of the workshop was, "The Workshop finds no reason for the diving community to prohibit reverse profiles for no-decompression dives less than 40msw (130fsw) and depth differential less than 12msw (40fsw)." They seem to be suggesting that the order of the dives can be ignored if you're within those depth guidelines. The authors of this guinea pig study (and you) would seem to disagree with that conclusion. I believe that's the "point" of this study. The Australian study did seem to ignore the inherent depth and time "penalty" for reverse dive profiles.

And a study to see what modificatons to RDP's are necessary to make them as safe as FDP's would indeed be interesting.

Bill

 

[gcbryan]

Interesting study but there is one part that I don't understand.


The application of “human” depth limitations, therefore, should carry much lower risks of DCS when applied to our experimental animals. Despite this, we have found significant risk of serious DCS when applying RDP schedule to these animals.

They didn't apply "human" depth limitations. They approximated guinea pig NDL's.

Am I missing something?

 

[Walter]

Bill, I disagree. Noone has said it's OK to ignore tables (assuming you are not using a computer) and tables do not allow the same total bottom time for reverse profiles. It's a point I use and demonstrate when teaching tables. Reverse profiles result in less bottom time.

This study doesn't demonstrate the danger of reverse profiles, only the danger of ignoring tables.

 

[drbill]

Walter's point is well taken. Since bottom time is my primary concern as a videographer, I prefer to maximize it through my dive planning. However, reverse profiles are a common part of my diving since I never know when, or at what depth, I will encounter an interesting subject.

Jon Hardy (formerly of NAUI and ScubaLab, and a resident of Catalina until his untimely death) did a fair bit of work on reverse profiles, so I've always felt comfortable doing them given the awareness of bottom time considerations.

 

[Dr Deco]

Hello readers:

This is an interesting article that BillP has brought up. There are several aspects to this.

Gas Loadings

One does not even need actually to calculate the gas loads to see that these are not equivalent dives. You cannot simply reverse the order of a series of dives without needing to make some modification of the gas loads. In some of these, you can expect that trouble will result. Divers recognize that deep-to-shallow dives result in more bottom time with small gas loads.

Since some of the animals had a DCS problem even on the forward profile [in some cases], we can expect that these are borderline dives for these Guinea pigs. The greater dose of nitrogen would be expected to produce greater DCS problems.

Small Animals

Another factor is the use of small animals for DCS studies. It has been known for several decades that these small creatures do not get “the bends” as do large animals [ three references below]. Probably this is the result of high perfusion rates to connective tissue. Therefore, when I did a study on diving and pregnancy, I used larger animals such as sheep and goats. They actually do get joint pain DCS.

Small animals load not nitrogen largely in muscle and fat tissues. These produce bubbles upon decompression that are released into the venous return, block the heart, and death is the end result. Clearly, this is not the same endpoint when we say that a diver got a “hit.”

In addition, small animals must be matched very closely in the weight range to get results that can be compared for to experimental runs. The weight range is actually about 20 grams; I do not know how closely the Australian group matched their animals.


Dr Deco :doctor:


References :book3:

Kindwall EP. Metabolic rate and animal size correlated with decompression sickness. Am J Physiol. 1962 Aug;203:385-8.

MR Powell. Leg pain and gas bubbles in the rat following decompression from pressure: monitoring by ultrasound: Aerospace Med., 43, 168-172 (1972).

MR Powell. Gas phase separation following decompression in asymptomatic rats: visual and ultrasound monitoring: Aerospace Med., 43, 1240-1244 (1972).

 

[BillP]

Good points brought up by all.

I’m not much of a mind reader, but IMHO this study was specifically designed to illustrate a point- that all else being equal (as far a depth and time are concerned) a reverse dive profile has an increased risk of DCS when compared to a forward dive profile. And this study appears to confirm that belief (at least for guinea pigs for the profiles they chose).

But as Walter rightly points out, all else isn’t necessarily equal in practice. Walter, you mention the importance of following dive tables. I wonder how many divers these days do limit their bottom time based on paper (or plastic) tables. Frankly, I do “assume” that many if not most divers are using a computer to plan their dives. I certainly haven’t crunched the numbers on all (or even many) platforms, but I had always thought that there were built-in depth and time penalties for reverse profiles in virtually all tables and at least most dive computers. If anyone knows of specific decompression algorithms that don’t have a depth/time penalty for a reverse profile I’d like to hear from them. But the designers of the study seem to believe they not only exist but are in fact common when they say:

The belief that FDP’s and RDP’s are analogous and therefore 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- irrespective of the order of exposures. This concept is inherent in many (emphasis added) decompression meter algorithms, especially those that deal with dissolved inert gas loads, as opposed to induced-bubble models.

Dr. Deco brings up good points too. Guinea pigs might not be the best decompression model for humans, but I think they were selected to be a good model to make a point (grin). I have to say I’m not sure what you mean when you say:

Since some of the animals had a DCS problem even on the forward profile (in some cases), we can expect that these are borderline dives for these Guinea pigs. The greater dose of nitrogen would be expected to produce greater DCS problems.

From my read of the paper, all of the animals that developed DCS were in the RDP group. Between series 1 and series 2 of the repetitive dive group two animals in the FDP group “appeared as if they may be developing DCS during the surface interval between dives one and two” but “In each case the animals soon appeared normal and completed the dives and were unaffected during the following 2 hours observation period.” The authors didn’t believe the symptoms were due to DCS in any of the FDP group animals. I suppose that’s arguable. Are these the "borderline" cases you mentioned?

Interesting study but there is one part that I don't understand.


The application of “human” depth limitations, therefore, should carry much lower risks of DCS when applied to our experimental animals. Despite this, we have found significant risk of serious DCS when applying RDP schedule to these animals.

They didn't apply "human" depth limitations. They approximated guinea pig NDL's.

Am I missing something?

That doesn’t seem to make sense on first read, does it? But what I think that they’re saying is that they did indeed use guinea pig tables for the dives, but they used the maximum and differential depth limits for humans as set by the workshop for reverse dive profiles. The authors seem to believe that guinea pigs should be “allowed” greater max depths or greater differential depths between dives than humans, but they still developed DCS when the stricter human limits were used.

The conclusions of the Smithsonian workshop are based on current practice and years of experience. People are doing reverse profile dives, and have been doing reverse profile dives for a long time, but the authors of the workshop's recommendations are not seeing an increased incidence of DCS in those dives. There’s a lot to be said for experience and observation. But I think the authors of the above study are saying that they would like to see some experimental data backing up those observations before giving a broad green-light for RDP dives. And they came up with a study that IMHO was specifically designed to show why they feel that way. The study was designed to raise questions, not give answers.

Like I said, food for thought.

Bill

 

[Dr Deco]

Hello readers:

In actuality, I would go with the experience gleaned from human divers rather than small animals. This is what the Smithsonian group did.

Small animals are notorious for their strange results because they do not actually develop decompression sickness as we recognize it in humans. Their small connective tissues do not get sufficient gas loads to cause joint pain. Rather they get large loads in the muscle and fat tissue and develop what I termed "decompression death.” This is actually cardiopulmonary collapse.

Until I see problems from human subjects, I would not have a large concern if one remains within the depth increment restrictions (40 fsw).

Dr Deco :doctor: