Reverse Profiles

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Permalink Reply by Lynne Flaherty on January 13, 2009 at 8:32am

There is a really fascinating discussion of this in the new book, Deco for Divers. It sounds very much as though Powell has either read the minutes of the entire AAUS symposium, or talked to some folks who were there, and the process by which the recommendation to limit reverse profiles to 40 foot differences was arrived at was something less than rigorously scientific. According to his account, there really were no data at all that reverse profiles were harmful, but the mathematical models being used by the bubble folks (Young and Weinke) said that there COULD be problems. The majority of the profiles they examined had depth differences less than 40 feet, so they agreed that that would be the recommended limit. But they didn't have a bunch of profiles with greater differences with DCS problems, to indicate that exceeding that difference would be dangerous. THAT came out of the mathematical models alone, according to Powell's article.

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Permalink Reply by Jeff Seckendorf on January 19, 2009 at 3:55pm

Hi Lynne,

I'm interested in this topic as it relates to bubble mechanics as we think we know them now, and that bubbles are always present. If that's the case, and we continue to do good, clean ascent profiles, doesn't it make sense that a deeper repetitive dive would cause any existing bubbles to compress that much more, making them easier to get rid of during successive clean ascents? I'm sure there are arguments to be made on both sides, but all the research on this seems really old.

Jeff

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Permalink Reply by Richard on January 22, 2009 at 1:45pm

Jeff Seckendorf said:

Hi Lynne,

I'm interested in this topic as it relates to bubble mechanics as we think we know them now, and that bubbles are always present. If that's the case, and we continue to do good, clean ascent profiles, doesn't it make sense that a deeper repetitive dive would cause any existing bubbles to compress that much more, making them easier to get rid of during successive clean ascents? I'm sure there are arguments to be made on both sides, but all the research on this seems really old.

Jeff

Superficially this makes sense.

However if you have created a bubble on dive1 (I have) it tends to irritate the surrounding tissues and create inflammation among other things (been there, done that). So its not a given that you will be able to offgas that already saturated area effectively on dive2 (also done that). And hence you may end up "more bent" after your 2nd deeper dive. You crushed the offending asymtomatic or subclinical bubble, that just didn't do much to eliminate the total gas load at the end of the day.

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Permalink Reply by James Mott on January 23, 2009 at 12:57pm

I don't think Jeff's assumption is that you are "really bent" after dive 1 moaning and numb, and then want to do a second dive deeper to clean up the DCS effects. I believe his discussion is assuming we are doing proper asents with as clean as possible result, to make a second dive that is deeper than the first, would it or would it not be proper. Most agencies are teaching if you did a 90 ft dive and a 130 ft dive on the same day to do the 130 ft first. Progressively getting shallower.
I think Jeff's questions is based around the idea that we don't recommend bouncing after a dive for obvious reasons and that would make the claim that if we were infact deeper, would it reset the bubble model and have just as clean of an ascent on the deeper second dive as it was on the first.


Richard said:

Jeff Seckendorf said:

Hi Lynne,

I'm interested in this topic as it relates to bubble mechanics as we think we know them now, and that bubbles are always present. If that's the case, and we continue to do good, clean ascent profiles, doesn't it make sense that a deeper repetitive dive would cause any existing bubbles to compress that much more, making them easier to get rid of during successive clean ascents? I'm sure there are arguments to be made on both sides, but all the research on this seems really old.

Jeff

Superficially this makes sense.

However if you have created a bubble on dive1 (I have) it tends to irritate the surrounding tissues and create inflammation among other things (been there, done that). So its not a given that you will be able to offgas that already saturated area effectively on dive2 (also done that). And hence you may end up "more bent" after your 2nd deeper dive. You crushed the offending asymtomatic or subclinical bubble, that just didn't do much to eliminate the total gas load at the end of the day.

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Permalink Reply by Richard on January 23, 2009 at 2:10pm

The fact is that we are almost always bubbling. And that while "deeper repetitive dive would cause any existing bubbles to compress that much more, making them easier to get rid of during successive clean ascents" probably works in jello or some other ideal/model tissue, it is unlikely to be true in tissues that have experienced subclinical DCS and some of the inflammatory responses thereof. Once you have a bubble, merely crushing it is only one step in the elimination process. And a subsequently cleaner MDL ascent may or may not be enough time for that gas to diffuse out through injured tissues. Relying on a deeper dive to "fix" a so-so ascent and/or mildly excess loadings from a previous dive is not a great idea.

That said, there is plenty of anecdotal evidence (mostly published by Rich Pyle) that IWR with even just air does not make clinically bent people worse.

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Permalink Reply by Lynne Flaherty on January 23, 2009 at 6:10pm

I think if the first dive was conducted properly, and bubbling is minimal, it makes awfully attractive sense that crushing the bubble will lesson the overall decompression stress. However, there are two issues with that that immediately come to mind: One is that crushing the bubbles can permit them to pass through the pulmonary filter, and subsequently expand on the arterial side of the circulation. The other is that you are going to have to do the deco on the second dive to get rid of the gas load you took on from that dive, and the gas load you hadn't unloaded for the first dive (what was in the bubbles) so the deco for the second dive probably should be quite a bit more conservative than for the first, single dive.

In other words, if you're assuming you're crushing bubbles with the second, deeper dive, then you weren't really "clean" when you got out of the water the first time (which I don't think many people ever are, looking at Doppler data).

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