In-Water Recompression, Revisited

[Akimbo]

Which leaves us with what people like Akimbo know to be true from having worked in environments where very early recompression is always available: very few divers who develop symptoms in these settings end up with long term problems and the most plausible explanation for this is the access to early recompression.

(general comment relating to the quote)
It is unfortunate that commercial diving operators tend to keep DCS incident reports as proprietary as local jurisdictions allow; but it is an understandable business reality. Even then, it would be hard to reach science-worthy conclusions because the "detailed reports" filed by dive-supers aren't very medically oriented and are often couched in a fog designed to limit liability and keep worker safety officials off everyone's backs. OK, "designed" is a little strong but "influenced" is probably understated.

The key question is unknowable... how many of the rapidly treated cases (like <15 minutes) would have progressed to really bad outcomes with significant treatment delays (like ~24 hours). Unfortunately, diving science has always been a bastard-child in the world's navies and to commercial diving companies where the raw data hordes reside. Science is only grudgingly supported because mission-critical objectives occasionally demand it.

 

[tbone1004]

@Kay Dee have you done any root cause corrective action *RCCA* analysis to figure out why you are bending people at such a high rate?
Based on your profile, you don't say how long you've been doing this, but diving CCR since 1995 which is in the very early days. Are those 10+ IWR instances early on in the days of technical diving or have any been recent? How many total dives is this a function of? You may be doing a lot more of those dives than we know so the percentages may actually be quite low.

@Duke Dive Medicine can correct me if I'm wrong, but the USN allows up to 2.3% DCS rate. That is very high, but is based on having a chamber on the boat that can get divers recompressed essentially instantly. IIRC the accepted DCS rate in the commercial diving world is around 1% and @Akimbo can probably weigh in better on that

The actual number in recreational is somewhere around .3% per a 10 year study by AAUS.
Based on DAN incident reports though, the actual number of DCS incidents was 18 in 2014 that were reported to DAN. It obviously isn't all of the DCS incidents worldwide, but as a percentage of total dives, or even total technical dives, it's pretty negligible.

 

[Dr Simon Mitchell]

This may be slightly off topic, but could you comment on what may be a related issue?

-snip- I think it is very possible that if I had gone to a hospital with a less-than-superior--hyperbaric physician, I would have been treated for DCS, and the usual fading of the symptoms during treatment might have been considered confirmation of that diagnosis.

Ha, you would not have been the first person with carpal tunnel to be recompressed after diving (or the last).

DCS, particularly the milder forms has many "non-specific" symptoms; that is, symptoms that are commonly caused by other things. Musculoskeletal pain is a classic example. Sorting out which divers have DCS and which don't is a significant challenge when they present solely with these sort of symptoms. It involves a very nuanced analysis of the dive(s) (profile, gases, rapid ascents etc), the exact nature of the symptoms (lots of potential distinguishing features here), their specific temporal relationship to the dive, the presence or absence of other plausible precipitating events, whether the symptoms occur at other times unrelated to diving, and sometime even a surrepticious evaluation of the psychological profile of the diver (anxious nervy type vs stoic tough guy) etc etc.

We have always accepted however, that sometimes we will get it wrong and recompress people who do not, in fact, have DCS. Everyone is moderately comfortable with that. Recompression in a chamber is fairly safe therapy; it is not like it is surgery or chemotherapy or other medical interventions with significant side effect profiles. So erring on the side of conservatism has been a common orientation.

Where things becomes problematic is when a diver presents with non-specific symptoms in a remote location from which it will cost a small fortune for an evacuation. We don't want to be doing that unnecessarily. That is why a huge effort went into developing a definition of "mild DCS" at a 2004 UHMS workshop. The definition comprised manifestations that we would be comfortable treating with first aid measures (including oxygen) but not recompressing even if the symptoms were due to DCS. This has prevented many unnecessary evacuations over the subsequent years.

This is a bit off topic but I would be happy discussing it further in another thread if necessary.

Simon M

 

[Dr Simon Mitchell]

I'm not sure if I understand this correctly: is there a plan to deliberately bend divers in a study?

Yes. The Navy has used clinical DCS as the primary outcome measure in several studies now, including the deep stops study, thermal study, heliox vs trimix study, and a gas switching study.

The point is that although some of their subjects develop DCS, they get recompressed immediately and the outcome is invariably good.

Simon

 

[Akimbo]

IIRC the accepted DCS rate in the commercial diving world is around 1% and @Akimbo can probably weigh in better on that

Sorry, but I'm not sure the data is available to make a blanket statement like that. For example, mixed-gas DCS in the British and Norwegian sectors of the North Sea is virtually zero since it is all saturation. The data for commercial air hits are probably pretty good but are hard to compare world-wide due to data reporting standards and allowable depth limitations. It (hit frequency) is surely much better today than 20 years ago due to improved algorithms and laptop computers to run them. For all practical purposes, there isn't enough Trimix data to be matter (from the commercial sector).

I'm not even confident that DCS incidence reporting in the US is reliable. Do you count adding a extra 20-minute cycle at 60' on a Sur-D-O2 run because the diver's elbow hurts? Everything hurts when you work hard. It is in everyone's best interest not to call it DCS instead of burning up some extra chamber time and O2 on the QT. Diver's don't want to get sent home and lose depth pay, supers don't need the paperwork, insurers really don't want to know unless there is little doubt, and it doesn't make dive company management look good.

There's a similar problem with the reliability of OxTox data of combat swimmers going too deep on O2 rebreathers. There are too many negative incentives for everyone involved to accurately report exceeding protocol plus they tend to operate in a pretty compartmented admin environment.

 

[Akimbo]

I'm not sure if I understand this correctly: is there a plan to deliberately bend divers in a study?

Yes. The Navy has used clinical DCS as the primary outcome measure in several studies now, including the deep stops study, thermal study, heliox vs trimix study, and a gas switching study...

Are you sure this is exactly what you intend to say? This could leave the impression that they intentionally bend divers for the purpose of treating them. Wouldn't it be more accurate to say that they expect to bend some percentage of divers as part of the process of determining limits? Basically the procedure is too conservative when nobody gets bent.

At the opposite extreme, I know of several studies that were cancelled when the percentage was too high (back to the drawing board). Over the years this process has provided more than enough opportunity for them to secondarily test DCS treatment procedures.

 

[tbone1004]

@Akimbo the commercial numbers were something I remember Dr. Pollack talking about in an interview with the body temperature studies to DCS and saying that certain commercial areas have different acceptance rates of DCS. In terms of the percentages and what gets reported, who knows, that is just what is published by DAN and from the AAUS studies so I'm positive that there are lots of incidents that aren't reported to DAN, but those would likely be the smaller ones.

 

[Akimbo]

This is a bit off topic but I would be happy discussing it further in another thread if necessary.

On the other hand, after OxTox, isn't diagnosis the biggest problem with IWR without a sat phone? It may actually be more critical than OxTox with older diving populations that can afford exotic trips and rebreathers. ie: IWR based on misdiagnosed DCS can be a real problem for stroke or cardiac victims. I think this is a pretty good place to have the discussion.

Edit: added "based on misdiagnosed DCS" above. It is just an inconvenience and and waste of Oxygen if the true DX is a pulled mussel.

 

[Akimbo]

I remember Dr. Pollack talking about in an interview with the body temperature studies to DCS and saying that certain commercial areas have different acceptance rates of DCS.

I think you're talking about the unexpected high hit-rate of hot water suit divers back in the 1970s. As I recall, the dive industry was concerned and provided data to hyperbaric researchers. To them, the relative increased hit rate is what mattered rather than absolute numbers. I understood that the companies selected the "comparable" data between divers that were actively heated (hot water suits) and those in wet and drysuits.

I could be mistaken but it was my understanding that all they wanted were factors so they could adjust their proprietary algorithms for divers that weren't freezing their butts off during in-water decompression. It was a long time ago and it didn't effect me that much because most of my focus was on saturation at that point.

 

[Miyaru]

Yes. The Navy has used clinical DCS as the primary outcome measure in several studies now, including the deep stops study, thermal study, heliox vs trimix study, and a gas switching study.

The point is that although some of their subjects develop DCS, they get recompressed immediately and the outcome is invariably good.

Simon

Maybe is the choice of words. ...showing that almost all cases of DCS presenting early and treated early in these programs get better with no long term problems.

Almost is not every case. I read it as when you're deliberately bending divers followed by immediate recompression, almost every diver has a good outcome. Almost = Not every diver. And that's where ethics come into play.
On the other hand, I agree that a valid study on recompression outcome requires comparable controlled cases of DCS. I am very interested in the results of your IWR review.