Decompression controversies

[David Doolette]

Agreed, but I think it's smaller than the systematic factors for an individual diver.
In a study with many individuals doing the same profile, there is variance in the population. Some have a PFO, some are fat, some smoke. If all run the same profile and 5% get bent, you might read "5% DCS risk" as "this profile is too tough for 5% of the population". That's exactly what you want to know when making a table.
For one individual diver this kind of "randomness" does not exist. Either you have such a risk factor or you don't. One person repeating the same dive over and over again should show more consistent results. But you hardly ever do that anyway, so in the end it's the profiles and dive conditions that vary, and when you get bent you can only guess if it was because of the profile, dehydration, being cold, ..., or just bad luck.

I have the opposite opinion. Some of the decompression trials conducted by the US Navy in the last couple decades are among the few studies where the exact same dive profile has been conducted hundreds (in some cases) of times under rigidly controlled laboratory conditions (depth, time, gas, work, temperature always practically the same). Because there are limited number of experimental subjects, some individual divers will dive the exact same profile on a few (or many) occasions. If ten divers undertake a dive profile one day, diver "A" might get DCS. On the next occasion the same ten divers undertake the exact same dive profile, diver "A" can be fine and diver "B" will get DCS, and so on. This sort of data is incontrovertible evidence that there is intra-individual variability in susceptibility to DCS (i.e. day-to-day variability - or "randomness" - in the same individual). The data that does not exist (as far as I know), is the same ten divers have not conducted the identical dive profile, say, 100 times, to identify if there is a diver "F" who gets DCS more often than everyone else, and a diver "J" who never gets DCS. Some inter-individual variability surely exists, we just have no idea how large it is, or if it is more important than the intra-individual variability.

Everyone thinks they are diver "J" until they get DCS.

David

 

[dmaziuk]

For one individual diver this kind of "randomness" does not exist. Either you have such a risk factor or you don't. One person repeating the same dive over and over again should show more consistent results.

For a well-calibrated perfectly spherical diver of uniform density perhaps. With an actual human body you get this:

... under rigidly controlled laboratory conditions (depth, time, gas, work, temperature always practically the same). Because there are limited number of experimental subjects, some individual divers will dive the exact same profile on a few (or many) occasions. If ten divers undertake a dive profile one day, diver "A" might get DCS. On the next occasion the same ten divers undertake the exact same dive profile, diver "A" can be fine and diver "B" will get DCS, and so on. This sort of data is incontrovertible evidence that there is intra-individual variability in susceptibility to DCS (i.e. day-to-day variability - or "randomness" - in the same individual).

 

[boulderjohn]

I found it bemusing that training agencies and dive computer manufacturers were so keen to adopt deep stops despite the lack of evidence of their effectiveness.

It is interesting, but I can see the dilemma. When you make changes to a course and build the course materials, it is a very expensive operation. You invest a lot of money up front in the planning and the course materials, and you hope it will all work out financially over time. Those books and materials need to last a number of years. With knowledge in a constant state of flux, they have to make a decision about what goes into the latest version of the course, knowing it might be a part of the curriculum for a decade or more. They can't write and print a new textbook every time a new study comes out.

I taught the academic portion of a trimix course this morning, using the 10-year old textbook. We discussed the fact that the course talks a lot about deep stops. The text says clearly that they are not proven. It says clearly that they have not had human testing. It says their use is debatable. The course standards, however, call for students to plan and execute some dives using deep stops, so not using them would be a violation of standards.

The book also talks about how quickly thinking can change, and it tells students that they must stay on top of the latest developments. I had already discussed this issue with these students when they were taking other tech courses, and they saw this video. We decided we would not use deep stops in our dives.

But what about the standards that call for them? No problem. I had already contacted the agency, and they confirmed that keeping current with evolving theory trumps standards, and we are free to ignore them when current research calls them into doubt like this.

 

[kensuf]

Teaching decompression theory this way makes some sense.

If you ask a random, non-diving person on the street to explain what's really going on inside a diver's body that leads to decompression sickness, the answer is likely to be "I don't know".

If you ask the same question of a typical scuba diving instructor, the answer will likely be that nitrogen is absorbed by body under pressure (a result of Henry's Law); and that if a diver ascends too quickly, the excess dissolved nitrogen in the blood will "come out of solution" in the blood to form tiny bubbles; and that these bubbles will block blood flow to certain tissues, wreaking all sorts of havoc.

Pose the question to an experienced hyperbaric medical expert, and you will probably get an explanation of how "microbubbles" already exist in our blood before we even go underwater; and that ratios of gas partial pressures within these bubbles compared with dissolved partial pressures in the surrounding blood (in conjunction with a wide variety of other factors) determine whether or not these microbubbles will grow and by how much they will grow; and that if they grow large enough, they may damage the walls of blood vessels, which in turn invokes a complex cascade of biochemical processes called the "complement system" that leads to blood clotting around the bubbles and at sites of damaged blood vessels; and that this clotting will block blood flow to certain tissues, wreaking all sorts of havoc. You will likely be further lectured that decompression sickness is an unpredictable phenomenon; and that a "perfect model" for calculating decompression schedules will never exist; and that the best way to calculate the best decompression schedules is by examining probabilistic patterns generated from reams of diving statistics.

If, however, you seek out the world's most learned scholars on the subject of decompression and decompression sickness, the top 5 or 6 most knowledgeable and experienced individuals on the subject, the ones who really know what they are talking about; the answer to the question of what causes decompression sickness will invariably be: "I don't know". As it turns out, the random non-diving person on the street apparently had the best answer all along.

Phys

 

[boulderjohn]

Teaching decompression theory this way makes some sense.



Phys

Ken forgot to credit the author on that piece, so I will add it. It comes from Dr. Richard Pyle. The web page he links to is from the Bishop Museum, where Pyle works. Although his name does not appear on that page, it looks like the kind of pages he was creating in those days, so I would guess it comes from him, too.

 

[lermontov]

It is interesting, but I can see the dilemma. When you make changes to a course and build the course materials, it is a very expensive operation. You invest a lot of money up front in the planning and the course materials, and you hope it will all work out financially over time. Those books and materials need to last a number of years. With knowledge in a constant state of flux, they have to make a decision about what goes into the latest version of the course, knowing it might be a part of the curriculum for a decade or more. They can't write and print a new textbook every time a new study comes out.

I taught the academic portion of a trimix course this morning, using the 10-year old textbook. We discussed the fact that the course talks a lot about deep stops. The text says clearly that they are not proven. It says clearly that they have not had human testing. It says their use is debatable. The course standards, however, call for students to plan and execute some dives using deep stops, so not using them would be a violation of standards.

The book also talks about how quickly thinking can change, and it tells students that they must stay on top of the latest developments. I had already discussed this issue with these students when they were taking other tech courses, and they saw this video. We decided we would not use deep stops in our dives.

But what about the standards that call for them? No problem. I had already contacted the agency, and they confirmed that keeping current with evolving theory trumps standards, and we are free to ignore them when current research calls them into doubt like this.

I think this is a great approach -i think its important to keep any course or teaching module open ended allowing for improvements and new techniques to be introduced- keeps any sport fresh and dynamic

 

[admikar]

I am not a deco diver, and have no plans becoming one in near future, but I would like to ask few questions, to see if I got the concept right.
I understand what GF's are. If we presume that deco time will be the same for both settings, my question is: is GF 40/70 safer than GF 30/85 (numbers are random). I know that higher GF low will help with slow tissues on gassing, but is it also safer to ascent "faster" at depth because of lower pressure gradient? Or is bubble formation only tied to delta P?

 

[Freewillow]

I am not a deco diver, and have no plans becoming one in near future, but I would like to ask few questions, to see if I got the concept right.
I understand what GF's are. If we presume that deco time will be the same for both settings, my question is: is GF 40/70 safer than GF 30/85 (numbers are random). I know that higher GF low will help with slow tissues on gassing, but is it also safer to ascent "faster" at depth because of lower pressure gradient? Or is bubble formation only tied to delta P?

i am not an expert, but my peronnal choice would be to consider GF 40/70 much safer. The reason is that GF 40/70 gives a TTS of 37 minutes for a dive @ 30M for 30 minutes, while the 30/85 gives only 23 minutes.

I would add that for this type of dive, I would be using GF 70/80, which gives a tts of 23 minutes.

 

[bada3003]

Teaching decompression theory this way makes some sense.
...
Phys

That's a good one.

There's also the saying that a little (and, perhaps, moderate) knowledge can be a dangerous thing. Kudos to the folks at NEDU for doing empirical studies on which all good science is ultimately based. Our tax dollars actually being put to productive use!

 

[rjack321]

I am not a deco diver, and have no plans becoming one in near future, but I would like to ask few questions, to see if I got the concept right.
I understand what GF's are. If we presume that deco time will be the same for both settings, my question is: is GF 40/70 safer than GF 30/85 (numbers are random). I know that higher GF low will help with slow tissues on gassing, but is it also safer to ascent "faster" at depth because of lower pressure gradient? Or is bubble formation only tied to delta P?

the ascent time won't be the same, 40/70 is going to have a lot more time shallow than 30/85.