"Undeserved" DCS hits

[TSandM]

That's Rick Murchison's quote, and it's one of my favorites.

 

[The Chairman]

He was probably quoting Rick!

 

[jtivat]

Actually, yes.

If you use more deco gas because your respiratory and cardiac rates are elevated, that will increase the elimination of inert gas acquired during the bottom phase. Obviously, intentional hyperventilation is different, and bad, for several reasons involving body pH, PCO2, and WOB, maybe even O2.

Pre-WWII, USN hardhat divers used to do jumping jacks on the dive stage during decompression to facilitate off gassing. Read some of Edward Ellsberg's works on diving in the 1920's. However, when it came to working this into tables it was too difficult to calculate. How much work is work? It resists quantitative analysis, which leads us back to the beginning of this thread.

This is not new science? It is basic decompression physiology, and if you don't understand it, you probably should not be conducting deco dives? You could earn yourself an unearned hit.

Cheers

JC

Well maybe you should write some deco software as you have it figured out better than those that have.

I have asked this question of people like Erik Baker and Doctors at DAN and they all seem to feel you are full of it!

After many of your posts here I am starting to feel you are full of yourself.

 

[jjones9527]

Well I just sat down and read this whole thread for the first time last night. I agree and disagree with a lot of whats been said, sometimes from the same poster

I do agree with JC that you should know your own body and be able to adjust your deco based on in dive variables (cold, stress, etc). Doing this is as simple as adding an extra few minutes to your bottom time when calculating your deco. As with anything, theres no hard and fast rules, which is why you need to know your body, know how you react to decompression, and adjust accordingly. This is where progressive experience comes in.

While there shouldn't be a stigma attached to someone who has a had a DCS hit, I don't think there should be a badge of honour either. I do think there are unexplained hits, however most, if not all of the cases I have seen were the result of an identifiable error on the divers part.

Those who are modifying their deco without a real rational reason or understanding are just moving the risks elsewhere. Slowing your ascent too deep, or doubling or tripling your deco may sound good on the surface, but you end up negating any schedule your chosen deco model was designed to give you. Haphazardly increasing your deco becomes unfeasible when your unmodified schedule is already an hour or two long.

When I did a deco course a few years ago, it basically consisted of read the manual, do the test, 4 dives, heres your card. The major knowledge passed on in the course was "Do what your computer tells you." I basically didn't bother with deco dives after that, because I felt I didn't have the knowledge to do it safely (and 100' on 32% provides lots of BT). I later did another tech course with a different agency, and the knowledge was astounding. I understood what deco was, different models, how to adjust them, etc. If you're a "follow the computer" person, perhaps you should rethink your training and understanding of decompression before messing around with what its telling you.

 

[boulderjohn]

This is not new science? It is basic decompression physiology, and if you don't understand it, you probably should not be conducting deco dives? You could earn yourself an unearned hit.

Here are a couple of old threads from the ask Dr. Decompression forum on the issue of breathing rate and DCS.


http://www.scubaboard.com/forums/ask-dr-decompression/199627-does-volume-air-breathed-change-dcs-risk.html

http://www.scubaboard.com/forums/ask-dr-decompression/169732-air-consumption-nitrogen-absorption.html

 

[Duke Dive Medicine]

This may be fatalisitic,but I have to give this some validity. This thread has gone on for some time, with a lot of responses,but how many posters on this thread have gotten bent,and from an "unearned hit"? I am not bashful to admit that a couple months ago I took a hit that exceeded what most cave divers blow off as a little skin bends. I developed subcutaneous emphysema along with large region of skin bends. The dive involved staged decompression,but nothing any different than I have done many times before. So the comment,you do this long enough you will get bent probably has some validity,just took me 1400 cave dives before I got the wake up call.

Are you sure it was subcu emphysema? If so, that's not from DCS, it's from a pulmonary overinflation injury and a different animal altogether. What were your symptoms?

 

[Cave Diver]

Let us say, that we have two identical divers making two identical dives with the same equipment. Diver 1 uses 2500psi of gas, while Diver 2 uses 1500psi of gas. So, Diver 1 breathed more gas, specifically more molecules of inert gas? So, would it not follow that Diver one had more molecules of inert gas in his/her body to get rid of? Duh????

I asked a similar question right out of my BOW class. The answer I got at the time was that your tissues will only absorb nitrogen at a given rate. If it takes 6 minutes for a compartment to fully saturate, you can't saturate it faster by breathing more. If I'm reading your response correctly, you're saying this premise is wrong?

 

[boulderjohn]

Are you sure it was subcu emphysema? If so, that's not from DCS, it's from a pulmonary overinflation injury and a different animal altogether. What were your symptoms?

I am reminded of something I mentioned something earlier. I read an article in (I believe) Dive Training a few years ago that said there was some indication that unexpected DCS hits might be initiated by minor lung overexpansion injuries and so may actually have aspects of both injuries. It did not say how one led to the other, and it gave no references to an article that I recall.

Do you have any information on this?

I would also assume that even if there is no direct connection between the two, it wold be possible to have a lung overexpansion injury and DCS on the same dive, wouldn't it?

 

[jtivat]

I asked a similar question right out of my BOW class. The answer I got at the time was that your tissues will only absorb nitrogen at a given rate. If it takes 6 minutes for a compartment to fully saturate, you can't saturate it faster by breathing more. If I'm reading your response correctly, you're saying this premise is wrong?

Same here but also what affects tissue loading is the PP of a given gas breathing faster does not affect your PP.

 

[rjack321]

I asked a similar question right out of my BOW class. The answer I got at the time was that your tissues will only absorb nitrogen at a given rate. If it takes 6 minutes for a compartment to fully saturate, you can't saturate it faster by breathing more. If I'm reading your response correctly, you're saying this premise is wrong?

This would occur IF on-gassing was substantively lowering the PP of N2 or He in the inspired gas as it dwells in the lungs. But the moles of gas actually entering the body is a miniscule fraction of the inspired moles and the fraction of N2 or He and hence the PP doesn't really decline due to differences in lung dwell time. So respiratory rate ends up being not relevant.