Decompression Tissue Question

[Ghost95]

Morning all. I didn't want to hijack another thread on decompression theory so I figured I'd start a new one.

I have learned and, in a former life, taught basic decompression theory as it relates to tables. I understand that all tables/computers use a theoretical model to represent tissue groups and that different models use different numbers of these groups to determine time to on gas and off gas nitrogen (primarily). I also understand that different "tissue compartments" on gas/off gas at different rates.

My question is what tissues do those compartments represent in the body?

I have read somewhere that blood is one type of tissue compartment and that it has very fast on and off gas times. That got me to thinking that if blood is a compartment, what other tissues are compartments are represented in models?

I thought of blood, cartilage, muscle, fat, lung, various organs, and maybe brain tissue. Is this what is meant by tissue groups or is it something else?

Also, since all gas is carried into the body by blood from the lungs, and blood on gasses and off gasses quickly, are slow compartments just tissues that have less circulation than other parts, for instance cartilage? Without adequate blood flow do they just have less ability/resources to move gas from one place to another?

I can teach a 7 year old HOW to use tables but what some of the theory represents is something I don't fully understand.

Anyway, this was just something I was thinking about while reading another thread.

Thanks for your answers.

 

[Angelo Farina]

The simple answer is: there is no exact correspondence between a "compartment", which is just a mathematical model, and a "tissue", which is a biological classification of cells.
Using a large enough number of "compartments", and tuning their mathematical properties, one attempts to "cover" ALL the human tissues.

 

[rongoodman]

I think it would be pointless to try to assign the compartments with their varying half-times to specific tissues or organs, beyond the fact that some on- and off-gas quickly and some more slowly. As you mentioned, the differences are probably related to perfusion. For the same reason, hypothermia is assumed to slow down off-gassing and to be avoided, while gentle excercise is sometimes seen as beneficial during deco stops

 

[TMHeimer]

This is stuff I have also wondered about. I understand it's all theoretical and an attempt to cover all tissues. To get to a point where it's not theoretical and we are able to tell exactly how much Nitrogen is in what tissues, they'd also have to figure out how to address the problem that, say, one person's bones are denser/less dense from another's.

 

[rjack321]

This is stuff I have also wondered about. I understand it's all theoretical and an attempt to cover all tissues. To get to a point where it's not theoretical and we are able to tell exactly how much Nitrogen is in what tissues, they'd also have to figure out how to address the problem that, say, one person's bones are denser/less dense from another's.

especially where they have fat cells. A person with more adipose cells might have fat in organs that have X half-life in one person but the extra fat makes them X+ z in another

 

[dmaziuk]

This is stuff I have also wondered about. I understand it's all theoretical and an attempt to cover all tissues. To get to a point where it's not theoretical and we are able to tell exactly how much Nitrogen is in what tissues, they'd also have to figure out how to address the problem that, say, one person's bones are denser/less dense from another's.

But if you go there, you also have to screen them for PFO and shunts whatever else may be lurking in there.

 

[GJC]

Even if we could determine an exact speed of on gassing and off gassing of every tissue in the body, there are other variables that can change it. Temperature, vasoconstriction, peripheral vascular disease, constriction from wet suits and straps, level of activity, and a lot of others factors, some we don't even know about yet. Nobody has figured out how to incorporate all this into an individualized dive plan yet.

Best we can do is to use the decompression models we have now and make changes when a bunch of people get bent, which is basically what we have been doing since humans first started getting bent after being exposed to pressure.

 

[KenGordon]

Morning all. I didn't want to hijack another thread on decompression theory so I figured I'd start a new one.

I have learned and, in a former life, taught basic decompression theory as it relates to tables. I understand that all tables/computers use a theoretical model to represent tissue groups and that different models use different numbers of these groups to determine time to on gas and off gas nitrogen (primarily). I also understand that different "tissue compartments" on gas/off gas at different rates.

My question is what tissues do those compartments represent in the body?

I have read somewhere that blood is one type of tissue compartment and that it has very fast on and off gas times. That got me to thinking that if blood is a compartment, what other tissues are compartments are represented in models?

I thought of blood, cartilage, muscle, fat, lung, various organs, and maybe brain tissue. Is this what is meant by tissue groups or is it something else?

Also, since all gas is carried into the body by blood from the lungs, and blood on gasses and off gasses quickly, are slow compartments just tissues that have less circulation than other parts, for instance cartilage? Without adequate blood flow do they just have less ability/resources to move gas from one place to another?

I can teach a 7 year old HOW to use tables but what some of the theory represents is something I don't fully understand.

Anyway, this was just something I was thinking about while reading another thread.

Thanks for your answers.

No, the compartments are theoretical, not representative. Also, not all models are so simplistic. Some are much more about the outcome statistics. Some have very few compartments but link them together in the model, for example the BSAC88 tables model perfusion (transport of nitrogen via blood flow) using three compartments.

Models are useful for reasoning about how behaviour might influence risk, but it is a bad plan to think they map well to reality. For example the popular ZHL16 model does not take account of bubbles at all, just overpressure limits and hopefully as a result implicitly bubbles, and who knows, maybe the bubbles don’t matter. So using this model as a reasoning tool will mean that an instructor doing 10 ascents teaching rescues skills in a one hour dive is perfectly fine.

 

[Ghost95]

Thanks for the all the replies.

I was just pondering where the models came from and figured that originally they may have been based on actual tissues. I wondered if there is still any of the actual tissue model left in the theory. It sounds like the shear numbers of observations have made this completely mathematical.

I figured it started something like, hey if you stay in this caisson for more than 4 hours you get this funny rash when you get to the surface but less than 4 hours you'll be ok, or 95% of you will anyway.

That would seem to be a limit based on a skin tissue group. I know that's a simplistic explanation and may be a little off but most mathematical models usually start with a real world model somewhere along the line. I was just curious if that was the case and if there were any real tissues still represented in the models.

Thanks again.

 

[Angelo Farina]

It all started from the Original Haldane's observation that, wathever the time of exposure, it was always possible to half the pressure without any DCS occurring. So he developed the theory that the human body "tolerates" a decrease in pressure so that the final pressure is half the pressure of the gas in solution in each tissue.
Of course slow tissues require several hours for reaching the same pressure as the external one (the so called "saturation"), whilst faster tissues reach saturation in minutes.
So Haldane developed his first tables based on this concept.
Later on, it was discovered that the ratio 2:1 for the pressure was not the same for all the tissues, and that it was safer to use smaller ratios for slower compartments.
What we use now are still models based substantially on the old Haldane's approach.