Decompression Tissue Question

[100days-a-year]

Are we assuming blood's ability to convey gasses from off gassing tissues remains constant in every type of tissue no matter it's location in the body? What if the blood has to pass through restrictions like capillaries? What causes DON?

 

[Seaweed Doc]

Yeah you are right. But the models are still based off of tissues and how they respond to pressure and gas changes... also nitrogen is used somewhat - in things like DNA synthesis, protein synthesis, and enzymes. But yeah, you pretty much repeated what I said, your ascent is limited by your most saturated tissue... This stuff is really interesting and confusing, I need to find a book.

Nitrogen gas can't be used by humans. We (and pretty much all organisms with nuclei in their cells) can't break the triple bond in N2. In our case, we get the nitrogen used in amino acids, nucleic acids, etc., from our food. Plants get their nitrogen from things like ammonia and nitrate in the soil. Some bacteria are capable of breaking the nitrogen bonds and convert N2 to ammonia (NH3).

In other words, the excess N2 you inhale has to off gas. Your body can't use it.

 

[dmaziuk]

Are we assuming blood's ability to convey gasses from off gassing tissues remains constant in every type of tissue no matter it's location in the body?

Yesno. Straight exponential model does, whereas linear-exponential will at some point cap the off-gassing. On the other axis e.g. DCIEM is a sequential model where gas passes from one tissue compartment to another whereas Buhlmann and other "parallel" models assume direct gas exchange between TC and "ambient pressure".

This is all part of why the relationship between "tissue compartments" and actual tissues is tangential at best.

 

[Scuba_Nick27]

Nitrogen gas can't be used by humans. We (and pretty much all organisms with nuclei in their cells) can't break the triple bond in N2. In our case, we get the nitrogen used in amino acids, nucleic acids, etc., from our food. Plants get their nitrogen from things like ammonia and nitrate in the soil. Some bacteria are capable of breaking the nitrogen bonds and convert N2 to ammonia (NH3).

In other words, the excess N2 you inhale has to off gas. Your body can't use it.

Yes you are right. I forgot that.

 

[Scuba_Nick27]

They really aren’t based on real tissues. “Tissue 5” in ZHL-16 isn’t an actual physical structure. It’s a model, not reality.

Nitrogen used in biological processes does not come from what you breathe in (N2). It comes from the food you eat, and it’s always in a different form than N2.

Yeah my bad about the nitrogen part. I definitely need to find some material about this... since now I'm really thinking about it. Does scubaboard have articles on this?

 

[Scuba_Nick27]

Response to pressure changes is very simplified, all it is is partial-pressure difference that equalizes at ln(2) rate. The key parameter is time constant, specifically the time it takes delta-P to drop to half its initial value. You fine-tune the model by futzing with the number "tissue compartments" and their half-times, trying to back-fit your theoretical "compartments" to empirical observations (as @Bernie_U posted).

You would typically have extra criteria to judge when you're "close enough". DSAT report, for example, mentions that 1990s USN tables were developed for 2% predicted DCS because the Navy considered 2% acceptable at the time.

I.e. you're actually looking at optimizing for multiple parameters, and the "real tissues" aren't even on the list.

They really aren’t based on real tissues. “Tissue 5” in ZHL-16 isn’t an actual physical structure. It’s a model, not reality.

Nitrogen used in biological processes does not come from what you breathe in (N2). It comes from the food you eat, and it’s always in a different form than N2.

So basically, the tissues, even though they aren't "real" still average for the average human being... and it doesn't matter how much, lets say, adipose tissue or muscle you've got, the algorithms still somehow account for that and are accurate down to 2%.

 

[PfcAJ]

So basically, the tissues, even though they aren't "real" still average for the average human being... and it doesn't matter how much, lets say, adipose tissue or muscle you've got, the algorithms still somehow account for that and are accurate down to 2%.

Most in-use models work pretty well for the average human, mostly by being quite conservative for the recreational diver. DCS rate are very low for rec diving.

 

[dmaziuk]

So basically, the tissues, even though they aren't "real" still average for the average human being... and it doesn't matter how much, lets say, adipose tissue or muscle you've got, the algorithms still somehow account for that and are accurate down to 2%.

2% was quoted for USN in the 90s, you have to remember that those people are young, fit, may need to get out of there in a hurry, and their boat has a chamber on board. A model developed for old overweight people leisurely diving hours away from nearest hospital will probably aim for lower predicted DCS incidence.

 

[NothingClever]

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?

I think your intuition is on track.

To be honest, I am really surprised by the number of answers here that casually ascribe Haldane’s fundamental analytical framework to “it’s just a theoretical model.”

I disagree with that based on a scholarly publication of proceedings from the 100th year celebration of Haldane’s landmark contributions to respiratory physiology and health protection. The symposium was co-sponsored by the Smithsonian and Trondheim University (Trondheim, Norway) in 2008.

I think Haldane had very specific types of tissue in mind when he established the first five compartments of tissues and that he used them methodically to establish controls in his studies. He was quite focused on the endothelium, the brain, the correlational aspect of velocity between diffusion and perfusion with the various tissues (so, lungs, blood, organs, muscles and other benefitting tissues) and the long term impact of oxygen toxicity on the brain. So extensive was his physiological studies that he used goat urine and it’s absorptive qualities with nitrogen to help determine one of the tissue half-lifes he developed. This use of goat urine was part of a much broader array of experiments with goats that both survived and perished in the decompression and recompression experiments. Those goats that perished were autopsied to specifically analyze tissues.

So, while I haven’t really answered your original question of “What exactly are the 16 tissues?”, I’ve tried to confirm you’ve got the bow of your ship pointed down the right channel.

I have other projects I’m working on but if I nail down the 16 tissues I’ll circle back here to list them. NOTE: Some computer manufacturers have used as many as 20 compartments. I’m not going to chase those as my hunch is their marketing and sales guys were making the decisions rather than any scholarly scientists.

I think the part that IS indeed theory is the determination of reliable, accurate and predictable outcomes while using a decompression model to account for and manage the rate of on-gassing and off-gassing from one individual to the next in various environments.

 

[Ghost95]

Thanks for the replies all. Interesting as always.

I always figured there were actual tissues at the root of the original calculations. I know that every thing is theoretical now but I just wondered what that theory had been extrapolated from. If I get time maybe I'll go through some of the references mentioned and a little research and see what other rabbit holes it leads me too.

Thanks again.