Guaranteed Aurgument....

[BIGJC]

If the time it takes to decompress from a dive is related to how saturated your tissues are then why does a 150lb man and a 190lb man have the same deco obligation from similiar profiles? shouldn't the larger man have a longer one? even if their body fat is the same, the larger man has more of every tissue...right????

 

[wreckchick]

The uptake and subsequent off-gassing is proportional across body size giving people with the same basic proportion of fat to muscle the same amount of time to off-gas.

I am the furthest thing from an expert ever, but the above makes sense to me!

Rachel

 

[Dr Deco]

Dear BIGJC:

Gas Transport

What you say would be true (larger vs. smaller) if gas was loaded from the outside (diffusion through the skin). The nitrogen is delivered through the blood system via capillaries and nitrogen then enters the tissue. The tissue and capillaries are not arranged based on body size when the body is large (i.e., capillary density is constant).

The important components in living systems are the uptake of oxygen and food and the elimination of carbon dioxide and cellular waste products. These occur in cells independent of the size of the [mammalian] organism.

Size vs. Size :book2:

Perfusion is body-size dependent across the animal kingdom, mice can decompress much faster than rabbits, and these in turn are faster than humans can. The spread in humans is very small.

Individuals in good physical condition do have a higher capillary density and the decompression obligation is related to fitness.

Conservatism

All tables and deco algorithms have such a margin of conservatism that somatic (body size) differences are not reflected in this factor. If a table was constructed for a given individual, such differences might become apparent.

Neurological DCS and Doppler Bubbles

There is some relationship to body fat in the venous bubbles. Again, there is such a degree of conservatism that it would not appear in tables released to a general population.

Dr Deco :doctor:

Please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[BIGJC]

OKay, I understand that the blood supply density allows off gassing but what about the so called "slow tissues" the ones that have very little blood supply, surely they are more effected??

It just seems like there are too many variables to be able to generate tables that fit everyones needs....


maybe I'm just being difficult....

 

[Dr Deco]

BigJC:

Tissue Gas Uptake in “Slow Tissues”

Slow tissues scale in the same fashion that fast tissue will scale. The capillary density is not a function of total size (for large animals). I believe (and this is by no means agreed upon by barophysiologists) that, for joint-pain DCS, the fast and slow tissues are the same and differ only in the perfusion at that moment. At NASA, with exercise during the oxygen prebreathe, we can vary the resulting DCS incidence. This can be viewed as a gradual shift of the “effective tissue halftime.” It varies from 36 to 360 minutes, the generally-agreed upon ten-fold increase of blood flow increase with maximal exercise.

Fat Tissue

Abdominal fat is a different issue, here we are generating new fat storage vacuoles (= subcellular lipid storage compartments). Surprisingly, when adding fat to your body, the number of fat vessels does not increase but rather the size of the vacuoles. This we can add a very large nitrogen storage depot with a long halftime. These fat compartments can release gas bubble into the venous system and lead to severe DCS.

Too Many Variables

You certainly have that correct. To obviate this problem, the scheme is to generate a table that is suitable for virtually everyone. Large conservatism is built into the table – or computer – to make a “one size fits all” device. For this reason, computers can vary considerably and still give a safe decompression. Naturally you cannot abandon your conservative algorithm and surface along with all the other divers, because your meter cannot adjust for a switch of algorithm. Possibly if I made the [hypothetical] Bends Buster decompression computer, it would allow for modifications on the fly.

Considerable time is spent by divers splitting hairs on different algorithms and models when such accuracy is not warranted. This is part of what is discussed in the Decompression Physiology class (see below). :book2:

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm