Deco & Body Position

[TexasMike]

Hello Docs!

Is there a body position that is more advantageous for off gassing nitrogen during deco and safety stops?

And what breathing pattern helps to offgass nitrogen faster? Long, slow, breaths or normal breathing?



--Mike (who wishes he could go diving in the Neutral Buoyancy Lab to "wreck dive" the space shuttle mock up there)

 

[svs3]

It is my understanding that body position can be very important. From a off-gasing pov, the difference in hydrostatic pressure bewteen your head and feet when decoing in an upright position and a horizontal one can be critical, especially at shallower depths. Rememeber, deco tables and computers are just "educated" guesses and more you can do to minimize the variables, like exposing yourself to a constant pressure (horizontal deco position), the more accurate the model will represent whats going in your body and better the model recomendations (ie length and depth of deco stop) will be. Research also shows mild movement of the limbs is also highly advantagous(sp?). George Irvine believes that body position is fundamental to effective deco technique and that highly displined deco is one of the primary reasons, if not the central reason, for the perfect safety record of the WKPP under his directorship.

To approach decompression with casual or sloppy mindset is to court serious trouble.

Cheers,
Sam

 

[Andreas]

and you can get the DIR2-tape and se Bill Mee and Geore Irvine explain it.
Get the tape at http://www.extreme-exposure.com/dir-vhs.shtml

/Andreas

 

[large_diver]

There is an interesting little debate about this topic in the August issue of Dive Training magazine (p. 14).

Green Manelishi, a frequent poster here, sent a letter to the editor stating his belief (following that of the WKPP and co.) that a horizontal position is the best choice for deco position.

The editor responded by saying while this position is "useful", it is a difficult position to maintain, provides minimal offgassing benefit and could result in a potentai injury (since the diver is somewhat less able to see obstructions above).

Would be interested in getting the opinions of DocV, DrDeco, et al on this one.

 

[Dr Deco]

Hello Mike:

Body position during decompression is an interesting concept, and is not one that I have thought about too much in the past. As the earlier responder remarked, a hydrostatic (= pressure) gradient exists between the head and the feet and this will be reduced if one is recumbent. This recumbency should promote blood flow in the legs because the pressure in the veins is not as great as when one is erect.

I suspect however, that the greatest effect on dissolved nitrogen washout will be from movement during decompression. Such movement is very beneficial in increasing the flow of blood through the moving tissue. This increase is from several mechanisms:

• Movement of muscles causes a contraction of the capillaries and a squeezing out of the blood (“the muscle pump”);
• Local increases in concentration of vasodilators (substances that cause the vasculature to open [dilate] );
• Increase activity of the central nervous system that signals the arteriole sphincters (= circular, muscular gates or valves) to open; and
• Increased rate of the heart and increases in its contraction.

Thus some physical activity during off gassing is good. (However, remember to avoid vigorous activity since it can lead to the development of tissue nuclei. This includes straining to reenter the dive boat.) This activity is good both at the stops while in water and when you reach the surface.

The rate of breathing is not actually very important since removal of gas at the lung is not a rate-limiting step. Gas exchange is only limited when one is exercising very hard. Becoming “winded” is, in part, a consequence of the limits of gas exchange in the lungs (and this really involves the release of carbon dioxide).

It is also important to recall that decompression schedules are meant to be a guide and are a “road map” of the partial pressures in your body. While it was postulated for decades that these “limits” represented tolerable gas supersaturations, it is not believed to be a fact today by barophysiologists. They do not represent or define bends/no-bends boundaries - - these are largely a pious fiction. Rather the “limits” are arbitrary regions where the nitrogen partial pressure is low and DCS is not probable (but it is possible).

You can limit your risk by the avoidance of those activities that promote the shift of your tissues into the “regions” of higher DCS probability. These actions include the generation of nuclei by strenuous physical activity and straining maneuvers.

Dr Deco