Breathing physiology... whats best for off-gassing

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It was a question, not an assertion. I do beleive that the concept of off-gassing (or on-gassing) PP of any gas in the blood has to do with the tension between the excess gas in the blood, the current PP of the gas being breathed (at the regulator) and the particular compartment values we are dealing with.

I asserted in the very beginning of this thread that there is no pressure gradient within the lungs, and Dr. Deco and you have confirmed that there is no PP gradient in our tissues due to a differential pressure outside of the body. Furthermore, as breathing is done more with the diaphragm pulling down, rather than the chest pulling out, our alveoli are probably normal with respect to being horizontal, right side up, or right side down. This is what I believed to be true from the beginning, but could not articulate it. Thanks for putting a handle on it! In other words... its either six of one or half dozen of the other!

I can see why that DM was acting so smug! :tease:
 
NetDoc once bubbled...
Furthermore, as breathing is done more with the diaphragm pulling down, rather than the chest pulling out, our alveoli are probably normal with respect to being horizontal, right side up, or right side down.

Actually, its a bit of both!
 
I post something from a doctor, and another doctor brushes it off as “opinion.” Then starts giving opinions. May I assume we can dismiss your opinions as casually as you dismissed the other doctor’s “opinions?”

Look, you all have come to EXACTLY the same conclusion I was making months ago, as far as I can tell you just don’t want to admit it.

There is no pressure gradient in the lungs (thank goodness Pete *finally* grasped this. Still think the bones protect all the soft tissues from the effects of pressure, or have we gotten past that as well??).

Thomas’ diagram_d.jpg is *exactly* what I’m talking about with one glaring error. It’s accurate if the diver is in a horizontal position where some of the lung is above the reg and some below (the axis is wrong, but that’s a nit). But in a vertical position even the very top of the lung is under ventilated (since the top of the lungs are below the regulator), and it just gets worse from there

So after agreeing with my original posts, to avoid coming out and saying that “a horizontal position is better” we invented a dance called “orientation is not the most important factor in offgassing.”

This is correct. No one EVER said it was the MOST important. It is however important and well-ventilated lungs with good gas exchange is very important to decompression, so a horizontal position is better than a vertical one.

Still don’t think so? Perhaps one of the doctors would like to start us out with a description of what’s happening mechanically and physiologically in the lungs when the diver tries to breathe through a 5 foot snorkel (they can’t) and we’ll work backwards from there.

The resistance I see to such a simple concept just boggles my mind.

Roak
 
http://www.scubaboard.com/showthread.php?s=&threadid=4911&highlight=smug+safety+stop

Not only does this demonstrate that they have horrible trim, but a vertical position is the worst position for gas exchange due to the hydrostatic head that’s developed between the top and bottom of your lungs. The alveoli at the bottom are subjected to a greater pressure (about .5 psi) and don’t expand as readily as the ones near the top. In a horizontal position this pressure difference is more than halved, and more alveoli can be used for effective gas exchange

So I am glad that YOU came around!!! :tease:
 
I don't tilt at shadows, exactly why do you think I've changed my tune?

Roak
 
roakey once bubbled...
I post something from a doctor, and another doctor brushes it off as “opinion.” Then starts giving opinions. May I assume we can dismiss your opinions as casually as you dismissed the other doctor’s “opinions?”
When I give an opinion, I give an opinion, nothing more, nothing less.

When I stated a fact on this thread, I made it clear that I was stating a fact.

In this country there are over 60M residents, and you can obtain a medical opinion from every one of those 60M people. Whether you accept that opinion is up to you, Roaky. In my opinion Dr Krebs is wrong. It is a shame you cannot get him to explain precisely why his opinion is the correct one, because without proof he has yet to convince this doctor, who also happens to have had an earlier career in environmental physiology.
Look, you all have come to EXACTLY the same conclusion I was making months ago, as far as I can tell you just don’t want to admit it.
That may be so, Roakey, but the conclusion that I eventually reached was based on an analysis of medical and physiological concepts, taking each concept to its ultimate conclusion. It is a shame no other doctor commented when I fell into the trap of looking at the ambient pressure gradient, which does exist, and an offgassing gradient, which I now believe does not.
There is no pressure gradient in the lungs (thank goodness Pete *finally* grasped this. Still think the bones protect all the soft tissues from the effects of pressure, or have we gotten past that as well??).

Thomas’ diagram_d.jpg is *exactly* what I’m talking about with one glaring error. It’s accurate if the diver is in a horizontal position where some of the lung is above the reg and some below (the axis is wrong, but that’s a nit). But in a vertical position even the very top of the lung is under ventilated (since the top of the lungs are below the regulator), and it just gets worse from there.
My friends call me Paul.

Now, I am not sure whether you believe we have an exoskeleton or not, Roakey, but the only part of the body that can actively be made to generate a pressure less than that of ambient pressure is the chest cavity, and then the intrathoracic pressure can only be reduced to about 0.3 bar (snorkelling depth). Muscular activity can be used, momentarily to increase the pressure within the abdomen and the chest cavity by a greater amount when straining but this increase is always referred to ambient pressure - exerted on the abdomen when the muscles are at rest, so I am not sure of the point you are trying to make.

I have already stated that vital capacity has frequently been shown to be reduced by immersion and tight fitting clothing etc. but this does not mean that the inate control mechanisms cannot generate adequate ventilation and respiration when diving. If this were not the case all divers would eventually succumb to carbon dioxide poisoning;- Respiratory control is mainly determined by the CO2 content of the venous blood.

It has been shown many times that in an effective zero gravity envirinment cardiac output increases because of the reduced work required for venous return. One effect of this is the diuresis generated by increased blood flow to the kidneys. It seems reasonable, at least to me, that since the entire cardiac output must pass through the lungs, the perfusion to all parts of the lung will be increased upon immersion. If all parts of the lungs are hypoventilated, as you state, this quite clarly leads to VQ mismatch and must result in CO2 retention.

However, this increase in pp CO2 generates a subsequent reflex increase in respiratory drive and respiratory minute volume;- due to an increased respiratory rate and an increase in tidal volume. (I suppose this is partly consequent upon a reduction in functional residual capacity on immersion in the erect attitude.) On the other hand, if this does not occur in the prone position, as you suggest, there is less VQ mismatch, less CO2 retention and therefore less respiratory drive. In the prone position the diver needs a lower respiratory minute volume in order to achieve the same reduction in pp CO2 to levels below the threshold for respiratory drive.

Since the excretion of both carbon dioxide and nitrogen are by the same mechanisms they both depend on adequate efficiency of the lungs and ventilation in scuba divers is perfectly adequate, whether in the erect or prone positions.
So after agreeing with my original posts, to avoid coming out and saying that “a horizontal position is better” we invented a dance called “orientation is not the most important factor in offgassing.”

This is correct. No one EVER said it was the MOST important. It is however important and well-ventilated lungs with good gas exchange is very important to decompression, so a horizontal position is better than a vertical one.
Roakey, you are suggesting that reflex respiratory drive is overidden in the vertical attitude. I, for one, fail to see how this can be so.
Perhaps one of the doctors would like to start us out with a description of what’s happening mechanically and physiologically in the lungs when the diver tries to breathe through a 5 foot snorkel (they can’t) and we’ll work backwards from there.
Very simply, the ambient pressure at a depth of five feet is greater than can be generated by the sum of surface pressure and the efforts of the muscles of inspiration acting against it, so no respiration at all is possible.

At shallower snorkelling depths the work required for inspiration is greater than it is on the surface, but if the lung can be ventilated, the reflex mechanisms controlling respiration will ensure that the minute volume is sufficient to excrete adequate amounts of carbon dioxide (and nitrogen).
The resistance I see to such a simple concept just boggles my mind.

Roak
Simple? Not if you control for all those variables!

Since the hydrostatic head within the pulmonary bed is present whether the diver is immersed or not I entirely fail to see its relevance. In fact my diagram D has the axes correct for the erect diver whether immersed or not but it can be reproduced with the axes turned 90 degrees with the diver prone, the gradient in such cases will be from the diver's back (top) to the front (bottom), not from head to toe. The VQ mismatch gradient will always be RELATIVE and in my opinion quite irrelevant.

Perfect trim most certainly is.

:rolleyes:
 
that I had changed "my tune" and had finally grasped a concept that I had actually presented to you in the original thread and which caused this thread to be generated?

I was merely pointing out that you had again twisted my words (and reality) so that you would look good at my expense. Not sure why you despise me so much, but it is obvious that you have a problem admitting that I am right every now and then. I think that Dr. Paul has presented a very sound reasoning as to why he believes what he does. You can dismiss him as you wish. Those that would like to fully understand will listen intently and learn from someone who is oviously more familiar with the physiology then either you or me. As for me, I choose to listen and learn from all of the doctors on this board. You however, may continue to tilt at those shadows as your pride dictates.
 
Off gassing at Depth
There is not any evidence that gas exchange in the tissues at any given depth is any different from another depth, that is, it is depth independent. This is why the half times for the compartments in tables and meters are constant for all depths. (The m-values will change, however.)
Dear Dr Thomas and Readers:

Gas Exchange Halftimes

We had been discussing gas exchange, and I made the remark above in reply to a question on gas exchange at depth. What was on my mind at that time was a change in gas exchange rates as they are expressed by the compartment half times. I stated that the exchange is not different at depth or on the surface and was referring to these half times.

The halftimes used in table calculations are 5, 10, 20 minutes et cetera up to several hundred minutes. These remain unchanged all the way from depth of hundreds of feet to the surface.

M-values

M-values (or maximum values) indicate the supersaturation limits (partial pressures of inert gas) for each half time compartment for a given depth. One knows when it is allowed to decompress from one depth to the next 10-foot shallower depth when a certain gas load is reached in that compartment. This is all by calculation, of course. The maximum loads vary by halftime compartment and they vary by depth.

Robert Workman, a captain in the US Navy who developed this method, determined from laboratory studies and earlier diving information, the magnitude of these partial pressures. The partial pressures allowed varied with depth, becoming a bit small as you went deeper. He found that he could project what the loads would be. This extrapolation produced a table of allowable supersaturations. After a certain depth, the increments became constant. Thus contemporary tables do not have “decompression ratios” but rather partial pressure differences.

These are related to the bubble micronuclei by Laplace pressure that has been discussed so much in this FORUM.


Dr Deco :doctor:
 
Holy S*%@ you guys are vicious!!! But hilarious and informative too, so its all good!!!!!!!
 
Q: “Have any studies been performed looking at the theory that a horizontal, verses a vertical position give the lungs better perfusion and results in better gas exchange?”

A: “Sounds good to me!”

-- Peter B. Bennett, Ph.D., D.Sc. at the “Safer Diving Through Education with DAN” forum on October 12, 2002 in Colorado Springs. (for more information, http://www.diversalertnetwork.org/about/welcome.asp)

He then went on to add, “I’ve already told you how I do my safety stop!” In reference to an earlier comment about light exercise at the stop and that he swims around [I assume in a horizontal position] at the stop. Of course he didn’t answer the question, which was disappointing.

Imagine, Irvine and Bennett agreeing on something! :)

Roak
 
https://www.shearwater.com/products/teric/

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