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 doctors 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 dont 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 Im talking about with one glaring error. Its 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 thats 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 whats happening mechanically and physiologically in the lungs when the diver tries to breathe through a 5 foot snorkel (they cant) and well 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.
