No-Limits Question

[Fitz-Clarke]

I should just clarify. I meant that reaching zero volume does not likely harm the lungs themselves. It is questionable if anyone has even reached such a depth to cause full alveolar collapse, but Herbert Nitsch and Patrick Musimu have probably come close. Negative upper airway pressure could cause injury though, depending on compliance properties of the chest and respiratory tract. There is insufficient data available to make that assessment with confidence. Gas exchange with the lungs would stop of course, but stored blood oxygen could act as a short reserve for a couple of minutes.

Yes, many divers have gone well beyond residual volume. Anyone who has gone deeper than about 50 metres has gone below their RV. You can calculate gas volume compression for any depth easily using Boyle’s law. Blood shift of course replaces some of the compressed air volume, but only partially depending on compliance and capacitance of the heart chambers and blood vessels in the chest. Estimating the volume of blood shift is complicated, as you need to know slope of the chest compliance curve at the low end, and cardiovascular properties. Blood shift can be as high as one litre at great depths. It comes to equilibrium when the pressure difference across the chest wall equals the average pressure difference across the vasculature.

 

[cebaztian]

I should just clarify.

Clarke, you have lost me. You say: Two people on earth MIGHT have reached colapsed lungs. Then you deliver a disclaimer saying there is insufficient data (is there any?), then you say colapsed lungs does not likely harm the lungs (so what is squeeze then?), you say blood reserve is a short reserv, but yet it is about 50% of oxygen stores, and you imply that you loose the O2 that was stored in the lung (but nothing dissapears, it just gets compressed, the O2 is still in the body, but I assume you mean that it is pushed up to places where there is no gas exchange), and when RV is mentioned, do you mean RV on land or the very variable volume of RV that occurs during dives, because of bloodshift. Isnt it so that the colapse is possible due to bloodshift. And isnt it so that if you know TLC and dry RV and come up after a dive without lung injuries (squeeze) then you can quite well estimate how much bloodshift you had? Its all very hypothetical, isnt it? A model? And too hard to understand for me what you are trying to "clarify". Can it be said in simpler language? Does it concern us that dives below 100?

Sebastian

 

[trux]

Well, I am not an expert, but will try to answer some of the questions:

Clarke, you have lost me. You say: Two people on earth MIGHT have reached colapsed lungs. Then you deliver a disclaimer saying there is insufficient data (is there any?),

One does not exclude other. Quite in contrary - we have insufficient data to tell whether the lungs were collapsed completely or only partially, but those two divers were in depths, where (with their lungs volumes), the complete lung collapses is likely.

then you say colapsed lungs does not likely harm the lungs (so what is squeeze then?),

Squeeze is definitely not equal to collapsed lungs. Squeeze is an injury when the tissue is damaged due to barotrauma caused by the difference of pressure inside and outside the lungs. If lungs (alveoli) collapse, and the remaining airways (trachea, bronchi) are still under the ambient pressure, there is no reason for a "squeeze" (barotrauma). You can suffer squeeze for example if your chest and diaphragm are not sufficiently flexible and hence your lungs cannot compress under certain volume (hence do not even come to the point of collapse), hence building underpressure, and leading to the barotrauma.

you say blood reserve is a short reserv, but yet it is about 50% of oxygen stores, and you imply that you loose the O2 that was stored in the lung (but nothing dissapears, it just gets compressed, the O2 is still in the body, but I assume you mean that it is pushed up to places where there is no gas exchange),

Again, I see not conflict in these claims - blood keeps only part of the oxygen available for the body. Under depth, the oxygen does not disappear. It is pressured in the remaining cavities - sinus, trachea, bronchi, etc. However, it cannot be used for the body in those cavities, because there is no gas exchange, and it cannot get into the blood until you ascent to depth where it gets back into alveoli.

and when RV is mentioned, do you mean RV on land or the very variable volume of RV that occurs during dives, because of bloodshift. Isnt it so that the colapse is possible due to bloodshift. And isnt it so that if you know TLC and dry RV and come up after a dive without lung injuries (squeeze) then you can quite well estimate how much bloodshift you had?

RV is not the threshold. You forget that the pressure can compress the lungs far beyond the RV. You will not manage to exhale actively below the RV in the lab, but it does not mean the volume cannot be reduced below it due to external pressure. It can, and how much only depends on your physiognomy, and the flexibility of your chest and diaphragm. So of course, blood shift compensates part of the remaining volume, but the flexibility (at some individuals) may compensate for a much bigger part than that.

Its all very hypothetical, isnt it?

I think it is all based on observations and measurements, so I would tell it is more than a simple hypothesis.

 

[trux]

(2) His large initial lung volume delays collapse until much deeper than would be experienced by someone with average lungs.
...
As for my statement that blood shift has little effect on collapse depth, this is because collapse depth is essentially governed by Boyle’s law which depends only on initial gas volume and the change in ambient pressure.

Dr. Fitz-Clarke, I am sorry, but these claims do not make much sense to me. In fact telling that big lungs will collapse later than smaller ones, violates the Boyle's law, doesn't it? If we really apply Boyle's law, small lungs (in a small body) of the same proportions as big lungs (in a big body), should collapse in exactly the same depth. I'd tell that the absolute volume of the lungs plays no role in this aspect. The only thing which is important is the shape of the lungs, better told the ratio between the collapsible part (primarily alveoli) and the rigid parts (i.e. trachea). The higher the ratio, the deeper the collapse will happen. But the lung volume alone tells us very little about the collapse depth, am I right?

 

[Will]

Is it possible to reverse pack to collapse depth? That is a great question. The answer is probably not. Only one study has looked at airway pressures during reverse packing, by Loring et al. Reverse packing by four elite divers in a dry lab found they got nowhere near this low volume before they experienced high negative airway pressures that were limiting. They only got about 0.3 to 0.4 litres out below RV. The problem is that when the volume drop is relatively rapid, as it is in reverse packing, expiratory airflow causes many of the alveolar ducts to collapse first, trapping air in the alveolar sacs. This is caused by a fluid dynamic phenomenon that I suspect is not seen to the same degree during a diving descent where the decrease in volume is much slower, and air flow within the lung is very small resulting in less air trapping.

I am fairly confident that with sufficient training you can shift more than 0.3-0.4L through reverse packing. This is one of the most important parts of my depth preparation, and I have practiced it for half an hour most days for the last 4 years. A simple test where I dive to 10m, reverse pack, then come to the surface and try to exhale using expiratory muscles (I can't) indicates that I am reverse packing to below half RV, so at least 0.8L. This is supported by being able to use my mouth to draw some air from my lungs at 100m (I have 8.5L TLC).

Like Fitz-Clarke has indicated, I think one of the most important parameters in deep equalisation is flexibility of the rigid airways (sounds like an oxymoron). The tracheal cartilaginous rings are unclosed C-forms, and the bronchioles have small cartilaginous plates - only the two bronchi have closed rings. So there is room to improve the compliance of this system, and I believe Lindholm has shown x-rays of tracheal flattening during reverse packing. When I do a deep RV dive (20m first attempt) I have to wait for several seconds upon surfacing before I can inhale again, and although this is probably mostly the glottis relaxing, it feels like something is reopening in my throat also.

 

[cebaztian]

I think it is all based on observations and measurements, so I would tell it is more than a simple hypothesis.

What about all the IF´s and the LIKELY´s in the texts above.

Squeeze is definitely not equal to collapsed lungs

No but I guess it is something that will happen before lung colapse, maybe not in theory, but in a majority of practical cases. Far more of the record dives include squeeze than we know, is my "guess".

If lungs (alveoli) collapse, and the remaining airways (trachea, bronchi) are still under the ambient pressure,

My guess is that they will not be "still under ambient pressure", unless there is a bloodshift that "reduces" normal RV.

blood keeps only part of the oxygen available for the body..... and it cannot get into the blood until you ascent to depth where it gets back into alveoli.

I think that at depth blood carries far more than 50% of the O2, I believe (guess) that it fairly quickly gets into the blood during descent.

RV is not the threshold. You forget that the pressure can compress the lungs far beyond the RV.

What I think (guess) is that this compression is possible due to bloodshift mostly, which I believe is a very quick process, even though some flexibility is needed to fill in the gap before onset of bloodshift.

You will not manage to exhale actively below the RV in the lab.

Yes you can, its fairly easy.

I am not the scientist here, but it still seem very hypothetical. But on the other hand, what I say is only based on "hunches"

Sebastian

 

[trux]

trux: Squeeze is definitely not equal to collapsed lungs
cebaztian: No but I guess it is something that will happen before lung colapse, maybe not in theory, but in a majority of practical cases. Far more of the record dives include squeeze than we know, is my "guess".

There is no reason why a squeeze should happen before lung collapse. As long as the ribcage and/or diaphragm are suffciently flexible, the lungs will collpase without creating any underpressure in lungs, hence there would be no reason for a squeeze (which is a barotrauma happening at relatively big pressure differences). That would happen ony if the flexibility (together with blood shift) were insufficent for compensating the pressure change (Boyle's law).


trux: If lungs (alveoli) collapse, and the remaining airways (trachea, bronchi) are still under the ambient pressure,
My guess is that they will not be "still under ambient pressure", unless there is a bloodshift that "reduces" normal RV.

It apparently depends on every individual and you can see it easily - some get squeezed in pretty shallow water, others can dive incredibly deep. So at some the flexibility may be sufficient for full collapse even without great blood-shift, at others where there is no flexibility, even huge blood shift may not be sufficient.


trux: blood keeps only part of the oxygen available for the body..... and it cannot get into the blood until you ascent to depth where it gets back into alveoli.
cebaztian: I think that at depth blood carries far more than 50% of the O2, I believe (guess) that it fairly quickly gets into the blood during descent.

No, not all oxygen gets into the blood during the descent. There is still a lot of it contained in the alveolar gas, hence cannot be used for the gas exchange at collapsed lungs.


trux: RV is not the threshold. You forget that the pressure can compress the lungs far beyond the RV.
cebaztian: What I think (guess) is that this compression is possible due to bloodshift mostly.

Why would be that? The ribcage and the diaphragm are flexible, and it is pretty obvious that with external pressure they may be deformed much more than you can achieve with simple muscle work, hence it is pretty evident that external pressure can press the lungs quite a bit below the RV. Blood-shift is certainly important too, but not the only factor reducing the volume below RV.


trux: You will not manage to exhale actively below the RV in the lab.
cebaztian: Yes you can, its fairly easy.

Since RV is defined as the volume remaining in lungs after forced exhale, you cannot exhale below it by definition. You can reverse pack, use vacuum cleaner, let someone jump on your belly, get blood shift, and/or dive to depth, to get below the RV, but that's not exhaling.

 

[Fitz-Clarke]

Yes, this can be confusing because we are talking about many things at once. I do not claim to have all the answers, but my work is based on physiological principles of how subsystems of the body interact at depth, with help from mathematical models to ensure the logic is sound and numbers are reasonable. The criticisms you raise are valid. Maybe we can sort things out through discussions like this.

I will try to clarify a few more things.

(1) What is the difference between squeeze and lung collapse? Well squeeze comes from negative pressure in the airways causing pain or small capillary bleeds. It can happen at any depth. It depends on gas volume and stiffness of the respiratory tract (airways, lungs, chest, diapragm). But note that stiffness changes with depth. As you go deeper, soft parts like the alveoli begin to collapse. The weaker parts are then gone. Now the remaining gas volume is in a stiffer system. As more and more alveoli and small airways collapse, the system gets even stiffer. The airway and chest pressure goes more negative. This is independent of blood shift. Yes you shift blood to compensate for lost volume, but the blood vessels are also stiff and they do not compensate completely. This is actually good because you don’t want your entire blood volume to end up in the chest! If airway pressure goes sufficiently negative in the meantime, you get squeeze.

Now if you go extremely deep, you can collapse all alveoli. This is what I mean by lung collapse. Boyle’s law tells us this must happen because there is a rigid volume that cannot collapse that must accommodate the remaining gas. Even if the trachea collapses partially, there are still other non-collapsible parts like the bronchi, pharynx, and sinuses. If you work out the volume of these parts from anatomy, you can predict the collapse depth for a given surface lung volume. It works best in a computer model that tracks all component properties more accurately with depth. My calculations show it to be very near the depths achieved by the two divers mentioned at 209 and 214 m.

Blood shift is of limited help because you can only shift about a litre into the chest before the vessels also get very stiff, and the chest is also getting smaller as you descend. Maybe you get squeeze in your airways, but maybe you don’t. It depends on many factors.

Did Herbert and Patrick collapse their lungs? I don’t know. I don’t have enough of their personal data to work with to make that prediction with confidence. I must assume standard values. That is my disclaimer.

(2) I do not mean to write off RV as being unimportant. But it is a dry land test of expiration limited by muscles. It does give an indication of the volume where your rib cage becomes very stiff. What is just as important is stiffness, not just the volume. You call it flexibility. I think we all agree on this. But it is almost never measured.

(3) You are right that some of my conclusions are based on mathematical models. I accept that they are not perfect. But it is possible to take what you know about the human body and convert this into a large mechanical system that obeys physical laws and is consistent with known physiology, cardiovascular system, chest, lungs, diaphragm, abdomen, tissue gas exchange, etc. You add components to it and calibrate it and test it where you have data. It passes all the tests with occasional minor adjustments that make it better.

Is the model missing something essential? Maybe. But unless we know what might be missing, we can still use it as a tool to generate ideas and discussion, and make predictions. Your comments are very useful. Your observations may lead to improvements.

 

[Fitz-Clarke]

Trux, you are right. When I said larger lungs allow a deeper collapse depth, I meant for a fixed rigid airspace volume. So collapse depth depends on the ratio (TLC+pack+Vo)/Vo. So higher TLC means greater collapse depth if rigid volume Vo is constant. If for example Vo = 0.5 litres and you start with 12 litres, then your lungs collapse at 12.5/0.5 = 25 atm or 240 m. The difficult part is estimating Vo because it is not truly fixed, but actually changes with depth. Even the stiffest airways do compress, and changing Vo has a large effect on collapse depth. I get it through a computer model because Vo is made up of multiple anatomical segments and airway generations that each have their own elastic properties. This allows me to get Vo as a function of depth that is calculated right down to the point of lung collapse.

 

[cebaztian]

Thank you for intresting discussion (and clarifications) (Trux/Clarke).

trux:
No, not all oxygen gets into the blood during the descent. There is still a lot of it contained in the alveolar gas, hence cannot be used for the gas exchange at collapsed lungs.

So any one, how much 02, due to compressed lung (increased Sp02), would move into the blood from lung (which at start I guess contains some 55% of the 02 resource) ? Would be interesting to know.

hence it is pretty evident that external pressure can press the lungs quite a bit below the RV. Blood-shift is certainly important too, but not the only factor reducing the volume below RV.

Right, would be intresting to hear som guess on how much contributes to what (bloodshift/flexibility) in for example a very flexible guy like Trubridge.

Could you not get all these volumes (sinus/trachea e t c, and RV after it has been flexed by pressure) by:
1) Measure on corpses.
2) Use pressure chambers with living freedivers.

How do we know 1 litre bloodshift is some kind of maximum?

Sebastian
PS Yes of course we reached below RV with the help of negative pack (I was one of the subjects in Lindholms xray of lung and trachea of neg packed lung).