Breathing physiology... whats best for off-gassing

[Uncle Pug]

I am not really familiar with rebreathers....
Is number one not an issue because of the counter lung????

And how old is that picture anyway!!!!

 

[The Iceni]

Yup!

1) As you know, when you use a rebreather the total volume of the lung and conterlung remains fairly constant with no change in bouyancy, whatsoever, at all stages of the respiratory cycle.

In theory, provided the counterlung is at the same depth as the lungs breathing effort is negligible because of this.

3) The change in bouyancy due to gas loss is considerbaly less as you only deplete the Oxygen according to your metabolic needs. I will have to do the calculations when I get home but I know a 3 litre cylinder (700 litres) will provide enough Oxygen for about seven hours of rebreather diving as opposed to OC my twin set which contains 5,500 litres of Nitrox 36 weighing in at about 6 Kg, most of which is "lost".

Rebreathers are not without their CONSIDERABLE problems, however.

My pic is about five years old, but I remain just as handsome and modest!

(HOW DO I CHANGE MY LOGIN and switch on private mail???)

 

[Uncle Pug]

Paul,
go to the top of the screen and click on User CP....
Change your email settings to receive...

You can send messages by clicking on the msg button at the bottom of other people's posts

 

[Dr Deco]

Dear Readers:

For the best in off gassing (though not necessarily station keeping), the best I believe would be to continue bicycle motions with the legs. Whether this is vertical or horizontal is not a big difference, but it is best to activate the “muscle pump.” This is the mechanism whereby the capillaries are compressed when the muscles contract. This increases the perfusion (= blood flow) through the muscle (as well as associated connective tissue).

This is a relatively mild activity that will increase the flow of blood but will not promote the growth of tissue micronuclei.

Dr Deco :doctor:

 

[roakey]

Lets see if this satisfies the naysayers/PADI is god types…

Undersea Journal, First Quarter 1987, page 46 or Best of the Undersea Journal page 9-5.

Excerpt from an article by Wolfgang Krebs, MD:

“When divers are in an upright position, the bottom of the lungs are in slightly deeper water than the top of the lungs. Although small, this pressure difference affects the lung function by causing the upper portion of the lungs to remain inflated while the lowermost portions collapse – even after moderate inhalation. Thus, during exhalation under water, many more bronchioles will collapse than would normal collapse at the surface.”

The article goes into something else that’s interesting. Dr. Krebs talks about surfactants in the alveoli/bronchioles and how a vertical position could in fact cause a lung overexpansion injury if contributory factors – such as an infection, dust or smoke (attention smokers) caused the collapsed bronchioles to stick shut. This could trap gas behind them, leading to an overexpansion injury upon ascent.

Similar to what can happen to a person with asthma.

So put this under the heading “Even PADI knew that a horizontal position in the water is best 15 years ago.”

Roak

Thank you CincyDiver for finding this article and bringing it to my attention.

 

[The Iceni]

roakey once bubbled...
“When divers are in an upright position, the bottom of the lungs are in slightly deeper water than the top of the lungs. Although small, this pressure difference affects the lung function by causing the upper portion of the lungs to remain inflated while the lowermost portions collapse – even after moderate inhalation. Thus, during exhalation under water, many more bronchioles will collapse than would normal collapse at the surface.”

Roaky, I am not sure that this is "data" as it reads like an opinion. Although it does expand a little on what I was saying earlier about lung zones.

On reflection it is obvious (dare I say that?) that the pulmonary arterial pressure is determined by the pressure generated by the right ventricle at the end of systole, which will be transmitted to all parts of the pulmonary bed equally because blood is a liquid not a gas and the diver is alive not dead. In addition, it is a fact that the intrathoracic pressure throughout exactly equals that at the oropharynx. The chest acts as a sealed box and whatever zoning takes place will be the same as it is on the surface and be due to gravity alone.

I looked at a schematic of hypothetical pooling in which the efficiency of the centre of the lung is 1.0. If gravity determines efficiency of the lung it reduces linearly due to V/Q mismatch both above and below this centre. It would appear that its effects on the efficiency of overall lung function are not great but horizontal does appear, at least theoretically, to be more efficient by about 1%. (I could not attach the spreadsheet)

I have to say that I still consider the tissue offgassing and not lung function to be the determining factor as I posted earlier.

However, after nearly another season of diving I find it much more comfortable to hold station during deco in the horizontal position because that is how my bouyancy and weight are trimmed and that, I feel, is the important factor.

:mean:

 

[The Chairman]

Thats a l-o-n-g time to be cogitating on something... strange thing was, as I was reading a rather lengthy tome by BW today, I was thinking of this very thread... and that I was gonna go and find it.

I will share an excerpt from the first paragraph that sort of sums it all up for me... but that will be AFTER my Venture Crew meeting (Go Scuba Ducks!) and might even be tomorrow.

 

[Genesis]

Take to a depth of your choosing a vessel (e.g. a clear, plastic bottle with a wide mouth.)

Fill with water.

Take a FULL breath.

Exhale into the vessel, displacing the water.

Mark the level of the water displaced. You now have a measurement of your lungs' ventilation capacity.

Now repeat at the other attitude (horizontal .vs. vertical)

That would settle this once and for all.

Why doesn't someone do the test, and report the results? If you calibrate the vessel with water after the fact, you can determine the amount of vital capacity in each instance, and the percentage change, if any.

 

[The Iceni]

Genesis once bubbled...
Why doesn't someone do the test, and report the results? If you calibrate the vessel with water after the fact, you can determine the amount of vital capacity in each instance, and the percentage change, if any.

Hi Genesis,

You are describing spirometry. If you take a look at the attachment to my post on lung volumes and breath holding in the thread at http://www.scubaboard.com/showthread.php?s=&threadid=13630 you will see the schematic of a spirometer, which was used by 19th century physiologists to do this very thing.

In fact immersion, tight wetsuts and diving equipment does reduce vital capacity but this is a constant factor.

The discussion is not about changes in vital capacity but on the relative efficiency of the lungs in the horizontal and vertical attitude and VQ mismatch. To the uninitiated I will try to explain

For efficient gas transfer the ventilation of the alveoli "V" and the perfusion of the pulmonary capilliaries "Q" must match. (Why Q? I have no idea!).

If there is too little ventilation to any area of the lung the blood going there cannot be fully oxygenated, equally if there is too little blood ithe oxygen in the alveoli cannot be absorbed as well and more is simply exhaled.

The converse is also true. If there is relatively too much ventilation to any area of the lung it is wasteful as the blood canot absorb all the available oxygen from it, equally if there is too much blood , that blood cannot be fully oygenated.

What applies for oxygen applies to any gas, including nitrogen.

The concept of zones is that at the bases of the lungs there is relatively too much perfusion (blood), while in the apices there is relatively too little, while in the centre of the lungs vetilation and perfusion match and this is where gaseous exchange is the most efficient.

The effect of VQ mismatch is the same as a shunt. A proportion of the blood goes through the lungs unaltered.

I doubt VQ differences from the horizontal and vertical positions can be singificant in offgassing for the reasons I have already given.

In my opinion Dr Wolfgang Krebs is wrong. There is no pressure gradient in the thoracic cage as it is held close to ambient pressure throughout, like a sealed box, by the diaphragm and intercostal muscles. And of course, the inhaled gas from the regulator, which prevents doming of the diaphragm and pooling of blood within the pulmonary tree as occurs in free divers.

I hope this makes sense but would be pleased to hear the opinions of the other doctors on board, if they disagree.

Kind regards

 

[Genesis]

So let me see if I get this...

What you're saying is that if there is an effect at all, some of the alevoli overexpand, and some underexpand, relative to normal expansion?

But if it were the case that gas exchange did not improve with more expansion, then deeper breathing (on the surface) would not be the response to exertion and a higher CO2 load, since greater expansion would not improve exchange. But we know it is the response and that it does improve exchange.

I'm not "getting it" in terms of why the attitude change from vertical to horizontal would lead to actual improvement in gas exchange in the lungs, and better deco....... (I realize that your belief is that it doesn't - I'm trying to figure out the whys and wherefores here )