Past Smoker

[trux]

I understand exactly what you mean, Dave. And you would be right in a static system with no consumption or no critical BO level of PaO2. In such system, with x mmHg on one side, and less on the other, you would not care about the slow diffusion, because in both systems the oxygen would continue diffusing from one side to the other until either equilibrium is established or all oxygen consumed. Yes, in such case in fact having slow diffusion would keep this process running longer.

We are not in such system, though - there is certain consumption rate, and there is certain critical level of PaO2. As long as the diffusion rate is bigger than the consumption rate, we indeed do not care - even the ex-smoker does well here. The problem starts when the diffusion rate drops below the consumption rate, because it results in lowering the PaO2 until the critical level is reached, and you black out.

And here we are again back where we started from, and what I wrote at the very beginning. The slower diffusion rate will result in reaching the BO level sooner.

 

[trux]

I go to bed now, but want to post another argument if you are still not convinced:

If you were right and the slower diffusion (or lesser efficiency) was not a problem (or was even an advantage), then by extrapolating your claim we'd came to the conclusion that you would do as well with just a single functional alveoli (assuming you manage to saturate your blood somehow before taking the last breath). I think we do not need any mathematical analysis, or plotting of graphs, to see with common sense that it would not work well.

Yes, it is an extreme example, but it work in the same way with inefficient alveoli. Just the threshold is not so apparent.

 

[Mullins]

My point is that, even with continuous consumption, diffusion may cease prior to BO due to the Bohr effect. I know arterial saturation is always dropping. But as I wrote just above, if increasing acidity keeps the effective gradient the same for a period of time, the slower system could catch up and reach equilibrium. This could happen before BO is reached.

Edit: in answer to the single-alveoli example - you're probablly right that it wouldn't work out, but only because the combination of decreasing saturation and increasing Bohr effect wouldn't hold effective arterial:alveolar gradient stable for long enough, biasing the much faster system. Even then, I'm actually not so sure it wouldn't work because you'd get very hypercapnic very early, forcing a bit DR....

Incidentally my own diffusion rate, as tested, is lousy - like that of a smoker Sleep well.

Just checked and my specific diffusing capacity was measured at 69% of expected, which is pretty low. For comparison, there was a study done on people with 'interstitial lung disease or lung volume restriction' and the median value for this group was 64% of predicted. Heh....

 

[devondave]

Wow guys, you lost me after the first three syllable word but great reading and a fine example of debating on db.

 

[trux]

Dave, even if the slower diffusion means faster growing of CO2, it only confirms what I wrote. With the growing CO2, the diffusion slow-down is amplified, so again, the PaO2 drops quicker. But of course there are many other factors having an impact, and as I wrote it in no way means the person can't hold the breath longer than the other one.

 

[Mullins]

Yeah the DR comment and my personal measurement are both kind of off the topic. My own case doesn't prove anything either way, because the low diffusing capacity may still be handicapping me for all I know. It was interesting when they told me about that measurement though...

An assumption I've been making is that diffusion ceases prior to BO in most people. That, due to the Bohr effect, there's a period at the end during which we're running down the arterial saturation without consuming any from the lungs. Do you know if this is accurate?

 

[trux]

Frankly told I don't really know, and did not yet see any detailed study on this. Personally I do not think the diffusion stops before the BO, but rather that the diffusion rate just drops below the consumption rate, and slowly continues dropping. It is the moment when the diffusion rate becomes smaller than the consumption rate that is critical. The exact curve will depend on many factors, but I would not tell it comes to a complete halt. Instead, the diffusion rate drops likely on a logarithmic curve (approaching the zero slower and slower). That's my uneducated guess only, though. I may be wrong here.

I've been thinking since a long time about creating a complex modelling software taking in account all of the complexity of a breath-hold, and allowing to play with different parameters, trying to find the optimal strategy for given conditions. I know that Dr. John Fitz-Clarke has a kind of a mathematical model of human breathing, but from what I understood it does not include all the complexity of parameters and factors, I'd like to see there (not only partial pressures, volumes, pH, Bohr and Haladane effect, but also the influence of the spleen, vasoconstriction, cardiac output, transpulmonary pressures and arterial compression due packing, partial vasoconstriction of carotides due to hyperventilation, influence of myoglobin, free calcium, acid lactic, ATP, etc, etc).

 

[Hydro-Mike]

Wow Thanks Dave and trux. Can I get the bathroom pass now.. Just kidding thank you for the great input, short answer I new it was just me sniveling. Onward and deeper Hydro-Mike