Breathing physiology... whats best for off-gassing

[Dr Deco]

Dear Readers:

This is a long post with so any questions that it truly makes my head swim. :burnout:

I believe that in the final analysis, what people wished to know was, does body position have an appreciable effect of inert gas elimination when the diver is in the water.

Ideally, your lungs should have blood flowing in capillaries around all of the alveoli. In practice, this is not exactly what happens. In an upright individual, the lungs are expanded more at the top of the chest cavity and collapsed somewhat at the bottom. The whole appears somewhat like a “Slinky” held in one hand and resting on the other. The alveoli are not as well ventilated (= air enters and exits) in the lower portions as the upper. The blood flowing past those these alveoli do not experience a good gas exchange with the blood and a “ventilation/perfusion inequality” arises. :grad:

The good news is that this makes little difference in healthy individuals. In divers, this compression can reduce gas exchange, but it would be quantitatively very small. Gas exchange halftimes in lung capillaries are on the order of seconds, while gas exchange halftimes for the body’s tissues are on the order of tens of minutes to several hours. On a practical level, I would not expect to see any real change in washout times in such a series arrangement (lung to tissue to lung).

Dr Deco
:doctor:

 

[roakey]

DrDeco,

A masterful job at telling Peter and I that we're both right and both wrong.

Though I think there's more to it than that, for the simple reason that it's easier to breather (for me) in a horizontal position, I'll stay that way. Besides, I'm trimmed that way for the dive, so it's my natural position in the water anyway.

Roak

 

[The Chairman]

Sorry for all of the guesses, but your answer speaks volumes in it's brevity! You actually know what you are talking about! I do have one more question... has any qualitative analysis actually been done in this area?

 

[Dr Deco]

There are many papers on the changes in gas exchange in the lung in the scientific literature but most involve pulmonary conditions that are pathological. These are not of value for recreational divers. ppp

The only study of which I am aware that treats normal lungs and gas bubble loads that are found in actual decompressions is a study performed at NASA in the early 1990s. A portion of it is retyped below.

[sp]W Norfleet, M Davis, and MR Powell. Absence of pulmonary dead space changes during hypobaric decompressions with high Spencer precordial Doppler grades. Undersea Biomed. Res. (Suppl.) 1992

[sp]METHODS: Subjects were exposed for 3 hours without oxygen prebreathe to 6.5 psi ambient pressure while breathing 100% oxygen. A protocol of mild exercise was performed. Tidal volume (VT) and the composition and volume of the expired gas were determined. The partial pressure of carbon dioxide in arterial blood was estimated through the determination of both the end tidal PCO2 and transcutaneous tcPCO2. Alveolar dead space was calculated by the Enghoff-Bohr equation. CONCLUSION: Alveolar dead space is not enlarged by the degree of embolization of the lungs with gaseous micronuclei produced by EVA-like hypobaric exposures.

The concern of this work was whether the large decompression bubble loads (Spencer Grade IV) found in astronauts during EVA could influence the gas exchange characteristics of the lungs. [These astronaut loads would be considered large when compared to sport scuba divers.] One method of checking this reduction of gas exchange is to determine the degree to which bubbles block the alveolar capillaries. The air will enter and leave the alveoli but, since there is not any perfusion (i.e., they are blocked by bubbles), the alveolar space is useless or “dead.” This would likewise hinder inert gas change, as one could well imagine. As the report indicated, a “dead space” was not detectable.

Divers can be assured that the lung will not normally be a rate-limiting factor for excahnge. The blood flow from the lower extremities to the lungs is improved in water since the blood “pooling” effect of gravity on the legs is not present. The hydrostatic pressure of the water will counteract this.

The most important factor is to maintain musculoskeletal activity when at the stops, both in water and when on the boat. This will insure that the so-called “muscle pump” is in operation. The contractions of the muscles cause contraction of capillaries and boost the local blood flow (along with ADP, hydrogen ions, heart rate increases, and innervation of the precapillary sphincters).

Dr Deco :doctor:

 

[neil]

Wow, all that great spirited discussion and then somebody who actually knows what they're talkin'
about comes around and spoils everything!

And do I read that upright might be better for offgassing, since it negates gravity pooling of blood, making more blood than normal circulate through the torso? Hmmm.

Originally posted by roakey
Pete,

If I take a balloon down to 33 feet, it halves in size because its walls do not offer any resistance to collapse.

Negative, mon frere, the VOLUME halves, not the size. It's half the size at 8 atmospheres, I believe. Did I really catch you math mavens on a technical point? Lemme see: If I take a balloon with a volume of 8 cu. somethings to 8 atm, it'll have a volume of 1, right? A cube with a volume of 8 has sides of 2x2, and a cube with a volume of 1 has sides of 1x1. A 1x1 cube is half the SIZE of a 2x2 cube. Not bad for the seriously math impaired. Calculating volume of balloons or spheres involves cube roots, and that makes my head hurt.

Neil
who went to music school cuz there was NO math.

 

[roakey]

Neil,

I'll give you a point for catching me on an ambiguity.

Am I talking volume or diameter when I refer to size?

I was talking volume, but I should have explicitly said so.

Roak

 

[Uncle Pug]

Originally posted by neil
And do I read that upright might be better for offgassing, since it negates gravity pooling of blood, making more blood than normal circulate through the torso? Hmmm.

Neil,
You're reading impaired too....
Here is the quote:

The blood flow from the lower extremities to the lungs is improved in water since the blood “pooling” effect of gravity on the legs is not present. The hydrostatic pressure of the water will counteract this.

Try again...

 

[neil]

Originally posted by Uncle Pug


Neil,
You're reading impaired too....
Here is the quote:

The blood flow from the lower extremities to the lungs is improved in water since the blood “pooling” effect of gravity on the legs is not present. The hydrostatic pressure of the water will counteract this.

Try again...

I'm not sure what the alleged problem with my reading comprehension is.
The subject of the first sentence, blood flow, is improved because the hydrostatic pressure of the water counteracts blood pooling. In sentence 2, "this" refers to blood pooling. That's how I translate those 2 sentences. So if one buys the theory that better blood flow to the lungs improves offgassing, (the good Doctor has cleared that up), then the best position might indeed be vertical. It's academic at this point, anyway.

And what's this about only getting ONE measly point for catching you in an ambiguity? You said size, I read size. Size is size. Nyah, nyah.

Neil

 

[roakey]

Originally posted by neil
And what's this about only getting ONE measly point for catching you in an ambiguity? You said size, I read size. Size is size. Nyah, nyah.
Neil

The Random House Dictionary of the English Language:

-Syn. SIZE, VOLUME, MASS, BULK, are terms referring to extent or dimensions of that which has magnitude and occupies space. SIZE is the general word.

Should have quit while you were ahead, having to look it up and finding that volume and size are synonyms just cost you a point.

Roak

 

[neil]

Oh foo! From a scientific standpoint, they are very different, so THERE, Mr. Smartypants! Some people just HAVE to be correct, all the time. Oh puleeeeze can I have the point back?

Neil