Tired after diving

[bleeb]

Great thread! I've learned a lot from it. I'd just like to ask one thing (out of curiosity), why does slower ascents lessen your fatigue? I can understand how dehydration or hauling heavy gear around cause fatigue but what is the mechanism that causes fatigue if your ascent is not slow enough?

I mean, I know the dangers of an ascent rate that's too fast but I'm assuming the argument here is not about air embolisms and barotrauma but something else?

Its about decompression. The biggest relative pressure change is closest to the surface, so thats also where its easiest to accidentally go up faster than youre able to decompress. The idea is that the better decompressed/offgassed you are when you surface, the less chance of fatigue or other DCS symptoms youll have..

To add to that:

One of the hypotheses that is being studied is the relationship between microbubbles and fatigue. As I understand it, microbubbles are not only small, they're near the size where the surface tension keeping them from growing can exceed the force exerted by the internal pressure. If you slow your ascent, the gas can diffuse from inside the bubble to your blood stream and then into your lungs faster than the bubble can grow in size. A few posts in this thread have indirectly referenced what happens when you don't succeed in making this happen (i.e. come up faster) in mentioning the possibility of fatigue really being a symptom of sub-clinical DCS. Unfortunately AFAIK, while the former (bubble size growth rates) is 'standard' gas dynamics, the latter (microbubbles/sub-clinical DCS causing fatigue) has not been scientifically proven.

Hopefully someone with specific references, or maybe even involved in such studies might be willing to explain in more detail the current state of research.

 

[Deefstes]

As I understand it, microbubbles are not only small, they're near the size where the surface tension keeping them from growing can exceed the force exerted by the internal pressure.

OK, what exactly does this mean? It looks like one of those sentences that I should understand but I just can't quite get my head around it.

 

[Tigerman]

OK, what exactly does this mean? It looks like one of those sentences that I should understand but I just can't quite get my head around it.

Its physics.. One of the things that makes diving so interesting and why the basic certification courses can be so short is that you dont need to understand the details of the physics to dive as long as you know what you need to do to dive safely, while learning more details makes you get some understanding of the complexity behind all the things we do..

 

[bleeb]

As I understand it, microbubbles are not only small, they're near the size where the surface tension keeping them from growing can exceed the force exerted by the internal pressure.

OK, what exactly does this mean? It looks like one of those sentences that I should understand but I just can't quite get my head around it.

Don't know how much of a technical education you have, so my apologies if this sounds like I'm talking down to anyone.

Bubbles initially form small and then grow by collecting gas from the surrounding liquid (if it's saturated) or as the pressure is reduced. However, the surface of the bubble is also subject to the surface tension of water, which acts as a force to minimize the surface area of the bubble. When a bubble is somewhere near a micrometer in diameter (I don't remember 'exact' figures) the force from surface tension can balance the force driving the bubble to grow from it's internal pressure being greater than the surroundings, if the pressure differential is low enough (i.e. ascent rate is slow enough). If you're then outgassing through your lungs and the nitrogen saturation in your bodily liquids is dropping, the near-100% nitrogen inside the bubble will tend to diffuse out of the bubble and back into the dissolved liquid phase. Ascend slow enough and this will be happening fast enough that the bubbles won't grow (much). The RGBM mathamatical model (Reduced Gradient Bubble Model) or variants attempts to include these effects in its calculations and is used by some dive computers to try to minimize or manage microbubble formation and growth.

However, some of the big unknowns on the human physiology side is how big and how many bubbles are actually how much of a problem.

Hope that's at least a bit clearer.

 

[Web Monkey]

I mean, I know the dangers of an ascent rate that's too fast but I'm assuming the argument here is not about air embolisms and barotrauma but something else?

The idea behind controlling your ascent rate is to minimize bubble formation. The 60'/min and later the 30'/min rate were supposed to take care of this. The 3-5 minute "safety stop" has the effect of slowing down your average ascent rate and was also designed to minimize bubble formation.

All the tables and ascent speed recommendations are based on data derived from animals, then people doing actual dives, and observing the results. Back in the dark ages, the observation was "is the diver bent?", while later there were Doppler studies and more detailed measurements, which were then used to create more conservative and hopefully more accurate tables and computer models.

However the catch is that all the deco models, tables and computer algorithms are just models and there is absolutely no guarantee that they apply perfectly to any individual, so if a slower ascent and/or longer stops and/or a different mix makes any particular diver feel better, it means that the model they used wasn't conservative enough for them.

Some computer manufacturers (like the Uwatec SmartCom and Galelio) have incorporated this into their software with a "conservativeness" setting that adds ascent rate changes and "level stops" stops even on supposedly no-deco dives to allow divers to adjust the model to match their particular physiology.

Terry

 

[cutlass]

O2 content has no practical bearing on CO2 retention.

I'm leaning toward a technical disagreement with this. IIRC, about 10% of cellular CO2 waste is directly dissolved into plasma. Approx 60% is in the plasma, converted into bicarbonate ions. The remaining 30% is bound into hemoglobin; actually, the amino acid "-globin" part. It's this fraction for which there is an interplay of O2/CO2 in the form of the Bohr and Haldane effects. The Bohr effect is that as more CO2 is in the bloodstream, the more oxygen leaves the hemoglobin. The Haldane effect is a continuation of this: as more O2 leaves, more CO2 moves in and more bicarbonate ions are formed. The reverse also holds: as hemoglobin saturates with O2, the more CO2 is unloaded.

 

[Deefstes]

Don't know how much of a technical education you have, so my apologies if this sounds like I'm talking down to anyone.
.
.
.
Hope that's at least a bit clearer.

Not at all, I asked the question so I gave you right to talk down to me Thanks for the good explanation.

Just to make sure I've got it straight. I understand how the surface tension could prevent the bubble from growing as big as the decrease in pressure would have dictated otherwise. But at some point the surface tension will not be strong enough anymore. So the idea is to remain before that point and keep the ascent rate slow enough so that the nitrogen diffusion out of the bubble (causing the bubble to shrink) makes up for the decrease in pressure (causing the bubble to expand).

Is this correct?

 

[24940]

Thanks for the explanation.

But, what's the bottom line on the impact of the effect you cite on CO2 retention? Is it really significant?

Thanks again.... I needed something to read up on this weekend.

I'm leaning toward a technical disagreement with this. IIRC, about 10% of cellular CO2 waste is directly dissolved into plasma. Approx 60% is in the plasma, converted into bicarbonate ions. The remaining 30% is bound into hemoglobin; actually, the amino acid "-globin" part. It's this fraction for which there is an interplay of O2/CO2 in the form of the Bohr and Haldane effects. The Bohr effect is that as more CO2 is in the bloodstream, the more oxygen leaves the hemoglobin. The Haldane effect is a continuation of this: as more O2 leaves, more CO2 moves in and more bicarbonate ions are formed. The reverse also holds: as hemoglobin saturates with O2, the more CO2 is unloaded.

 

[cutlass]

Bottom line is that for a normally healthy person in an average environment, elevated CO2 level is a short-lived effect typically related to exercise level. If it's up, grab a rest and give the lungs and blood circulation a few minutes to catch-up unloading. (IIRC, an average resting person's cells will generate about 200ml/min of CO2 waste; this amount is almost entirely matched and unloaded by the lungs.)

IMO, "CO2 retention" implies an abnormal environment (breath-holding, defective rebreather or bad gas mixes) or disease process (usually lung function but could extend to blood circulation or chemistry disorders whose list of causes can be hideously long). CO2 levels are an indicator of body acidity or pH level. Overall, the body tolerates only a fairly narrow pH range, roughly 7.36 to 7.44, and will adjust itself to keep it there -- or die. The first line of defense for excess acidity is to increase breathing and heart rate and depth. Next would be to try to discourage further activity which would add to the load of CO2 waste: Acid buildup impairs muscle function and changes body chemistry to affect neurologic function which includes warnings of coordination deficits and, possibly, blunted motivation (sense of overall pain or fatigue or the "blahs"). The kidneys will also ramp up to unload excess acid but this response may take many hours; compare this to the lung response which may take only minutes.

 

[bleeb]

Just to make sure I've got it straight. I understand how the surface tension could prevent the bubble from growing as big as the decrease in pressure would have dictated otherwise. But at some point the surface tension will not be strong enough anymore. So the idea is to remain before that point and keep the ascent rate slow enough so that the nitrogen diffusion out of the bubble (causing the bubble to shrink) makes up for the decrease in pressure (causing the bubble to expand).

Is this correct?

Yup. That's pretty well it, as far as I understand the basics.