Bubble model vs. Gradient Factors redux

[Storker]

the lack of bubble size tests should not deter us from testing.

No, but it should certainly deter us from using bubble size as an argument or a parameter.

If you can't measure it, it doesn't count.

the strength of ZHL was based on numerous animal and human tests

Quite. It's based on, and validated by, in vivo studies. Which is a rather strong argument for its applicability.

 

[Umuntu]

And we shouldn't forget that the strength of ZHL was based on numerous animal and human tests long before VGE studies (showing bubble quantity) became available. So, the lack of bubble size tests should not deter us from testing.

I seem to recall that in one of the studies they had difficulty to get the rats bent. So, I'm not sure animal physiology (?) is necessarily comparable to human physiology.

 

[EFX]

No, but it should certainly deter us from using bubble size as an argument or a parameter. If you can't measure it, it doesn't count.

In regards to Haldanean models please direct me to a study that measures the pressures within tissues. Oh, we haven't measured those pressures. I guess the studies don't count.

 

[lexvil]

I seem to recall that in one of the studies they had difficulty to get the rats bent. So, I'm not sure animal physiology (?) is necessarily comparable to human physiology.

It’s very hard to get a rat to dive once you put an HP 100 on him.

 

[rhwestfall]

It’s very hard to get a rat to dive once you put an HP 100 on him.

So, that is where SpareAir came from....

 

[dmaziuk]

I seem to recall that in one of the studies they had difficulty to get the rats bent. So, I'm not sure animal physiology (?) is necessarily comparable to human physiology.

If "the goats are easy to work with and have body mass similar to man's" was good enough for Haldane, it's good enough for me.

 

[Storker]

In regards to Haldanean models please direct me to a study that measures the pressures within tissues.

"Tissue pressure" is neither a meaningful nor a measurable parameter. It's just a representation of the concentration of dissolved gas normalized against the Henry's constant. So if the "tissue pressure" of N2 is 0.79 ata, that just means that the amount of nitrogen dissolved in the tissue is equal to the amount of nitrogen dissolved in a liquid in equilibrium with air at 1 ata. It can not be measured, it's just a numerical representation of the modeled amount of nitrogen dissolved in a tissue represented by compartment number X.

Tissue pressure is just a mathematical construct to make the comparison between the modeled amount of dissolved gas and the partial pressure easier.

 

[scubadada]

@EFX

So what would it take to test the bubble model vs. dissolved gas, one or more of the profiles as described in post 66, or something similar for a different proflile?

 

[Brett Hatch]

No that's not the goal..

The goal is to not get bent. The bubbling is a reasonable proxy for decompression stress. Bubbling is a mediocre proxy for DCS. So this graph is 2 steps away from the objective. The presumption is that 30min bubble scores are equated with faster tissues which are more tolerate of over pressure gradients in theory. And the 90min bubbles scores are from slow tissues that are not as tolerate of overpressure gradients. But without any DCS we cant really say which one better addressed the ultimate goal.

Hi @Brett Hatch
Bubbling may be a marker for risk of DCS but that is all. As there were no episodes of DCS, we can't tell. Take NEDU for example, there were episodes of DCS and an excessive number in the bubble model group. As bubbling is only one marker, it would be smart to check other valid markers, perhaps like the chemokines measured in the Spisni study, see Bubble model vs. Gradient Factors redux

Thanks both of you for responding, and for reminding me that the purpose here is to increase safety by reducing the DCS incidence rate, not to decrease bubble quantity, bubble size, etc. However (and correct me if I'm wrong) there is some correlation here, even if it's a bit tenuous or noisy or not the ideal thing to measure. My question is, taking as a given that they aren't actually measuring the thing we really care about, what is the best way to interpret the measurements?

On reflection, given that we're coming up on 100 posts discussing how to interpret these measurements and what measurements would be better to take next time, perhaps it's naive to think that this question has an easy answer.

Bubble detection requires transthoracic echocardiography. This cannot be done underwater during ascent. Doing it every 15 min starting at 30 min is pretty intensive. I might have liked to see the 15 min grade.

Neat! The fact that these measurements are done by EKG does explain why they can't easily be done during ascent on real dives. I would be interested to hear of studies which try this with simulated dives. From some of the other posts here, it's my understanding that EKG's are useful for counting bubbles, but not so much for measuring their size, which some models try to take into account. Is that correct?

As well, an important part of the Buhlmann model is not just the number of bubbles, but where they come from. As @rjack321 pointed out, fast tissues' M-value lines have larger coefficients than those of slow tissues. I suppose the EKG can only measure where the bubbles are, not where they came from, so there is another downside to this measurement tool.

 

[scubadada]

...Neat! The fact that these measurements are done by EKG does explain why they can't easily be done during ascent on real dives. I would be interested to hear of studies which try this with simulated dives. From some of the other posts here, it's my understanding that EKG's are useful for counting bubbles, but not so much for measuring their size, which some models try to take into account. Is that correct?

As well, an important part of the Buhlmann model is not just the number of bubbles, but where they come from. As @rjack321 pointed out, fast tissues' M-value lines have larger coefficients than those of slow tissues. I suppose the EKG can only measure where the bubbles are, not where they came from, so there is another downside to this measurement tool.

No you don't understand bubble detection, nothing to do with the EKG. For transthoracic echocardiography, a transducer is placed on the chest wall and can detect the bubbles, in the right side of the heart Transthoracic echocardiogram - Wikipedia. The bubbles in the right side of the heart come from the venous system. A connection between the right and left sides of the circulation, such as a PFO, can result in arterial side emboli.