Diving with gradient factors for a new recreational diver

[doctormike]

Unqualified opinion alert...
The algorithms cannot cater for external circumstances.

Well, a mathematical model can technically account for any variable you include in the algorithm. So if you could measure and quantify individual variables like hydration or right to left shunt fractions, you could have an algorithm that more accurately predicts DCS risk.

The ones in current dive computers take into account depth, time and mix. Then, they allow for fudge factors that people can modify to better match their own perceived or known risk factors and risk tolerance.

But the point is that ALL scuba algorithms do one thing - let a diver predict DCS risk and create a safe ascent profile. If the underlying clinical data is drawn from a population that has 25% PFOs, then that will be reflected in whatever the model generates.

 

[Celt]

This could be a buddy problem though. The OP said he doesn't have a regular buddy (and he isn't a solo diver). He would need to explain ahead of time and get the buddy's assent. Even then, it would be kind a jerk thing to ask of an insta-buddy.

That would be the buddies problem, I’d conduct my assent as I saw fit. If I needed to slow or make a stop that’s what I’d do. An insta-buddy certainly wouldn’t influence me.

 

[scubaozy]

There is a DAN presentation on a research on actual divers about GF and deep stops and what they found was the when bubble wave hits was just shifted and they did not know how they could interpret this (timestamp roughly 45:00 in below link).
If that statement is true, do you folks feel safer if you can set this?

Link to the presentation:

 

[rjack321]

There is a DAN presentation on a research on actual divers about GF and deep stops and what they found was the when bubble wave hits was just shifted and they did not know how they could interpret this (timestamp roughly 45:00 in below link).
If that statement is true, do you folks feel safer if you can set this?

Link to the presentation:

Its only 7 divers and 14 dives.... and they are changing more than one variable at a time and puzzled that they can't isolate the cause of the differences. This is not really robust science...

Assuming the figure is later supported by a study which actually differentiates the high and low GFs with a multifactorial design:
The bubble wave is probably shifted longer because the 20/85 profile has more time deeper, so there is more slow tissue on-gassing, those tissues offgas slower and you end up with more bubbles at 60+mins into the SI. The 50/75 profile is shifted shallower, there is less slow tissue on-gassing during the middle/lower portions of the deco profile, so there is less of a gas load to get released later in the SI.

 

[boulderjohn]

The algorithms were not designed for people with PFOs.

The algorithms are just that; mathematical models. They don't know what state the diver's in and there's no magical fiddle-factor to cater for unknown issues.

I agree with Dr. Mike above. The algorithms were created based on observations of divers doing dives. They did not filter out people with PFOs, so we have to assume that the percentage of test divers with PFOs was the same as the general population.

 

[Wibble]

Who were the study set? A wide cross section of the population, or taken from fit young men? Certainly some of the fit young men may have a PFO, but the "fit" may well exclude people who may have PFO symptoms.

Am not disagreeing, am genuinely interested in the algorithm studies to set the bend/no bend limits. NEDU — Navy Experimental Dive Unit — famously used young servicemen which definitely isn’t the typical population of divers (many of us in our sixties or more).

 

[boulderjohn]

Who were the study set?

It depends upon which of the many studies done over more than a century you are talking about. Haldane used goats. I am not aware of the prevalence of PFOs in goats. I am sure the early navy studies used fit sailors, but I don't know the composition of the studies done by Buhlmann, DSAT, DCIEM, etc. I am quite sure none of them tested their populations to see if they had PFOs.

 

[doctormike]

Who were the study set? A wide cross section of the population, or taken from fit young men? Certainly some of the fit young men may have a PFO, but the "fit" may well exclude people who may have PFO symptoms.

I am definitely not an expert in this field, so feel free to correct me if I'm wrong. However...

There are very rarely PFO symptoms until the system is stressed, by introducing something into the circulation that can cause trouble if it bypasses the lungs.

For non-divers, that's a blood clot, where a PFO can change a pulmonary embolism into a cerebral embolism. Big difference! A small blood clot in the lungs may well be undetectable. A small blood clot in the brain can be lethal.

For divers, the thing that bypasses the lung filter is not a clot but a bubble. And it's not just the issue of embolism, it's clearance of gas. A blood clot that ends up in the lungs will just stay there. A bubble will eventually be offgassed. We know that everyone bubbles to some degree on ascent (with doppler studies), so that means that those venous bubbles are being cleared in the lungs. If they aren't cleared because of any sort of right to left shunt (like a PFO or a pulmonary AVM) means that they persist in circulation, and can cause clinical symptoms even in the absence of a stroke.

Also, often PFOs are not open until there is an increase in the pressure gradient, like with straining.

So, yes... early work in decompression algorithms favored fit young navy divers in the study population. But I would bet that they would have a similar prevalence of PFO as the general population...

 

[tursiops]

Maybe I'm biased, but it seems that a remarkable number of "undeserved hits" have been associated with PFOs detected after the fact.

 

[Wibble]

Question about PFOs: are there many cases of "late onset" PFOs in divers? Meaning that a diver who was doing decompression diving for some time before there was an 'incident' where the PFO was discovered.

Logically a person with a PFO who gets into diving and progresses on to decompression diving would have had their 'incident' before they were doing serious depths.

I suppose I'm asking if PFOs can spontaneously happen, or are they congenital -- with you since birth?