Nitrox for "safety margin"

[String]

Originally Posted by Captain Calamity

Sometimes this gets misinterpeted as meaning that no evidence is the same thing as no benefit.

Surely you cant claim there is a benefit unless you can prove one?

[Big Kahuna]

Originally Posted by String

There do seem to be several claims around regarding nitrox that to me at least seem unproven. Another is the "less tired after nitrox". Obviously this is hard to measure but some people claim a great reduction, others (me included) notice none at all. It could be real, could be placebo effect or could simply by a small difference masked by other factors such as long hard day, no food etc.

I started diving Nitrox on Dive 35 and have pretty much used some form of nitrox mix on every dive since. As you say it may be the placebo effect, or that as I am getting more experienced things are becoming less physically and mentally taxing (i.e I have calmed down a little and my brain is not racing 10 to the dozen trying to remember everything i had been taught etc).

I tend to feel (JMHO) less tired after 3 dives in a day on nitorx than I did diving air. As I said just my opinion though.

[Lazlo]

I think it's a totally fair question. We should be guided by actual evidence, not hearsay and conjecture. Plenty of things seem obvious but later turn out not to be.

For example, one thing to consider would be the cumulative effect of breathing elevated ppO2 (above 0.5 bar). This is known to cause problems at least in the short term. Does anybody know of any studies which show one way or another, what the long term effects are?

Does anybody know any study which does a proper risk analysis of O2 toxicity vs DCS risk?

As people have often said, DCS is survivable, Ox-tox will kill you. But those people don't tend to provide any real evidence other than anecdotal.

[tom]

Originally Posted by String

Surely you cant claim there is a benefit unless you can prove one?

it's beneficial for me not to stab myself in the face with a garden fork. Do I need to run a controlled experiment to prove that?

[rs3100]

You can't statistically significantly reduce a risk that isn't in itself statistically significant.


What you can show, statistically, is that Nitrox reduces the odds of DCI over Air, but the difference may not be statistically significant (95%).

If you have 300,000 air dives and 300,000 nitrox dives and of those 200 air dives and 100 nitrox dives result in dci, that's a rate of 0.67 per 1,000 dives on air and 0.33 per 1,000 dives on nitrox. Each in itself would not be a significant risk.

However, when compared, those on Air have a OR of 2.001 (95%CI 1.574 - 2.544) - a statistically significant difference.

Obviously these figures have been made up, but it is possible to show a significant difference between two gases that in themselves have no significant risk. Proper evidence will be very difficult to come across.

[Captain Calamity]

Originally Posted by String

Surely you cant claim there is a benefit unless you can prove one?

No, but you can use existing knowledge to conclude that there is highly likely to be a benefit.

In my day job when we really want to know if treatment X is better than treatment y we turn to meta-analysis of double blind randomised controlled studies. This requires thousands of patients per study (in order to get a statistically significant result e.g. one that is likey to be due to the different treatments rather than just chance). Each patient is unaware which treatment they are taking, as is the doctor who examines them to assess the effects of that treatment, so that any bias is excluded. Finally, a group of independent experts check that each trial is scientifically sound before they are all combined to give an overall result. These can take decades and cost millions.

You tend to find drug companies funding this sort of research, but then again they stand to make the money back. I just can't see who would fund research to prove that nitrox dived on air tables is safer as there is little financial incentive to do so.

CC

[String]

Guess the only people that might fund something like it would be DAN.

The point i'm making is a theoretical benefit doesnt always translate to a real world benefit when you start factoring unknowns like how a human body actually works etc.

Im seeing a few agencies claim this safety margin thing but from what i can find, not one of those claims has actually been verified with facts or evidence.

[Mal Bridgeman]

Although it would be good to get statistical gurus to comment on this I think it's a nigh on impossible prove.

People who dive Nitrox tend to do it when they have some experience so comparing a control group becomes virtually impossible. What Nitrox do you use in order to measure 22%, 23%, ...... 32% or 36% ? Shallow dives, deep dives, first dive of the day, last dive of the day, all dives etc etc etc so again it becomes more difficult to the point of impossible to measure, I think.

If I might paraphrase Mr. Pooley (and I am not sure if he was quoting someone else) ..... "I do not need to be hit on the head by a brick, to know I do not want to get hit on the head by a brick"
Mal

[sccs]

Originally Posted by String

Guess the only people that might fund something like it would be DAN.

The point i'm making is a theoretical benefit doesnt always translate to a real world benefit when you start factoring unknowns like how a human body actually works etc.

Im seeing a few agencies claim this safety margin thing but from what i can find, not one of those claims has actually been verified with facts or evidence.

if we ignore all the other contribitry factors, fitness, hydration, ect...
doesnt excessive nitrogen loading lead to increased risk of dcs, therefore any reduction in nitrogen, is benificial,

as for proof dosent the increased no stop time provided by nitrox over air
prove something its self, so if you dive nitrox on air tables then you are benifiting from lower nitrogen loading thats fact.

the only thing that makes me think is the divers who dive nitrox on air tables then use the nitrox surface intervals and claim they still have the same margin as using the air surface interval

is it of benifit to diver x then yes it will be benificial as it will reduce the amount of nitrogen loading during the dive, how ever will it reduce the odds of dcs in diver x, yes it will as they now have a lower nitrogen loading

would i receive same amount of benifit as diver x, then yes i will,
will i be in the same risk group as diver x prob not, as my fitness level ect, are different,

in the end make your own mind up.

personally i know its of benifit to myself vs diving air, and anything that reduces risk has to be a good thing.

[Bantam]

The way I understand it is like this.

You do a dive to 30m, for 30 minutes and you're breathing air.

You get (using vplanner cos I don't have my VR2 in London) a total dive time of 55 minutes to the surface and you start offgassing at 21.8m

Then run the same dive on 32% with a stage of 50% for deco.

Total run time is 35 minutes, with offgassing starting at 17.1m, because you've already got an equivalent air depth of 24m for your max depth of 30m anyway. (In actuality, Vplanner doesn't even make you stop on this dive apart from a 2 min stop at 3m!)

So if you were to do the Nitrox dive but on air tables, you'd end up doing an extra 20 minutes of deco with massively increased O2 levels which will end up making you not only ongass less, but offgass more readily.

End result is that your medium tissues probably won't be as saturated to begin with anyway.

EDIT - I always find it fascinating to monitor the tissue loading graph on the VR2 as that is what it thinks is happening to the body. Obviously it's designed for ultra-fit generic navy-types, so a fat bloater like myself will have slightly different on-and-off gassing profiles, but it interesting to see what types of activity (saw tooth profiles and so on) really twist it's noodle and which tissues it seems to think will get loaded.

But as we all know, deco is a theory. You won't get any cast-iron evidence, but it seems pretty clear cut to me that running a dive on Nitrox using Air tables puts a huge amount of safety into the equation, and I know for a fact that PADI states this as the case in their course material, as do DAN according to an article that appeared in their magazine a few years back. I'll see if I can dig it out somewhere.

HTH.