Air Consumption while on Nitrox - A theory

[Jimbo200SX]

Firstly - apologies if this is in the wrong place!

We were discussing this yesterday, and it would be nice to have any opinions/knowledge/scientific evidence people may have.

Whilst diving on Nitrox, it was argued that air consumption should be less due to a higher oxygen content being provided, hence reducing the urge to breathe, however, working on the basis that the urge to breath stems from increasing CO2 levels in the body, if you are receiving a higher O2 content, would you producing more CO2, increasing the urge to breath, causing you to breath more heavily, using more gas in the process.

I'm aware that of the air we breath only a fraction of the oxygen content within it is actually used, however is this an absolute amount i.e. would we use the same amount of O2 within the gas we breath, no matter what the mix, or is it a percentage of the available O2.

It seems that if its an absolute, in theory we should have the same consumption of air, but if its a percentage, then in an enriched mix, air consumption would be higher, due to increased CO2 levels creating a greater need to breath.

In your opinions is there any validity to this theory, or am i completely barking up the wrong tree?!

P.S. Would partial pressures of O2 also have a bearing on this?

[big_si]

It may well be that breathing rate would go down?

I read recently that for some breath hold attempts they hyperventilate on 100% O2 to 'completely' purge their system of CO2 so that the build up to a level to prompt breathing takes longer...

Whether this applies with normal respiration though I'm not sure? :-/

[nickb]

... if you are receiving a higher O2 content, would you producing more CO2...

I'm no physiologist, but that sounds like bollocks to me Jimbo

[Jimbo200SX]

Yeah i heard about that on the david blaine attempt, sounds like cheating to me!

An interesting point though, so potentially the increased O2 levels might well 'push out' the CO2, reducing the need to breath.

[weazelz]

increasing CO2 levels in the body, if you are receiving a higher O2 content, would you producing more CO2, increasing the urge to breath, causing you to breath more heavily, using more gas in the process

I would have expected CO2 production to be dependent on work-rate, so independent of O2 intake

[nickb]

I would have expected CO2 production to be dependent on work-rate, so independent of O2 intake

Exactly

[Jimbo200SX]

So with an increased work rate, the additional oxygen available in a nitrox mix would have no effect on the CO2 produced, therefore either there is no direct relationship between O2 absorbed and CO2 produced....
...or the amount of O2 absorbed is an absolute amount regardless of mix, therefore, CO2 produced for the same workrate on a different would be the same, hence air consumption would be the same?

[NotDeadYet]

if you are receiving a higher O2 content, would you producing more CO2, increasing the urge to breath, causing you to breath more heavily, using more gas in the process.

CO2 production is dependant on work rate, not how much O2 there is in the lungs.

I read recently that for some breath hold attempts they hyperventilate on 100% O2 to 'completely' purge their system of CO2 so that the build up to a level to prompt breathing takes longer...

Nope. CO2 is being produced all the time so breathing O2 isn't going to get rid of it. The O2 is to increase the oxygen saturation in the blood. Hyperventilation drops the CO2 levels, hence the pH of the blood, which in turn reduces the urge to breathe so that O2 will run out before the urge to breathe kicks in... Hence the need to increase the O2 saturation.

It's all bollocks anyway as AIDA is the recognised body for verifying records and Blaine's methods are outside AIDA standards. He's also a twat. The official static records are impressive in themselves without having to cheat.

I've never noticed any difference in SAC when on nitrox.

[uwila]

For someone who is reasonably fit, I don't believe there will be any difference. As stated earlier, urge to breathe is dependant upon CO2 levels. However, for anyone who has breathing or circulatory difficulties, a richer mix allows that person to get the O2 they require with a normal breathing pattern - ie not gasping away like a steam train. They use less gass when breathing 100% for example, but I can't see how this is relevant to anybody who dives.

Freedivers purge CO2 in order to delay the point at which the CO2 levels compell breathing to restart. They need to use 100% otherwise they would become victims of shallow water blackout on their ascent.

Cheers, CHris

[Richard Mason]

So with an increased work rate, the additional oxygen available in a nitrox mix would have no effect on the CO2 produced, therefore either there is no direct relationship between O2 absorbed and CO2 produced....
...or the amount of O2 absorbed is an absolute amount regardless of mix, therefore, CO2 produced for the same workrate on a different would be the same, hence air consumption would be the same?

That's right, basically you are respiring rather than combusting.