[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? :-/

 

[1693]

... 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

 

[1693]

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.

 

[5363]

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.

 

[nigelH]

I'm going to disagree with the whole premise of the first post.

O2 inhaled has nothing to do with O2 consumed. We make O2 available to the tissues that need it but they still only use what they want. Increasing the ppO2 will not change the O2 consumption nor the CO2 production rate.

'Desire to breathe' depends on blood acidity which depends primarily on CO2 levels. However you need to move N litres of gas through your lungs to get a 'flush' so more CO2 to get rid of means more litres of gas (at what ever is ambient pressure) to keep the same amount of CO2 travelling outwards for the same level of breathing comfort.

Hence changing the mix you breathe won't affect your gas consumption for a given depth one iota. The simple law of surface consumption multiplied by ambient pressure is as good as you are going to get. You can skip breathe and push up your CO2 levels and save some flushing gas at the expense of the classic CO2 headache or rebreathe it and let the scrubber take the strain.

The big thing that drops O2 consumption and hence CO2 production is relaxing. If I'm nice and warm and dangling on a string waiting an hour for a computer to announce the deco is finished the SPG on the O2, only a 3L tank, just stops moving. One day I'll get so relaxed I'll fall asleep, lose my hold of the reel and drift away to be remembered only as an awful warning against the evils of solo deco diving.

 

[Rydive]

Everybody knows that we only consume a percentage of the O2 we breath on air. So, why would we think that breathing a richer mix would increase the consumption of 02. If our tissues required more 02 there is plenty availabe in the air we breath.

02 cunsumed, not breathed = co2 produced

 

[Jimbo200SX]

That was the question as to whether the amount of O2 able to be absorbed was an absolute, or a percentage of the O2 available, therefore, if a higher percentage of O2 is available then a higher percentage of the total gas could be absorbed.

It seems that this is an absolute, rather than a percentage, hence irrespective of any relationship (Or lack of) between O2 absorbtion and CO2 production, nitrox would not affect air consumption as we are still absorbing the same amount of O2. I think.....

 

[Scuby]

It seems that this is an absolute, rather than a percentage, hence irrespective of any relationship (Or lack of) between O2 absorbtion and CO2 production, nitrox would not affect air consumption as we are still absorbing the same amount of O2. I think.....

Aye.

Of course there could be (and probably are) very small effects in gas consumption on nitrox vs air, maybe because you feel more relaxed on nitrox knowing you have a nice safety margin, or because you usually dive shallower on nitrox (less kit, less narcosis, less stress, warmer, etc), or anything like that. No easy way of reliably testing any of those though (at least not able to prove the causality) so stick with the assumption of "gas consumption doesn't change" and you'll be fine :angel:

David

 

[Mal Bridgeman]

I am sure that when Nitrox was first taught to the "mass market" this was one of the stories put about to increase its attractiveness.

I have been unable to find proof of it though .... I think it's another of the those diving falsehoods, like you must do your deepest dive first, it won't hurt when you take your mask off or 40m in the Red Sea is "different" to 40m in the UK..... but that does not stop people believing them

Mal

 

[Okeanos]

my best SAC rates have been on nitrox dives!

 

[Tunicates]

The amount of o2 you can draw from the inspired air is dependant on your red blood-cell count. The molecule responsible for o2 uptake is haemoglobin - found only in red blood-cells - and each molecule has only so many sites to bind o2 onto. Its more about how good a carriage system you have rather than how many things you have available to carry.

Cyclists and other athletes going for extreme records often train at altitude so as to boost the numbers of red blood cells in their body. Presumably one would uptake more o2 in the inspired air than normal if one had spent 2 weeks previously up a mountain.

 

[Foggy]

The amount of o2 you can draw from the inspired air is dependant on your red blood-cell count. The molecule responsible for o2 uptake is haemoglobin - found only in red blood-cells - and each molecule has only so many sites to bind o2 onto. Its more about how good a carriage system you have rather than how many things you have available to carry.

Cyclists and other athletes going for extreme records often train at altitude so as to boost the numbers of red blood cells in their body. Presumably one would uptake more o2 in the inspired air than normal if one had spent 2 weeks previously up a mountain.

Correct.

But as stated above the CO2 is produced by respiration. It is metabolic, and depends on work rate.

It is the CO2 level that triggers the breathing response (unless you are a heavy smoker).

Now Hyperventillation (think of lots of breathing out as overdumping CO2) will reduce CO2, BUT uses more gas (breaths in for every out).

I have never noticed a difference in SAC on Nitrox.

Now relaxation - meditation / Yoga - have been shown to reduce SAC. Articles in magazines and just look at the freedivers crowd.
Some people relax more when less concerned about nitrogen (deco) comitments, and this may be the cause of anecdotal evidence for reduced SAC.

Cheers, Paul

 

[nigelH]

Presumably one would uptake more o2 in the inspired air than normal if one had spent 2 weeks previously up a mountain.

No.

You can only take up more if you have more 'empty' Hg molecules. Your body only uses so much so you only have that number 'empty'. Having more full ones just going round and round does nothing beneficial.

Having more carriers is good if you are going to exceed the 'normal' flow rate but diving is hardly energetic. We hit lactic acid build up long before we get muscle oxygen starvation. That one only features in extreme endurance sports and then only at the very top levels.

 

[A. Berk]

my best SAC rates have been on nitrox dives!

Which may be due to no other reason than the reduced narcotic presence of N2 causing more relaxation/less anxiety.

kerBo :redface: