Using Absolute VO2 to compute theoretical RMV for emergencies?

[VikingDives]

I was thinking about rock-bottom gas calculations, and the one sticking point that I've run into is the arbitrary "oh crap" factor that goes into the calculation. What I'm calling the "oh crap" factor is the factor by which RMV is multiplied to account for increased breathing in an emergency.

This led me to think that as an athlete, I usually know what my VO2 Max is, and that's a measure of milliliters of oxygen per kilogram...

So here's the question: If I convert VO2 max to my known absolute VO2 (litres of air per minute), wouldn't that be a better factor to use as an "oh crap" number than some arbitrary multiplication factor.

For example, My VO2 max is 49 and I weigh 78 kilos, so (49 x 78) /1000 is 3.51 Litres per minute. Very dull video on the math
Am I missing something here? This isn't my strong suit, so I could be completely out in left field...

 

[bradlw]

without wrapping to much of my brain bandwidth around it...(and I am by no means knowledgeable about VO2 other than a basic understanding of the gist)
and I didn't watch the video
I don't think you're taking account of the partial pressure of the O2 in your mix
this marching on my probably incorrect assumption that VO2 max is YOUR theoretical consumption of O2 in the blood when exerting yourself....
and there's probably some efficiency factor because your lungs probably aren't transporting every available O2 molecule to your blood, etc...

 

[johndiver999]

I was thinking about rock-bottom gas calculations, and the one sticking point that I've run into is the arbitrary "oh crap" factor that goes into the calculation. What I'm calling the "oh crap" factor is the factor by which RMV is multiplied to account for increased breathing in an emergency.

This led me to think that as an athlete, I usually know what my VO2 Max is, and that's a measure of milliliters of oxygen per kilogram...

So here's the question: If I convert VO2 max to my known absolute VO2 (litres of air per minute), wouldn't that be a better factor to use as an "oh crap" number than some arbitrary multiplication factor.

For example, My VO2 max is 49 and I weigh 78 kilos, so (49 x 78) /1000 is 3.51 Litres per minute. Very dull video on the math
Am I missing something here? This isn't my strong suit, so I could be completely out in left field...

so you use 3.5 liters per minute of oxygen and then you just adjust for depth and mix? you don't consume all the oxygen from an inhalation, only a relatively small portion of it, so you would seem to have to add the oxygen extraction/absorption ratio as well?

 

[Bowers]

The logic would be sound if you also added your dive buddies max to plan for a gas sharing contingency

Edit: i read more in depth and notice your referring to oxygen and not tidal volume. Just because you can only use so much oxygen doesn’t mean thats all you can move through your lungs (this math may be better with rebreathers). For open circuit you need to consider the max tidal volume x max respiratory rate that you may achieve. This respiratory minute volume can then be expressed as cfm or lpm. Then multiply that by max depth in atmospheres to account for it being wasted faster at depth. Take that and double it so you can share with a buddy. That will give you a number you can multiply by the number of minutes you’ll need (ie how long to fix problem then slowly surface). All this comes together to give you a rock bottom value that you should have remaining when you begin ascent.

Example:

The average tidal volume is 500ml but we are capable of moving up to 2000ml per breath. The average rate is 16 breaths/min but in cases of stress we may breath in excess of 30 bpm.

So..
2000ml x 30breaths/min = 60,000 ml/min (60liters)
If i dive to 66’ im at 3 ata.
60 liters/min x 3 ata = 180 liters/min at depth
Double it to cover your buddy= 360 liters/min
There are 28 liters per cubic foot so here in the USA we continue with 360 liters\28=13 cf
If i want 1 minute to fix a problem and 2 minutes to surface then to cover that 3 minutes i need 3 x 13 = 39 cubic feet of air (which is 1500psi in an al80).
There is a slight built in conservancy since my consumption decreases as i get shallower.

This is not what you are required to surface with but the minimum reserve you should have before beginning to ascend. It’s ok to breathe the tank down a bit more on the way back to the boat. You could do the math a couple times and see what you would be comfortable with at different depths. For example, i may be ascending from 66’ with half a tank but then willing to explore at 20’ until im down to 500 psi before surfacing.

 

[Alekseolsen]

There's some 15-18 percent oxygen in the air you exhale at the surface, so you would likely have to multiply your 3.5 by ~3 for it to make sense mathematically. Also "**** hits the fan" may cause hyperventilation, which I guess utilise way less of the O2 in the inspired gas

 

[VikingDives]

I don't think you're taking account of the partial pressure of the O2 in your mix
this marching on my probably incorrect assumption that VO2 max is YOUR theoretical consumption of O2 in the blood when exerting yourself....

Ah, good point.

so you would seem to have to add the oxygen extraction/absorption ratio as well?

I think this would also be correct...

The logic would be sound if you also added your dive buddies max to plan for a gas sharing contingency

I use the diver with the highest rmv to calculate for both divers... but your point is taken.

There's some 15-18 percent oxygen in the air you exhale at the surface, so you would likely have to multiply your 3.5 by ~3 for it to make sense mathematically. Also "**** hits the fan" may cause hyperventilation, which I guess utilise way less of the O2 in the inspired gas

Good points all. I still have a sneaking suspicion that someone like an anesthesiologist is going to comment here and blow this theory up, but to keep the formula going:

My VO2 max is 49 and I weigh 78 kilos, so (49 x 78) /1000 is 3.51 Litres per minute.

At the surface, on air, we have a PPO2 of .21 (we can worry about other pressures and mixtures later), so we divide 3.51 litres, by .21 = 16.7 litres of air. Then we assume that (unresearched) since only 3 percent of the O2 is being used in a given breath, that would be (3.51 / .21) 1.97 = 32.9 litres per minute.

A quick googling turns up this, which says you can consume 100 litres of air per minute during exercise... so I must be missing something.

 

[L13]

This is a complete mistake. RMV has very little to do with O2 consumption. RMV is gas consumed, not O2 consumed. The "need to breath" is driven by 3 main factors: CO2 buildup, breathing habits, and mental state(calm,panicked, narced, paranoia, etc.). VO2 has nothing to do with the latter 2 factors, and is only indirectly related to the first.

An average person uses a PPO2 of ~0.04 per breath, with a minimum usable inspired PPO2 of ~0.16. Using air at 100ft you would be breathing in PPO2 ~0.88, and breathing out PPO2 ~0.84. In that case the air you are breathing out still has enough O2 in it to be breathed ~17 more times! (but does displace N2 in the inspired air, extending NDL/reducing Deco). With 32% nitrox, you have enough oxygen for ~28 more breaths. But, you can't reuse that air because now it contains CO2 that must be removed.

VO2 max does indicate the maximum amount of CO2 that must be removed since the O2 is consumed by combining it with "food" to produce CO2 + energy. But things like state of mind, breathing effectiveness, gas density, etc. will dominate breathing by directly or indirectly affecting CO2 sensitivity and CO2 removal effectiveness.

VO2max/(VO2 average on a dive) might help some by indicating the increase in CO2 removal require. But it still doesn't account for all the other factors.

 

[broncobowsher]

As other stated, not going to work out very well.

It actually does work somewhat good on a rebreather as you are consuming O2 and the rest of the gas just goes round and round again (except for the bit that goes in or out of tissues). 3.5 Liter of O2 means about 3.5 liter of CO2 produced, and that may get into issues of over breathing the scrubber.