Effective O2 percentage with added O2??

I'm trying to determine the effective oxygen concentration in the air I breathe with my CPAP given 3 Liters/minutes O2 flow.

Here's what I think is true - please post corrections if you see errors. An explanation of the error would also be appreciated.
1) Normal O2 concentration in room air is 21%
2) Minute Vent value is in liters/minute
3) Unintentional leak rate is in liters/minute
4) Supplemental oxygen is 100% oxygen or close enough to not matter
5) Supplemental oxygen flow rate adds to the overall flow through the CPAP

Here are my calculations:
Total room air flow = Minute Vent + Unintentional Leak Rate or 12.2 + 9.9 lpm or 22.1 lpm (for me last night, data from Encore)
Total room O2 flow = O2% * Total Flow or 22.1 * 21% or 4.7 lpm
Supplemental O2 flow = 3 lpm (measured on my O2 concentrator)

Final O2 % for combined air flow = (total room O2 flow + Supplemental O2 flow)/(Total room air flow + Supplemtal O2 flow)
or (4.7 + 3.0)/(22.1 + 3.0)) or 30%

That seems way too high so I must have some of the assumptions wrong or some of the calculations wrong or both.

Any help from an actual RT (or someone who actually knows about these things) would be greatly appreciated.

Great question. You’ve piqued my interest! I don’t know anything about your calculations, and look forward to any insight others give.

This is interesting...

http://www.ucdenver.edu/academics/colle ... herapy.pdf

Only the part of the O2 that is used up in the lungs, is the effective part, the rest is like the gas tank in your auto storage and grade of fuel. It must be used to be effective. Jim

The math is way way more complicated than you think. First, are you using an oxygen concentrator, which has an output way lower than 100% O2, or a pure O2 tank? The net flow into a room is the flow in less the flow out, and you have no way to estimate the flow out, through registers doorway, leaks, etc. The total volume of air in the room will have an effect on the growth rate of the O2 concentration. Then, taking into account those factors it is a differential equation to determine the growth rate of the O2 concentration, and the steady-state value. And don't forget that you are removing some of the O2, so that your exhaled O2 concentration must be taken into account. The way to set this up is a compartmental model, with all the inputs, and all of outflows, then a dynamic balance to arrive at the steady state concentration.

What is the reason that you're using an oxygen concentrator? If you have plain Sleep Apnea and no other conditions, you shouldn't need it.

TASmart wrote: ↑

Fri May 25, 2018 6:43 pm

The math is way way more complicated than you think. First, are you using an oxygen concentrator, which has an output way lower than 100% O2, or a pure O2 tank? The net flow into a room is the flow in less the flow out, and you have no way to estimate the flow out, through registers doorway, leaks, etc. The total volume of air in the room will have an effect on the growth rate of the O2 concentration. Then, taking into account those factors it is a differential equation to determine the growth rate of the O2 concentration, and the steady-state value. And don't forget that you are removing some of the O2, so that your exhaled O2 concentration must be taken into account. The way to set this up is a compartmental model, with all the inputs, and all of outflows, then a dynamic balance to arrive at the steady state concentration.

Output should be 90% or the machine is broken, 90% is NOT way less than 100%, at least it wasn't taught that way in my math class. Jim

My O2 unit is in the next room, hose running under the door, so my room builds up O2 levels also. It hasn't started a fire yet!

JDS74 wrote: ↑

Fri May 25, 2018 8:53 am

4) Supplemental oxygen is 100% oxygen or close enough to not matter

Actually about 95, if you're lucky (assuming it's coming from a concentrator, since what it actually does is filter out the nitrogen).

JDS74 wrote: ↑

Fri May 25, 2018 8:53 am

5
Here are my calculations:
Total room air flow = Minute Vent + Unintentional Leak Rate or 12.2 + 9.9 lpm or 22.1 lpm (for me last night, data from Encore)
Total room O2 flow = O2% * Total Flow or 22.1 * 21% or 4.7 lpm
Supplemental O2 flow = 3 lpm (measured on my O2 concentrator)

Final O2 % for combined air flow = (total room O2 flow + Supplemental O2 flow)/(Total room air flow + Supplemtal O2 flow)
or (4.7 + 3.0)/(22.1 + 3.0)) or 30%

That seems way too high so I must have some of the assumptions wrong or some of the calculations wrong or both.

Any help from an actual RT (or someone who actually knows about these things) would be greatly appreciated.

Actually, all you need to do is figure the *total leak* on your machine, which fortunately, yours shows, and add the 3lpm O2.

So if you're venting 70lpm, then you've got 70lpm at around 21%, plus 3lpm at around 95%.

your MV doesn't factor in, because the O2 is being added upstream, to the total airflow.

At least, that's how I figure it... I'll leave it up to you to do the maths

. wrote: ↑

Fri May 25, 2018 6:43 pm

The math is way way more complicated than you think. First, are you using an oxygen concentrator, which has an output way lower than 100% O2, or a pure O2 tank?[

Not if the concentrator is working properly... and many these days have O2 sensors that alarm, (the one on the concentrator here alarms if the O2 percentage is below 85%, normal operation is 85%-95.6%

. wrote: ↑

Fri May 25, 2018 6:43 pm

The net flow into a room is the flow in less the flow out, and you have no way to estimate the flow out, through registers doorway, leaks, etc. The total volume of air in the room will have an effect on the growth rate of the O2 concentration. Then, taking into account those factors it is a differential equation to determine the growth rate of the O2 concentration, and the steady-state value. And don't forget that you are removing some of the O2, so that your exhaled O2 concentration must be taken into account. The way to set this up is a compartmental model, with all the inputs, and all of outflows, then a dynamic balance to arrive at the steady state concentration.

Perhaps if OP were to hermetically seal himself into a room, you might have a point...

In real life, you don't.