# Formula For Air/o2, No2/o2 Mixtures

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• Specializes in Nurse Practitioner/CRNA Pain Mgmt.

Hi guys,

Hope someone can help me with this question.

Does anyone know how to manually calculate (if there's a formula) the percentage of gases, especially room air and oxygen or roomair, oxygen & NO2, that the patient is getting through the anesthesia machine? Without looking at the monitor's inhaled oxygen, air, or NO2 readings?

Especially when someone asks, "Give the patient 33 percent FI02 of air/02 mixture". How do you guys figure out where to set the air flow meter and 02 flow meter without looking at the monitors?

Does my question make sense? Sorry, I just woke up.

Thanks!

70 Posts

There must be an easier way but........to figure out how much you are giving CURRENTLY you could.....

2 liters o2= 100% *2 (200)

3 liters air= 21%*3 (63)

2 liters nitrous= 0%

total flow is 7 liters total % o2 is 263

263/7 = 37% 02 delivereed

I'm a second semester student in a front loaded program so forgive me for asking....... Will the machine tell you this or not?

.

268 Posts

Yeah, that's how I learned it also. It's easier to calculate N2o with O2 because N2O doesn't contain any O2 so ratios are easy to determine. I just thought it out a little bit. In order to have less than 50% O2 your air would have to be at least double liter flow of O2. I have .5L O2 and 2.5L Air giving 34% O2. Anybody know of an easier way?

430 Posts

Yeah, that's how I learned it also. It's easier to calculate N2o with O2 because N2O doesn't contain any O2 so ratios are easy to determine. I just thought it out a little bit. In order to have less than 50% O2 your air would have to be at least double liter flow of O2. I have .5L O2 and 2.5L Air giving 34% O2. Anybody know of an easier way?

Don't know if this will help but it works for me, break the air into fifths (1/5 is roughly the O2) so 1 liter of O2 and 1 liter of air equals 60% FiO2. Your mix is now 6 parts O2 of 10 parts or 60%.

Mike

430 Posts

You asked the question with .5 L of O2 and 2.5 air so that gives you 1/3 of the mix is O2 (1/5 of 2.5 is .5, .5 + .5 = 1, 1/3 = 33 % FiO2)

Mike

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