Respiratory Therapist/PaO2/FiO2


David wrote at 2007-04-23 05:19:42
Hey there,

I just wanted to add on that to Calculate the PF ratio, it is as simple as dividing the PaO2 by the FiO2. Therefore, if someone's PaO2 is 100 and they are on room air (i.e. most healthy people at sea level) than their PF ratio would be 100/.21 = 476.

Another example:

PaO2 = 60

FiO2 = .50

PF ratio = 60/.50 = 120 >> ARDS (Assuming other clinical manifestations are also met for ARDS).

Hope this helps,


ITU Dr Hugo wrote at 2007-05-01 00:16:36
Calculating the PaO2/FIO2 is simple.

Take the PaO2 in mmHg and divide it by the FIO2 (eg if the patient is on 60% oxygen then the FIO2 is 0.6).

Example: A patient has a PaO2 of 80mmHg whilst on 50% oxygen. His PaO2/FIO2 is therefore 80 divided by 0.5 which is 160.

Calculating the PaO2/PAO2 is more complex as you need to calculate the estimated PAO2 and is therefore rarely done.

Just one other thing that is rarely explained - little letters refer to liquids and big letters refer to gases so PaO2 stands for arterial (partial pressure of oxygen) whilst PAO2 refers to Alveolar (partial pressure of oxygen). FIO2 is the Inspired fraction of oxygen.

RTJen wrote at 2007-05-20 16:33:33
Hi there,

I would also add that if they are not on 100% O2 or room air this equation would not be accurate because of the varying O2's in the devices.  

bjornisthespoon wrote at 2007-12-22 22:14:58

    The PaO2/FIO2 ratio is derived by taking the PaO2 derived from an ABG, for example pH=7.45, PaCO2=40, PaO2=80, and HCO3=22.  The PaO2 of 80 is divided by the FIO2 that the patient is being oxygenated with.  So again using an example, A/C mode, rate of 12, tidal volume of 500mL, FIO2 of 0.5 (or 50%), and PEEP of =5.  The practitioner would literally divide 80 by 0.5 and the result would be 160 for the PaO2/FIO2 ratio.  Not such a great number.  You would anticipate with a high FIO2 of 50% to receive a higher oxygen content than 80mmHg.  I hope this was helpful.

JP Souza wrote at 2009-06-06 14:34:55
you should divide the PaO2 value in mmHg and divide it by the FiO2. For instance:


FiO2=50% i.e. FiO2=0,50

PaO2/FiO2= 60/0,50 = 120

Resp.GSU wrote at 2009-07-26 22:39:31
Hello, I am a respiratory therapy student at GSU.  PaO2/FiO2 will show you the effectiveness of how well the O2 therapy is going.  A value less than 200 is critical and requires immediate attention.  This means that very little O2 is getting to the arterial blood.  Do not expect substantial results by increasing FiO2 above .60 if PaO2 is less than 70.  The only answer is PPV...try BiPAP first...monitor for signs that MV is needed.  There's so much more to say but this may just be enough for you.

Jason wrote at 2009-11-15 03:23:58
PaO2/FIO2 ratios are calculated by dividing the PaO2 by the FIO2. For instance, if the PaO2 is 100 and the FIO2 is 21%, the calculation is 100/.21=476. If the PaO2 is 100, but the FIO2 is 50%, the calculation is 100/.50=200. Below 300 is indicative of ALI, and below 200 is ARDS. As stated by Neal, you will need a PaO2 from an ABG. We would like to see PaO2 stay between 80-100. Less is not enough oxygen and more can cause oxygen toxicity, but we shouldn't be concerned if the level is slightly high in critically ill patients on oxygen.

Jason RRT

Dave wrote at 2011-11-17 20:44:20
I realize this question is a few years old, but since this comes up pretty early in a google search of PaO2:FiO2, I thought I should add that the ratio is calculated by hand very easily.  At least with the blood gas machines I've seen, none of them calculate this for you.

As an example, if a patient's PaO2 is 90 (this value will be provided as part of the blood gas) and their FiO2 is 0.5 (they are breathing 50% oxygen), then the ratio is 90:0.5 (which is the same as 90/0.5) = 180.  ie, that patient falls into the ARDS category (ratio less than 200).  

Elicia wrote at 2013-02-24 01:08:20
I am currently researching this topic for class. You calculate the PaO2/FIO2 ratio only after you have obtained an ABG and you are able to obtain the PaO2 value. Then you divide the PaO2 value by the FIO2 percentage.

Ex: your PaO2 is 98 mmHg and your FIO2 is 35% or 0.35. So 98/0.35= 280mmHG is your PaO2/FIO2 ratio. According to ARDS NET guidelines, this would classify the patient as being in ALI.

Hope this helps!

Elicia, BSN


ARDS Network study (2000). New England Journal of Medicine; 342:1301-1308.  

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Neal Okerson


I should be able to answer most questions about anatomy and physiology of heart and lungs. I also enjoy answering questions about understanding diseases. I should also be able to explain the goals and objectives of various pulmonary related treatments. Not an M.D., but worked in the field (Certified Respiratory Therapy Technician) since 1990. Registered Respiratory Therapist since 1995. B.S. degree in Health Education (1987). Currently working (since 1993) as the Director of Clinical Education in an AMA approved Respiratory Care Program in Western KY.

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