9- PF Ratio

KYJ9- PF Ratio

In this episode of Knowing Your Jargon we take a look at one of those funky respiratory terms called the PF ratio.

Correctly referred to as PaO2/fiO2 ratio, this calculation is a number derived from dividing the pressure of oxygen in arterial blood (in mmHg) with the concentration of oxygen that the patient is breathing.

PaO2 is normally 80-100 on room air. The fiO2 (fraction of inspired oxygen) of room air is 0.21 which is just another way of saying 21%.

To calculate the PF ratio you just divide paO2 by fiO2.

80/0.21 = 380
100/0.21= 475

Normal PF ratio is therefore 380-475.

As the number drops= badness
Infact the PF ratio is used to diagnose acute lung injury
(ALI= PF < 300 ) and the dreaded
Acute Respiratory Distress Syndrome (ARDS = PF <200)

So let's look at some predictions.  Assuming normal is 400.

1.  Assuming you have your patient on a Hudson simple face mask at 6 lpm.  It assumes oxygen delivery of 50% which is an fiO2 of 0.5.

If a blood gas was taken you would expect a paO2 of at least 200 (200/0.5=PF ratio of 400)

Predict a paO2 for any given patient by starting with a PF of 400 and multiplying it by their oxygen concentration.

Eg 400 x 0.5 predicts that s patient on 50% oxygen should have a PaO2 of about 200.

Eg - on an oxylog (air mix) at 60.5% oxygen, you'd expect that your patient should have a paO2 of at least 242 mmHg.

400 x 0.605 = 242

So next time you look at a set of ABG results, and think your patient is well oxygenated because their paO2 is between 80-100 mmHg,  consider the oxygen that they are breathing, calculate that little PF Ratio and just be sure. Remember 380-475 is normal.

Breathtaking hey?