PF ratio and why we need it

#KYJ -  PF Ratio

In our respiratory seminar today we discussed PF ratio. I thought that is might be an interesting 'Knowing your jargon' topic.

PF ratio or P:F is a calculation to determine severity of lung injury.

The P stands for PaO2

The F stands for fiO2

So PaO2:fiO2

Normally on air, a persons PaO2 is 80-100 mmHg. Let's say 90.

The fiO2 on room air (21% oxygen) is 0.21

So a PF ratio of 90:0.21 is calculated as 90/0.21=428

Therefore a PF ratio of >400 is considered normal.  If a person develops an acute lung injury or acute Respiratory Distress Syndrome (ARDS), their PF Ratio is markedly diminished. 

The 100/50 rule is a diagnostic example. If a person's PaO2 is 100 while they are breathing 50% O2, then the PF is at the diagnostic point for ARDS.

(<200)

100mmHg / fiO2 of 0.5

100/0.5= 200

You would expect their PaO2 to be at least 200mmHg if breathing 50%. 

The PF ratio is a good way to determine if the ABG oxygen value is consistent with the concentration that the person is breathing.

Critical values

PF ratio >400 is normal

<300 = Acute lung Injury (ALI)

<200 = Acute Respiratory Distress Syndrome (ARDS).