Evaluate your patient’s P/F ratio :

P/F ratio = PaO2 / FiO2 ratio.

It is a great way to index your patient’s oxygenation status.

It is cheap and not complicated but tells a lot about your lung in 30 seconds.

**HOW TO CALCULATE P/F RATIO :**

Divide PaO2 by FiO2 (in decimal format).

Normal P/F ratio value : 500-600 (on any FiO2).

Normal patient : (breathing room air)

PaO2 = 100 , FiO2 = 21% >> PaO2 / FiO2 ratio = 100 / 0.21 = 500.

Normal patient : (breathing 100% FiO2 via NRB (non-breather)).

PaO2 = 600, FiO2 = 100% >> PaO2 / FiO2 ratio = 500 / 1.00 = 500.

**WHAT ARE DIAGNOSTIC VALUES :**

P/F ratio > 500-600 = Normal.

P/F ratio < 300 = ALI.

P/F ratio < 200 = ARDS.

P/F ratio < 150 = AHRF (acute hypoxemic respiratory failure).

**WHAT IS THE AVERAGE SURGICAL PATIENT’S P/F VALUE :**

P/F ratio on arrival from operating theatre / room : PaO2 = 250, FiO2 = 100% >> P/F ratio = 250s.

P/F ratio 4 hours after surgery : PaO2 = 120, FiO2 = 50% >> P/F ratio = 250s.