Tricks of the trade : P/F ratio without knowing an ABG.

Look at your patient’s SpO2 > this will allow you to approximate both your patient’s  PaO2 and P/F ratio.

If your patient’s spo2 = 90%, your patient’s PaO2=60mmHg.

If your patient’s spo2=99%,  your patient’s PaO2=90mmHg.

So if your patient’s SpO2 = 97% and the FiO2=40%, you can estimate the P/F ratio as ~80/0.4=200.

If your patient’s SpO2=100%, your PaO2 can be anywhere from 90 to 600 > this is no help because you cannot approximate, let alone pinpoint, the pao2.

So, your patient with an SpO2=100% is not very telling of the patient condition.

If your patient’s spo2=100% and the pao2=100, on an fio2=40%, the p/f ratio=100/0.4=250 (=ALI = acute lung injury).

if your patient’s spo2=100% and the pao2=240, on an fio2=40%, the p/f ratio=240/0.4=600 (= normal).


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