Tag: p/f ratio
Most important item(s) to evaluate patient’s pulmonary status : P/F ratio
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.
Pulmonary physiology – Alveolar-arterial gradient – depicted
Depiction of the alveolar-capillary unit representative of the alveolar-arterial gradient and easily and quickly
defined by the PaO2 / FiO2 ratio (P/F ratio).
As the distance between the alveolus and the capillary increases, it is referred to as an “widening ” of the A-a gradient.
Many things can occupy this potential space : water, blood, infection (bacteria) as well as thickening membranes.
Tricks of the trade : PaO2 vs. SpO2
Always keep SpO2 < 100% (with rare exception).
An SpO2 between 92% to 99% says something about your patient’s condition.
SpO2 PaO2 FiO2 P/F ratio
100 % 600 100% 600
100% 500 100% 500
100% 400 100% 400
100% 300 100% 300
100% 200 100% 200
100% 100 100% 100
99% 90 100% 90
98% 87 100% 87
97% 85 100% 85
96% 82 100% 82
95% 80 100% 80
90% 60 100% 60
Tricks of the trade : the P/F ratio
Normal P/F ratio=600.
As a generalization :
everybody breathing room air (21%) will have an PaO2=100. P/F ratio=100/0.2 = 500.
everybody breathing 100% oxygen via NRB will have an PaO2=500. P/F ratio=500/1=500.
so now, you can figure out the max PaO2 on any FiO2.
FiO2 P/F ratio PaO2
100% 600 600
90% 540 600
80% 480 600
70% 420 600
60% 360 600
50% 300 600
40% 240 600
30% 180 600
21% 120 600
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).