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 : 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).