## 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 : 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).