## 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.