The P/F ratio is a powerful objective tool to identify acute hypoxemic respiratory failure when supplemental oxygen has already been administered and no room air ABG is available, or pulse oximetry readings are unreliable.
The diagnostic criteria for acute hypoxemic respiratory failure is:
The P/F ratio indicates what the PaO2 would be on room air (if patient was taken off oxygen):
The P/F ratio should not be used to diagnose acute on chronic respiratory failure since many patients with chronic respiratory failure already have a P/F ratio < 300 (PaO2 < 60) in their baseline stable state which is why they are treated with chronic supplemental home oxygen.
Note that PaO2 and pO2 are synonymous.
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“P” represents PaO2 (arterial pO2) from the ABG. “F” represents the FIO2 – the fraction (percent) of inspired oxygen that the patient is receiving expressed as a decimal (40% oxygen = FIO2 of 0.40). P divided by F = P/F ratio.
PaO2 = 90 on 40% oxygen (FIO2 = 0.40): 90 / 0.40 = P/F ratio = 225.
A P/F ratio of 225 is equivalent to a pO2 of 45 mmHg, which is significantly < 60 mmHg on room air.
A nasal cannula provides oxygen at adjustable flow rates in liters of oxygen per minute (L/min or “LPM”). The actual FIO2 (percent oxygen) delivered by nasal cannula is somewhat variable and less reliable than with a mask but can be estimated as shown in the Table below as the accepted clinical standard for the conversion. The FIO2 derived from nasal cannula flow rates can then be used to calculate the P/F ratio. Note: Assumes room air is 20% (0.20) and each L/min of oxygen = +4% (0.04).
Example: A patient has a pO2 of 85mmHg on ABG while receiving 5 liters/minute of oxygen. 5 L/min = 40% oxygen = FIO2 of 0.40. The P/F ratio = 85 divided by 0.40 = 212.5.
When the PaO2 is unknown because an ABG is not available, the SpO2 measured by pulse oximetry can be used to approximate the PaO2, as shown in the Table below.
*Note the SpO2/PaO2 conversion becomes unreliable when SpO2 is > 98%, but PaO2 of 110 mmHg for 97% may be used as a substitute avoiding an overestimation.
Patient has a pulse oximetry SpO2 of 95% on 40% oxygen: SpO2 95% is equal to PaO2 of 80mmHg. P/F ratio = 80 divided by 0.40 = 200.
Although the patient may be stable and asymptomatic while receiving 40% oxygen, the patient still has severe acute respiratory failure. If oxygen were withdrawn leaving the patient on room air, the PaO2 would only be 40 mmHg (much less than 60 mmHg criteria on room air for acute respiratory failure). Clinicians commonly lose sight of this fact.
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