by Richard Pinson, MD, FACP, CCS
Diagnoses of acute hypoxemic respiratory failure is based on two things:
- Symptoms: acute or subacute ranging from increasing shortness of breath, increased (>20) or decreased (<10) respirations to cyanosis, retractions, accessory muscle use depending on severity and finally respiratory arrest.
- Blood gas measurement:
- PaO2 <60 mmHg on room air arterial blood gas (ABG), or
- O2 saturation <91% by pulse oximetry (SpO2) on room air, or
- PaO2/FIO2 <300 on supplemental oxygen
Coders and documentation specialists should review available blood gas values and laboratory values for any hint that a patient may have acute hypoxemic respiratory failure that needs to be documented and a query submitted. This article will explore the different measures of oxygen saturation you are likely to see in a record. (In this article, the expressions PaO2 and pO2 are used synonymously.)
What is the P/F ratio?
P/F ratio is the shortened expression for PaO2/FIO2 ratio taken from blood gas measurements. It is a powerful objective tool used to identify acute hypoxemic respiratory failure when supplemental oxygen has already been started and no ABG or pulse oximetry readings on room air are available, or at any time during hospitalization when a patient is receiving oxygen.
The P/F ratio indicates what the PaO2 would be on room air if oxygen discontinued:
Physicians often have the false impression that because the patient’s symptoms are relieved and the pO2 and/or SpO2 have improved with supplemental oxygen, respiratory failure has resolved. Even though symptoms are relieved and oxygenation improved or normalized with oxygen administration, the underlying respiratory failure persists which would become obvious if oxygen were removed. A P/F ratio < 300 on oxygen confirms continuing hypoxemic respiratory.
For example, although a patient may be stable and asymptomatic with a pO2 of 90 mmHg while receiving 36% oxygen, she still has acute respiratory failure because the P/F ratio is 250. If supplemental oxygen were withdrawn, the room air pO2 would only be 40 mmHg.
Caution: The P/F ratio should not be used to diagnose acute on chronic respiratory failure since all patients with chronic hypoxemic respiratory failure already have a P/F ratio < 300 (PaO2 < 60 mmHg) in their baseline stable state which is why they are treated with chronic supplemental home oxygen.
The P/F ratio is also used as a diagnostic criterion for sepsis and severe sepsis in both the Sepsis-2 (SIRS) and Sepsis-3 (SOFA) definitions. For the benefit of clinicians and as a finding upon which a query can be based, it should be shown on the ABG report but usually is not. A minor software modification to add a data field is easily performed for such a simple calculation.
How do I calculate the P/F ratio?
“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.
pO2 = 90 on 40% oxygen (FIO2 = 0.40): P/F ratio = 90 / 0.40 = 225.
A P/F ratio of 225 is equivalent to a pO2 of 45 mmHg, which is significantly less than 60 mmHg on room air.
Translating SpO2 (pulse oximetry) to pO2
When the pO2 is unknown because an ABG was not performed, pulse oximetry readings (SpO2) can be used to calculate the P/F ratio. The SpO2 can be used as a surrogate to approximate the pO2 as shown below:
SpO2 = 95% on 40% oxygen (FIO2 = 40%)
Using the above table, SpO2 95% = pO2 of 80 mmHg
P/F ratio = 80 / 0.40 = 200
Translating oxygen flow in L/min to FIO2
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. The FIO2 derived from nasal cannula flow rates can then be used to calculate the P/F ratio.
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 oxygen at 5 L/min by nasal prong.
5 L/min = 40% oxygen = FIO2 of 0.40. The P/F ratio = 85 / 0.40 = 212.5 (<300).
The 2021 CDI Pocket Guide, both print and electronic “Unbound” versions, contain more information about blood gases, the P/F ratio and their application to all forms of respiratory failure.
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About the Author
Dr. Richard Pinson, MD, FACP, CCS
Dr. Richard Pinson is a physician, educator, administrator, and healthcare consultant. He practiced Internal Medicine and Emergency Medicine in Tennessee for over 20 years having board certification in both. Dr. Pinson is a Fellow of the American College of Physicians and former Assistant Professor of Clinical Medicine at Vanderbilt, and has assisted hospitals as a physician consultant for the past ten years. Dr. Pinson’s personal understanding of the mindset of both doctors and managers allows him a unique vantage point from which to promote cooperation between physicians, hospital staff, and administrators toward achieving beneficial and practical changes.