The P_aO₂/ FiO₂ ratio, or P/F ratio, is a useful tool for ARDS and respiratory failure, especially during the COVID-19 pandemic. Using the P/F ratio you can estimate the severity of hypoxemia and trend progression of respiratory failure — even if formal blood gas determination is not readily available.

Severe hypoxia/hypoxemia is common with COVID pneumonia. This has resulted in having to care for large numbers of hypoxic/hypoxemic patients on oxygen therapy simultaneously. Tracking P/F ratios can help you distinguish which of your patients on oxygen are stable, which ones need intervention to avoid intubation, and which ones should be intubated early.

What Is the P/F Ratio?

The P/F ratio is the arterial partial pressure of oxygen (P_aO₂) divided by the inspired oxygen concentration (FiO₂).

If our healthy patient’s P_aO₂ is 100mmHg when breathing room air with a FiO2 of 0.21, then the normal P/F ratio will be

P_aO₂ divided by FiO₂ or100 divided by 0.21 = about 500.

Note that I’m rounding the numbers to make comparisons less complicated.  If our COVID patient’s P_aO₂ is 60mmHg while breathing room air, then his P/F ratio is:

P_aO₂/ FiO₂ or 60/0.21 = about 300.

A normal P/F Ratio is ≥ 400 and equivalent to a PaO2 ≥ 80 mmHg on room air.

Okay, P/F is easy to calculate, but how does it help us? There are many health providers learning how to apply respiratory physiology in the trenches of the pandemic. To understand how P/F helps, let’s quickly review some important respiratory physiology facts.

What Is Respiratory Failure?

Respiratory failure is defined as:

• P_aO₂ < 60 mmHg (corresponding to oxygen saturation, or SpO₂, of 90%)
• P_aCO₂ > 50 mmHg with a pH < 7.35 (hypercarbic)
• P/F ratio (P_aO₂/ FiO₂) < 300 for a patient on oxygen

What’s The Difference Between Hypoxia and Hypoxemia?

The distinction between hypoxemia and hypoxia is important.

• Hypoxemia refers to a SUBnormal concentration of oxygen in the blood relative to the concentration of oxygen being inhaled.
• Hypoxia refers to an insufficient amount of oxygen in the tissues.

When caring for a patient with respiratory insufficiency or failure we want to provide enough oxygen and ventilatory support to keep the P_aO₂ above 60, (SpO₂ above 90%) because those values should still provide adequate oxygenation to the tissues.

How Can You Spot Hypoxemia?

The normal P_aO₂ is roughly equal to 5 times the FiO₂ written as a percent. Thus a healthy patient breathing 40% FiO₂ will have a P_aO₂ of about 200 mmHg (5 X 40 = 200). Healthy Patient A, breathing room air, or 21% FiO₂, will have a P_aO₂ of about 100 mmHg (5 X 21 = 105). Knowing this relationship alerts you to whether a patient is or isn’t relatively hypoxemic relative to the FiO₂.

Patient B is breathing a FiO₂ of 50% oxygen who has a P_aO₂ of 100 is relatively hypoxemic — his P_aO₂ should be about 250 with that FiO2 instead of 100. But he’s not hypoxic, since a P_aO₂ of 100 provides an adequate amount of oxygen to his tissues.

On the other hand, a patient with a P_aO₂ of 50 is both hypoxic and hypoxemic. His tissues are not getting enough oxygen.

Hypoxemia can progress to hypoxia if the patient’s condition worsens, or you take the oxygen away too soon.

PaO2 vs. Oxygen Saturation?

We can measure SpO2 noninvasively with a pulse oximeter, while we need to do a blood gas analysis to measure the PaO2. Monitoring SpO₂ is very helpful, but it is not a very sensitive tool. Let’s quickly review the difference between P_aO₂ and SpO₂ to show why. For a more complete discussion of oxygen saturation vs PaO2 click here:

What’s The Difference Between Oxygen Saturation and PaO2?

What Is Arterial PaO2

P_aO₂, put simply, is a measurement of the actual oxygen content in arterial blood. Partial pressure refers to the pressure exerted on the container walls by a specific gas in a mixture of other gases. In other words, if a gas like oxygen is present in an air space like the lungs and also dissolved in a liquid like blood, and the air space and liquid are in contact with each other, the two partial pressures will equalize.

What Is Oxygen Saturation?

Oxygen saturation is the percent of Hemoglobin (Hgb) binding sites in the blood that are carrying oxygen. Hemoglobin is a chemical molecule in the red blood cell (RBC) that carries oxygen on specific binding sites. Each Hgb molecule, if fully saturated, can bind four oxygen molecules. We can measure how many of these binding sites are combined, or saturated, with oxygen. This number, given as a percentage, is called the oxygen saturation or SPO2. When all the Hgb binding sites are filled, Hgb is 100% saturated and SpO2 is 100%.

Is An Oxygen Saturation Of 100% Always Normal?

No, it’s not. Let’s look again at our patient breathing 50% FiO2 who has a P_aO₂ of 100mmHg.

We know that someone breathing room air at 21% oxygen should have a P_aO₂ of about 100mmHg (5 X an FiO2 of 21% = 100). So if the patient is breathing 50%, then we know that his PaO2 should be about 250mmHg. It’s not, so therefore something is very wrong.

The Oxygen-Hemoglobin Dissociation curve shows the relationship between arterial PaO2 and oxygen saturation.

If you look at the Oxygen-Hemoglobin Dissociation Curve, a P_aO₂ of 100 and 250 both have an O2 sat of 100% because both provide enough oxygen molecules to fill all of the Hgb binding sites. SpO2 by itself can’t distinguish between those two very different patients.

P/F Helps Identify Severity of Hypoxemia

However, you can use the P/F ratio to tell you what the PaO2 of the patient on oxygen would be if you take them off oxygen — WITHOUT taking them off oxygen to test.

Example: As we saw above, Patient B, has a PaO2 of 100mmHg while receiving oxygen at FiO2 of 50% or 0.5

P/F = 100/O.5 = 200

But now you can use his P/F to calculate what his PaO2 would be on room air (FiO2 0.21)

“New P”  = “P/F ratio” times “New F”

“New P” = 200 X 0.2 = 40 mmHg

Note that Patient B on oxygen might possibly be stable and asymptomatic, but he’s severely hypoxemic. You definitely should not reduce the oxygen therapy. You must watch this patient carefully!

 Let’s look at another example. Patient C also has a PO2 of 100 breathing 30% FiO2 and we want to see what it might be on room air.

P/F = 100/0.3 = 330

“New P” = 330 X 0.21 = 66

Not great, but the PO2 is above 60, equivalent to a SPO2 of about 93%.

Both patient “B” and “C”, as well as our healthy Patient A, had PO2s equal to 100mmHg, and thus would all have an SPO2 of 100%. But you can see the P/F lets you compare the relative status of their oxygenation. “A” is normal, “C” is doing okay, and “B” is in pretty fragile condition.

Remember in the definition of respiratory failure, one of the criteria was a P/F less than 300 on oxygen. So P/F also gives you a quick evaluative measure of your patient’s status.

You can trend P/F in a single patient over time to see if it is improving or deteriorating with treatment over time.

What If You Don’t Have a Blood Gas!

These are tough times when we have potentially too many critical patients at one time and too few reagents, health providers, and technicians. You may be able to easily check SPO2 but not have access to frequent blood gas measurements.

If you have an SPO2 and you know FiO2, you can still use P/F.

SPO2 to PO2 Conversion

 You can use the Oxy-Hemoglobin Dissociation Curve to estimate your PO2 from the SPO2 if the measurement is below 100. You can also find these values in chart form. Here is one version of the chart.

Once you have the PaO2 corresponding to your patients SPO2, plug it in the P/F formula.

FiO2 Conversion from Liters Per Minute Via Nasal Prongs

What happens if your patient is on nasal prong oxygen? You can estimate the value of the FiO2 from the liters per minute being administered by using the following formula. Assume:

• room air contains 20% FiO2 (rounded off to make the math easier)
• each additional liter of oxygen adds 4% (0.04)

Example: Patient D has a PaO2 of 75 while receiving 5 liters/min of oxygen by nasal prongs.

FiO2 = 20 + 5 X 4 = 40%

This would yield a P/F ratio of 75/0.4 = 187

Summary

Knowing the P/F ratio lets you:

• Recognize severity of hypoxemic respiratory failure in a patient on oxygen therapy.
• Easily calculate the predicted PaO2 on room air in a patient receiving oxygen— without having to remove that patient from oxygen to test.
• Trend patient status over time as treatment progresses.
• Triage advanced care among multiple critically ill patients when resources are limited
• Recognize need to intensify treatment to avoid intubation.
• Recognize the patient who should be intubated early.

This pandemic is a difficult time. I am afraid that this is just the end of the beginning of what will be multiple surges of patients. We all need to be alert. We all need to be kind to each other. And we all need to watch over each other to stay safe.

May The Force Be With Us

Christine E. Whitten MD, author
Anyone Can Intubate 5th Edition: A Step By Step Guide
and
Pediatric Airway Management: A Step By Step Guide

Note. To assist with learning and teaching airway management during this pandemic crisis my books are available from amazon.com AT COST. My videos are available on YouTube for free viewing.