How to Validate a Diagnosis of Acute Respiratory Failure

One of the most important pieces of clinical evidence that assists in validating a diagnosis of acute respiratory failure are the P/F ratio. The P/F ratio has been used for years in critical care and pulmonary medicine as one of the determinations for acute lung injury and ARDS. The Infectious Diseases Society of America and American Thoracic Society recognizes a P/F ratio as one of the 10 criteria for “severe” pneumonia. It has been used in the International Sepsis Definition criteria in 2001 and the Surviving Sepsis Severe Sepsis Guidelines in 2008 and 2012. It is also one of the SOFA criteria in Sepsis-3. It is easy to see that the P/F ratio has a history of credible followers that continues to this day.

The P/F ratio is a powerful tool to identify acute hypoxemic respiratory failure at any time while the patient is receiving supplemental oxygen. The power of the P/F ratio is its ability to predict what the pO2 would be on room air based on the arterial pO2 measured while the patient is receiving supplemental oxygen. The P/F ratio is calculated by the arterial pO2 divided by the FIO2 or fraction of inspired oxygen, which is expressed as a percentage or decimal.

The P/F ratio is one of the SOFA score diagnostic criteria for Sepsis-3. A P/F ratio of 300-399 indicates hypoxemia and equals 1 point on the SOFA scale. A P/F ratio <300 equals 2 points on the SOFA score (if the baseline is above 400) and represents acute hypoxemic respiratory failure. Although ICD-10-CM codes don’t exist for degrees of respiratory failure outside of ‘acute’, a P/F ratio <250 is approximated to a pO2 <50 mmHg on room air (and clinically defined as severe respiratory failure), and a P/F ratio <200 is approximated to a pO2 <40 mmHg on room air (and clinically defined as extreme respiratory failure)

When the arterial pO2 is not available, such as when an ABG was not performed, the pulse oximetry readings (SpO2) can be used to approximate the pO2 in order to calculate the P/F ratio as shown in the tables below:

SpO2 (percent)	pO2 (mmHg)
86	51
87	52
88	54
89	56
90	58
91	60

SpO2 (percent)	pO2 (mmHg)
92	64
93	68
94	73
95	80
96	90
97	110

Case Scenario (using the above tables): Pulse oximetry of 96% on 40% O2 per ventimask.

The data from the above tables show a pulse oximetry of 96% is approximated to a pO2 of 90 mmHg.

pO2 of 90 ÷ FIO2 of .40 = P/F ratio of 225

The P/F ratio is the pO2 of 80 divided by the FIO2 of 0.40 = 200.

The P/F ratio is relatively easy to calculate if the patient is on supplemental oxygen administered by mask. However, supplemental oxygen administered by the nasal cannula adds an additional step in the process. Even though a nasal cannula delivers oxygen at a somewhat variable and less reliable FIO2 than a mask, the FIO2 can be roughly estimated using the table below.

Flow Rate (liters/min)	FIO2
1	24%
2	28%
3	32%
4	36%
5	40%
6	44%

Case Scenario (using the above table): Pulse oximetry of 97% on 6L O2 per nasal cannula.

The data from the previous tables show a pulse oximetry of 97% approximates to a pO2 of 110 mmHg. The data from the above table shows nasal cannula oxygen of 6L/min is estimated at 44% FIO2.

pO2 of 110 ÷ FIO2 of .44 = P/F ratio of 250

The P/F ratio is the pO2 of 80 divided by the FIO2 of 0.40 = 200.

This blog began by stating that the P/F ratio was “one of the most important pieces of clinical evidence…”, and it’s worth emphasizing that acute respiratory failure is more than a P/F ratio. There is a saying among clinicians; “Treat the patient, not the numbers.” If the patient is truly in respiratory failure, clinical evidence consisting of more than a P/F ratio would exist in the record.

Wishing you success in CDI!