ACPHOSPITALIST November 2013 7

REVISITING RESPIRATORY FAILUREPart two of a two-part series

By Richard Pinson, MD, FACP

Last month’s column addressed some challenges in the diag-nosis of respiratory failure, including correct documentation

terminology to meet the diagnostic criteria for acute and chron-ic respiratory failure. We left off last month with the P/F ratio,one of the four defining criteria for acute respiratory failure:

■ pO2 <60 mm Hg or SpO2 (pulse oximetry) <91% breath-ing room air

■ pCO2 >50 mm Hg and pH <7.35 (hypercapnic)■ P/F ratio (pO2 / FIO2) <300■ pO2 decrease or pCO2 increase by 10 mm Hg from base-

line (if known).

P/F RATIO

The P/F ratio equals the arterial pO2 divided by the FIO2 (thefraction of inspired oxygen expressed as a decimal) the patientis receiving. It is most familiar, and commonly used, in the con-text of acute lung injury (ALI) and acute respiratory distress syn-drome (ARDS) where a P/F ratio <300 is diagnostic of acute lunginjury or mild ARDS, <200 is consistent with moderate ARDSand <100 indicates severe ARDS.

However, the ratio simply expresses a physiologic relation-ship between inspired oxygen and the arterial partial pressure.The P/F ratio is therefore applicable to any circumstances where

this relationship is impaired. In the case of pneumonia, forexample, the Infectious Diseases Society of America and theAmerican Thoracic Society recognize a P/F ratio <250 as one ofthe 10 criteria for “severe” community-acquired pneumoniathat may require admission to intensive care. Similarly, theInternational Sepsis Definition criteria (2001) and the SurvivingSepsis Severe Sepsis Guidelines (2008 and 2012) use a P/F ratio<300 as an indicator of acute organ (respiratory) failure.

The power of the P/F ratio is its ability to predict, based onarterial pO2 measured while the patient is receiving supplemen-tal oxygen, what the pO2 would be on room air:

■ P/F ratio <300 is equivalent to a pO2 <60 mm Hg onroom air (acute respiratory failure)

■ P/F ratio <250 is equivalent to a pO2 <50 mm Hg onroom air (severe respiratory failure)

■ P/F ratio <200 is equivalent to a pO2 <40 mm Hg onroom air (extreme respiratory failure)

The arterial pO2 measured by arterial blood gas (ABG) is thedefinitive method for calculating the P/F ratio. However, whenthe pO2 is unknown because an ABG is not available, the SpO2can be used to approximate the pO2, as shown in Table 1.Estimating the pO2 from the SpO2 becomes unreliable when theSpO2 is greater than 97%.

The P/F ratio is much more than a diagnostic and prognos-tic tool. It’s a valuable clinical measure of the patient’s respira-tory status while receiving supplemental oxygen, allowingphysicians to prospectively monitor the degree of hypoxemia,quickly detect early progression of respiratory failure, and inten-sify treatment to avoid impending intubation. For example,imagine a patient with:

■ initial SpO2 of 86% on room air, equivalent to pO2 of 50mm Hg. P/F ratio = 250 (severe respiratory failure),

CODING CORNER

Table 1. SpO2 to pO2 conversion

SpO2 pO2(%) (mm Hg)

85 50

86 51

87 52

88 54

89 56

90 58

91 60

92 64

93 68

94 73

95 80

96 90

97 110

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