Respiratory
PaO₂/FiO₂ (P/F) Ratio
Assesses ARDS severity by comparing arterial oxygen to inspired oxygen.
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Parameters
PaO₂/FiO₂ (P/F) Ratio Result
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Formula
P/F Ratio = PaO₂ / (FiO₂ % / 100)Clinical Pearls
The P/F ratio is quick to calculate at the bedside and dictates step-up therapy in ARDS, including when to initiate prone positioning (usually considered when P/F < 150).
Limitations
The ratio assumes a linear relationship between FiO2 and PaO2, which is not strictly true at the extremes of the oxygen-hemoglobin dissociation curve. Requires an arterial blood gas (ABG) for PaO2.