The arterial partial pressure of oxygen (PaO2) provides some information on pulmonary gas exchange, especially when viewed relative to the oxygen concentration in the inspired air.
PaO2-to-FIO2 ratio =
= (PaO2) / (FIO2)
where
• FIO2 is the fractional concentration of inspired oxygen, ranging from 0.21 for room air to 1.0 for 100% oxygen
Interpretation:
• in patients with ARDS, the ratio does not vary much over changes in FIO2, as the PaO2 increases or decreases similar to the change in FIO2
• in patients with significant shunting, the ratio is lower at high FIO2 since the PaO2 does not rise as fast as the increase in FIO2
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