A-a Oxygen Gradient
PaO2 (partial pressure of arterial O2): mmHg
PaCO2 (partial pressure of arterial CO2): mmHg
Atmospheric Pressure:
Inspired Oxygen Concentration (FiO2): Room Air (21 percent)
A-a Gradient = mmHg

A-a (O2) = (FiO2%/100) * (Patm - 47 mmHg) - (PaCO2/0.8) - PaO2
(all units mmHg)

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