A-a gradient

A-a O2 Gradient = [ (FiO2) * (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] - PaO2 from ABG

Aa Gradient = (150 - 5/4(PCO2)) - PaO2
Normal Gradient Estimate = (Age/4) + 4

The 5 Causes of Hypoxemia, #1-3 have an elevated A-a Gradient:
V/Q Mismatch (ex: PNA, CHF, ARDS, atelectasis, etc)
Shunt (ex: PFO, ASD, PE, pulmonary AVMs)
Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA)
Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc)
Low FiO2 (ex: high altitude)