Units: US SI

A-a O2 Gradient

Atmospheric Pressure (760 mm Hg or 101.33 kPa @ sea level) mm Hg
PaO2 (arterial O2) mm Hg
PaCO2 (arterial CO2) mm Hg
FIO2 (fraction inspired O2 - room air is 21%) %
Age (for calculating normal A-a Gradient) years
A-a O2 Gradient (Alveolar - Arterial) mm Hg
Expected A-a Gradient Based on Age mm Hg
A-a O2 Gradient = [ (FiO2) * (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] - PaO2 from ABG

Normal Gradient Estimate = (Age/4) + 4
The 5 Causes of Hypoxemia, #1-3 have an elevated A-a Gradient:
  1. V/Q Mismatch (ex: PNA, CHF, ARDS, atelectasis, etc)
  2. Shunt (ex: PFO, ASD, PE, pulmonary AVMs)
  3. Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA)
  4. Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc)
  5. Low FiO2 (ex: high altitude)

Equations

Scores