Alveolar-arteriolar (A-a) gradient =
(FiO2 x (Patm-47 mmHg) - (PaCO2 /0.8) - PaO2

Causes of A-a gradient:
  • V/Q mismatch (eg, PNA, CHF, ARDS, atelectasis)
  • Shunt (eg, PFO, ASD, PE, pulmonary AVMs)
  • Alveolar hypoventilation (eg, interstitial/environmental lung dz, PCP, PNA)
  • Hypoventilation (eg, COPD, CNS disorder, neuromuscular disease)
  • Low FiO2 (eg, high altitude)
FiO2 = fractional inspired O2 (0.21 at sea level);
Patm = atmospheric pressure (760 mm Hg);
PaCO2 = CO2 arterial partial pressure (40 mm Hg);
PaO2 = O2 arterial partial pressure (90 mm Hg);