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  P(A-a)O2 = PAO2 - PaO2;  PaO2, PaCO2: Calculated from ABG:
  PAO2 = (712 x FIO2) - (PaCO2/0.8);  or  (712-pCO2)-pO2
  Normal P(A-a)O2 = 12 mm Hg in the young Adult to 20 mm Hg at age 70 yo;
  In pure Ventilatory Failure the P(A-a)O2 will remain 12 to 20 mm Hg; in oxygenation failure it will increase;

Interpretation of alveolar-arterial oxygen tension difference.