How is the alveolar-arterial PO2 gradient measured in the pathophysiology of respiratory failure?

Updated: Apr 07, 2020
  • Author: Ata Murat Kaynar, MD; Chief Editor: Michael R Pinsky, MD, CM, Dr(HC), FCCP, FAPS, MCCM  more...
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The efficiency of lungs at carrying out of respiration can be further evaluated by measuring the alveolar-arterial PO2 gradient. This difference is calculated by the following equation:

PAO2 = FiO2 × (PB – PH2 O) – PACO2/R

where PA O2 is alveolar PO2, FiO2 is fractional concentration of oxygen in inspired gas, PB is barometric pressure, PH2O is water vapor pressure at 37°C, PACO2 is alveolar PCO2 (assumed to be equal to PaCO2), and R is respiratory exchange ratio. R depends on oxygen consumption and carbon dioxide production. At rest, the ratio of VCO2 to oxygen ventilation (VO2) is approximately 0.8.

Even normal lungs have some degree of V/Q mismatching and a small quantity of right-to-left shunt, with PAO2 slightly higher than PaO2. However, an increase in the alveolar-arterial PO2 gradient above 15-20 mm Hg indicates pulmonary disease as the cause of hypoxemia.

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