RN Tools
Atmospheric pressure*
mm Hg
Use 760 mm Hg (101.33 kPa) at sea level
PaO₂*
mm Hg
FiO₂*
%
Use 21% for room air
PaCO₂*
mm Hg
Age (for expected A-a gradient)
years
Fill out the fields above to see your result.
Assesses for degree of shunting and V/Q mismatch.
A-a O2 Gradient = [ (FiO2) × (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] – PaO2 from ABG
H2O pressure = 47 mmHg
Normal Gradient Estimate = (Age/4) + 4
The 5 Causes of Hypoxemia
| Causes of Hypoxemia | A-a O2 Gradient Shift | |
|---|---|---|
| 1 | V/Q Mismatch (ex: PNA, CHF, PE, ARDS, atelectasis, etc) | Elevation |
| 2 | Shunt (ex: PFO, ASD, pulmonary AVMs) | Elevation |
| 3 | Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA) | Elevation |
| 4 | Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc) | Depression |
| 5 | High altitude | Depression |
The A-a Gradient can help determine the cause of hypoxia; it pinpoints the location of the hypoxia as intra- or extra-pulmonary.
After calculating the A-a Gradient and determining if it is elevated or normal, one can rule out several causes of hypoxia.