A-a O₂ Gradient
A-a O₂ Gradient
years
%
mmHg
mmHg
A-a Gradient:
–
–
–
Assesses for degree of shunting and V/Q mismatch.
Why Use
The A-a Gradient can help determine the cause of hypoxia; it pinpoints the location of the hypoxia as intra- or extra-pulmonary.
When to Use
Patients with unexplained hypoxia. Patients with hypoxia exceeding the degree of their clinical illness.
Formula
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
Advice
After calculating the A-a Gradient and determining if it is elevated or normal, one can rule out several causes of hypoxia.
More Information
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