A-a gradient :Formula

It is a useful tool in evaluating how well a patient is oxygenating.
A-a gradient = PAO2 – PaO2 (mmHg)
={(760-47)FiO2 - PaCO2/0.8}-PaO2

* Normal = 10-20 mmHg
Or Normal =age/4 + 4


Elevated A-a gradient is caused by:
V/Q mismatch (eg. Pneumonia, CHF, ARDS)
R-L shunt (eg. PE)
Diffusion abnormalities (eg. Interstitial lung dz)


The 5 Causes of Hypoxemia, #1-3 have an elevated A-a Gradient:
V/Q Mismatch (ex: PNA, CHF, ARDS, atelectasis, etc)
Shunt (ex: PFO, ASD, PE, pulmonary AVMs)
Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA)
Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc)
Low FiO2 (ex: high altitude)