Causes of A-a gradient:
- V/Q mismatch (eg, PNA, CHF, ARDS, atelectasis)
- Shunt (eg, PFO, ASD, PE, pulmonary AVMs)
- Alveolar hypoventilation (eg, interstitial/environmental lung dz, PCP, PNA)
- Hypoventilation (eg, COPD, CNS disorder, neuromuscular disease)
- Low FiO2 (eg, high altitude)
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