Calculate A-a gradient = PAO2 - PaO2. PAO2 = (FiO2 x (Patm - 47)) - (PaCO2 / 0.8). Normal A-a gradient on room air: age/4 + 4 mmHg (up to approximately 25 mmHg). Elevated gradient indicates V/Q mismatch, shunt, or diffusion impairment.
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