A-a Gradient Field Guide: When Three Numbers Decide Hypoventilation vs V/Q Mismatch
When ABG returns PaO2 in the 50s, the A-a gradient splits hypoventilation from V/Q mismatch, shunt, diffusion block, and low mixed-venous O2 in one equation. PAO2 = FiO2 * (Patm - 47) - PaCO2 / 0.8; A-a = PAO2 - PaO2; normal A-a approximates (Age + 10) / 4 on room air.