Calculates the Alveolar-Arterial Gradient to assess degree of shunting and V/Q mismatch.

A-a O2 Gradient = [ (FiO2) * (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] - PaO2 from ABG

A-a O2 Gradient [mmHg] = ( (c/100 * (a - 47) - d/0.8)) - b
Expected A-a Gradient [mmHg] = e/4 + 4