Alveolar-arterial (aa) Gradient (Maa) calculator Formula:
   aag = FiO2 * (Atm - 47) - PaCO2 * 1.25 - PaO2
Where:
   FiO2: fraction inspired O2, room air is 21%
   Atm: Atmospheric Pressure, 760mmHg at sea level
   PaO2: Arterial O2
   PaCO2: Arterial CO2
   aag: A-a O2 Gradient