Alameda County Medical Center / Highland General Hospital
Trauma Service

Formulas and calculations

 

Alveolar Gas Equation  
   
PAO2 = PIO2  -  (PaCO2 / R)

            

= FIO2 (Patm  -  pH2O)  -  (PaCO2 / R)
   
where:
  • PAo2 = alveolar partial pressure of oxygen 
  • PIO2  = inspired partial pressure of oxygen
  • PaCO2 = arterial partial pressure of carbon dioxide
  • FIO2 = fraction of inspired oxygen
  • Patm = atmospheric pressure (at sea level = 760 mm Hg)
  • PH20 =  partial pressure of water in alveolus, 100% saturated (at seal level = 47 mm Hg)
  • R = respiratory quotient (normally 0.8)