AaDO2

AaDO2 - Alveolar-arterial oxygen tension difference

• Measures the difference in the partial pressure of oxygen between the alveoli and the arteries. This measures the efficiency of the oxygen exchange between the lung alveoli and the pulmonary capillaries.

• Is a calculated component of APACHE II score when FIO2 being administered to a patient is >=50%.

• To calculate the AaDO2:

• • AaDO2 =(( FIO2 X 713) - PO2) - PaCO2
  • Example with FIO2 50%, PO2 85 and CO2 40:
  • 231.5 = ((0.5 X 713)- 85) - 40))

• • Select the ABG (arterial blood gas) with the WORST AaDO2 or the WORST PO2, which ever gives you the most APACHE II points in the first 24 hrs of ICU admission.

NOTE: the value 713 mmHg is the barometric pressure(Pb)in this formula.

• See APACHE Scoring table
• See APACHE FIO2 Table