AaDO2

AaDO2 - Alveolar-arterial oxygen tension difference

• See APACHE Scoring table
• See APACHE FIO2 Table

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.

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

• To calculate the AaDO2:

• • AaDO2 =(( FIO2 X 713) - PaO2) - PaCO2

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

• if FIO2 being administered to a patient is <=49%, select the ABG with the WORST PO2.
• if FIO2 being administered to a patient is >=50%, select the ABG result with the WORST AaDO2.
• For APACHE II, select the ABG (arterial blood gas) with the WORST AaDO2 or the WORST PO2, which ever one gives you the highest APACHE II points in the first 24 hrs of ICU admission.

NOTE: I put this detail here because I get asked this time an again.

• the value 713 mmHg is the barometric pressure (Pb) used in the AaDO2 formula.
• Normal Sea level Pb is 760 mmHg. (Winnipeg is close to sea level. We don't adjust for different elevations around the city nor do we adjust this value for the constant changes in Pb).
• The barometric pressure is corrected for water vapor pressure (Pbc).
• (vapor pressure is 47 mmHg @ a temperature of 37C).
• (760-47)= Pbc of 713 mmHg.

• TOstryzniuk 20:25, 8 January 2009 (CST)