A-a Gradient
Calculate alveolar-arterial oxygen gradient to assess gas exchange.
ABG Values
mmHg
Partial pressure of oxygen in arterial blood. Normal: 80-100 mmHg on room air.
mmHg
Partial pressure of CO2 in arterial blood. Normal: 35-45 mmHg.
%
Fraction of inspired oxygen. Room air = 21%. Enter as percentage.
years
Patient age. Expected A-a gradient increases with age.
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A-a Gradient
0.0 mmHg
Normal
Alveolar O2 (PAO2)
0.0 mmHg
Arterial O2 (PaO2)
80 mmHg
Expected A-a (age-adjusted)
12.5 mmHg
Status
Normal
Alveolar Gas Equation
PAO2 = FiO2 × (Patm - PH2O) - (PaCO2 / RQ)
= 0.21 × (760 - 47) - (40 / 0.8) = 0.0 mmHg
A-a Gradient = PAO2 - PaO2
= 0.0 - 80 = 0.0 mmHg
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