Alveolar Gas Equation & A-a Gradient
Input Parameters
Formulas
Alveolar Gas Equation:
PAO2 = FiO2 × (Patm − PH2O) − (PaCO2 / RQ)
A-a Gradient:
A-a = PAO2 − PaO2
Expected A-a Gradient (adults):
Expected = (Age / 4) + 4
PH2O = 47 mmHg at body temperature. Normal A-a gradient in pediatrics is typically < 10 mmHg on room air.
A-a Gradient Interpretation
| < 10 mmHg | Normal |
| 10 – 20 mmHg | Mildly elevated |
| 20 – 40 mmHg | Moderately elevated |
| ≥ 40 mmHg | Severely elevated |