Alveolar–Arterial Oxygen Gradient
Input patient age, arterial blood gas values, inspired oxygen, and altitude to compute the A–a gradient and compare with expected normal values.
Room air ≈ 21%. Adjust if patient on supplemental oxygen.
Adjusts atmospheric pressure (approximation).
Results
Alveolar PaO₂
151.6 mmHg
A–a gradient
76.6 mmHg
Severely elevated gradient—consider shunt, diffusion impairment, or severe V/Q mismatch.
Expected (age-based)
15.3 mmHg
Equivalent to 15.3 mmHg
Atmospheric pressure (adjusted): 760 mmHg.
Interpretation:
- Normal A–a gradient suggests hypoxemia primarily due to hypoventilation or low FiO₂.
- Elevated gradient indicates V/Q mismatch, diffusion impairment, or shunt.
How to Use This Calculator
Collect ABG and ventilator data
Obtain arterial blood gas values (PaO₂, PaCO₂), FiO₂, and patient age. Adjust FiO₂ for supplemental oxygen.
Account for altitude
Higher altitudes lower atmospheric pressure, increasing the expected gradient. Enter meters above sea level.
Interpret relative to age
Compare calculated A–a gradient to age-based normal values to gauge severity of gas exchange impairment.
Formula
Alveolar gas equation: PAO₂ = FiO₂ × (Patm − PH₂O) − (PaCO₂ × FiO₂ × (1 − R⁻¹))
Equation uses respiratory quotient (R) ≈ 0.8 and water vapor pressure 47 mmHg at body temperature.
Frequently Asked Questions
When is A–a gradient most useful?
Differentiating hypoxemia causes (e.g., V/Q mismatch vs hypoventilation) in acute respiratory distress, COPD, or pulmonary embolism evaluation.
What increases A–a gradient?
Diseases causing shunt, diffusion limitations, or V/Q mismatch—such as pneumonia, ARDS, pulmonary edema.
Why adjust for altitude?
Lower atmospheric pressure reduces alveolar oxygen, altering expected gradient. This calculator approximates the change.
Can I use kPa instead of mmHg?
Yes—toggle units to kPa; the calculator converts values automatically.
What if FiO₂ is unknown?
Estimate based on oxygen device (nasal cannula, mask) or assume room air (21%) if no supplemental oxygen applied.