Alveolar-Arterial (A-a) Oxygen Gradient
Calculates the A-a oxygen gradient to assess for causes of hypoxemia. Normal gradient increases with age and FiO2. Elevated gradient suggests V/Q mismatch, shunt, or diffusion limitation.
mmHg
Arterial partial pressure of oxygen from ABG.
decimal fraction (0.21-1.0)
Fraction of inspired oxygen. Room air = 0.21.
mmHg
Arterial partial pressure of CO2.
mmHg
Sea level = 760 mmHg. Adjust for altitude.
years
For age-adjusted normal gradient calculation.
Enter ABG values to calculate A-a gradient
References
- Alveolar-arterial (A-a) oxygen gradient - MDCalc — MDCalc (2025)
- Evaluation of Hypoxemia - UpToDate — UpToDate (2024)
Clinical Notes & Warnings
- Normal gradient increases with age and FiO2.
- At 100% FiO2, normal gradient is higher: (Age/4) + 12 approximately.
- On room air, gradient >10-15 mmHg suggests abnormality in young adults.
- Does not distinguish between V/Q mismatch and true shunt without further testing (100% FiO2).
- A-a gradient can be normal in pure hypoventilation (e.g., opioid overdose, neuromuscular disease).
- At high altitude, use actual atmospheric pressure.
The A-a gradient helps differentiate hypoxemia mechanisms: normal gradient = hypoventilation or low FiO2; elevated gradient = V/Q mismatch, shunt, or diffusion limitation. On 100% oxygen, if gradient persists, indicates true shunt (>10% of cardiac output).