A-a Gradient
Alveolar-arterial oxygen gradient for hypoxemia evaluation
Calculator
Why use it
Distinguishes pulmonary from non-pulmonary causes of hypoxemia. Helps narrow the differential diagnosis of hypoxemia. Important in PE evaluation — PE typically elevates the A-a gradient.
About
The alveolar-arterial (A-a) oxygen gradient measures the difference between alveolar (PAO₂) and arterial (PaO₂) oxygen tension. An elevated A-a gradient indicates a pulmonary cause of hypoxemia (V/Q mismatch, shunt, diffusion impairment), while a normal gradient with hypoxemia suggests hypoventilation or low FiO₂.
When to use
- Evaluation of hypoxemia on ABG
- Differentiating causes of respiratory failure
- PE workup — A-a gradient is typically elevated
- Assessment of gas exchange efficiency
Formula
Pearls & pitfalls
- Normal A-a gradient increases with age: expected normal ≈ (Age/4) + 4.
- On room air (FiO₂ 21%), A-a gradient < 10-15 mmHg is normal for young adults.
- The A-a gradient is most useful on room air — becomes less reliable at high FiO₂.
- An elevated A-a gradient with normal CXR should raise suspicion for PE.
- In pure hypoventilation (e.g., opioid overdose), the A-a gradient remains normal.
- At altitude, the reduced atmospheric pressure changes the alveolar gas equation values.
Next steps
Normal A-a gradient with hypoxemia: Consider hypoventilation (CNS depression, neuromuscular disease) or low FiO₂ (altitude). Elevated A-a gradient: V/Q mismatch (PE, pneumonia, asthma), shunt (ARDS, AVM), or diffusion impairment (ILD).
Evidence
Based on the alveolar gas equation developed in the 1940s-1960s. Fundamental concept in pulmonary physiology used universally in clinical medicine. Age-adjusted normal values validated in multiple studies.
References
- Kanber GJ et al. The alveolar-arterial oxygen gradient in young and elderly men during air and oxygen breathing. Am Rev Respir Dis. 1968;97(3):376-381. — link