The A-a gradient is the difference between alveolar (PAO₂) and arterial (PaO₂) oxygen tension. It helps determine the cause of hypoxemia.
Formula
PAO₂ = (FiO₂ × [Patm − PH₂O]) − (PaCO₂ / 0.8)
A-a Gradient = PAO₂ − PaO₂
At sea level: Patm = 760 mmHg, PH₂O = 47 mmHg
Expected A-a Gradient
Expected A-a = (Age / 4) + 4
Interpretation
- Normal A-a gradient + hypoxemia → Hypoventilation (e.g., opioids, neuromuscular weakness) or low FiO₂
- Elevated A-a gradient + hypoxemia → V/Q mismatch (PE, pneumonia, COPD), shunt (ARDS, pulmonary edema), or diffusion impairment
Clinical Use
- Evaluating pulmonary embolism
- Distinguishing hypoventilation from intrinsic lung disease
- ICU oxygen monitoring