A normal gradient points to hypoventilation or low inspired oxygen; a wide gradient points to lung disease
Expansion
A-a gradient = alveolar PO2 (calculated) - arterial PO2 (measured)
Normal is under about 2 kPa (15 mmHg) in a young adult, widening with age.
Normal gradient with hypoxaemia means the lung is transferring oxygen normally, so the problem is upstream:
- Hypoventilation (opioids, neuromuscular disease, airway obstruction)
- Low inspired oxygen (altitude)
Widened gradient means a gas exchange problem within the lung:
- Ventilation-perfusion mismatch, by far the commonest
- Shunt
- Diffusion impairment
The distinction is clinically powerful. A patient with hypoxaemia, hypercapnia and a normal gradient has pure hypoventilation and needs ventilation, not oxygen alone. A patient with hypoxaemia and a wide gradient has intrinsic lung pathology.
Administering high concentrations of oxygen then separates shunt, which responds poorly, from mismatch, which responds well.