Low inspired oxygen, hypoventilation, diffusion impairment, V/Q mismatch, shunt
Mnemonic
Five mechanisms, distinguished by the A-a gradient and the response to oxygen:
| Cause | A-a gradient | Corrects with oxygen |
|---|---|---|
| Hypoventilation | Normal | Yes |
| Low inspired oxygen | Normal | Yes |
| Diffusion impairment | Raised | Yes |
| V/Q mismatch | Raised | Yes |
| Shunt | Raised | No |
“Only shunt fails to correct with oxygen”, because blood bypasses ventilated alveoli entirely, so no amount of inspired oxygen reaches it. This is the single most useful discriminator at the bedside.
A normal A-a gradient with hypoxia means hypoventilation or altitude, which is why the gradient is calculated before anything else: it separates a lung problem from a ventilation problem.
V/Q mismatch is much the commonest mechanism in practice, and shunt is best thought of as its extreme, where V/Q equals zero.
Expansion
Five mechanisms:
- Low inspired oxygen - altitude, or a hypoxic gas mixture. Normal A-a gradient
- Hypoventilation - opioids, neuromuscular weakness, obesity. Normal A-a gradient, with a raised carbon dioxide
- Diffusion impairment - fibrosis, emphysema. Widened gradient; rarely a cause at rest
- Ventilation-perfusion mismatch - the commonest mechanism: asthma, chronic obstructive pulmonary disease, pneumonia, pulmonary embolism. Widened gradient
- Shunt - blood bypassing ventilated alveoli: consolidation, atelectasis, pulmonary oedema, cardiac right to left shunt. Widened gradient
Two discriminating tests:
- A-a gradient: normal in the first two, widened in the last three
- Response to 100 per cent oxygen: corrects everything except a true shunt, because shunted blood never contacts alveolar gas at all
A right to left shunt of about 30 per cent cannot be corrected by any inspired oxygen concentration.