Determinants of cerebral blood flow
Four factors dominate, and three are directly manipulable:
| Factor | Effect | Bedside relevance |
|---|---|---|
| PaCO₂ | The most potent influence. Flow changes by roughly 2–4% per mmHg across the physiological range. | Hyperventilation lowers ICP quickly by vasoconstriction — at the cost of the flow it constricts away. A rescue measure, not a maintenance strategy. |
| PaO₂ | Little effect until roughly 50–60 mmHg, below which flow rises sharply. | Avoid hypoxaemia; supranormal oxygen buys nothing. |
| Cerebral metabolic rate | Flow is coupled to metabolic demand. | Sedation, temperature control and seizure treatment all reduce demand, and therefore flow and blood volume. |
| Perfusion pressure | Buffered by autoregulation while it is intact. | Once autoregulation fails, flow tracks pressure directly in both directions. |