Hyperventilation to PaCO2 of 30 mmHg is occasionally used as a temporising measure in acute intracranial hypertension. Its mechanism of ICP reduction and its limitation are:

• A Increases cerebral metabolic rate → vasoconstriction; limited by O2 toxicity
• B CSF drainage via choroid plexus inhibition; limited by rebound CSF production
• C Reduces cerebral oedema via osmotic effect; limited by electrolyte imbalance
• D Cerebral vasoconstriction via hypocapnia → reduced CBF and CBV; limited by cerebral ischaemia if prolonged ✓

Explanation

Hypocapnia from hyperventilation causes cerebral arterial vasoconstriction (CO2 is a potent cerebrovascular dilator); this reduces cerebral blood flow and cerebral blood volume, thereby rapidly lowering ICP within minutes. However, prolonged hyperventilation (PaCO2 <30 mmHg for >2–4 hours) causes significant cerebral ischaemia because vasoconstriction outweighs metabolic demands, especially in already injured brain tissue. Additionally, compensatory CSF bicarbonate buffering occurs over hours, normalising pH and blunting the vasoconstrictor effect. It is therefore used only as a temporising bridge while more definitive ICP reduction is achieved.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.