A ventilated patient with severe traumatic brain injury and rising ICP is hyperventilated to a PaCO2 of 28 mmHg. The immediate fall in ICP occurs because:

• A Hypocapnia constricts cerebral arterioles, reducing cerebral blood volume ✓
• B Hypocapnia directly reduces CSF production by the choroid plexus
• C Respiratory alkalosis increases the osmotic gradient for brain water loss
• D Low PaCO2 improves cerebral perfusion pressure by raising arterial pressure

Explanation

Cerebral arteriolar tone is exquisitely sensitive to PaCO2, with CBF changing by roughly 2 to 4 percent per mmHg change around the normal range. Hypocapnia causes vasoconstriction, shrinking intracranial blood volume and lowering ICP within minutes. However, excessive hyperventilation risks cerebral ischemia, so it is reserved for acute herniation. CSF production and osmotic gradients are unaffected, and PaCO2 does not raise arterial pressure.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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