Prolonged hyperventilation lowers intracranial pressure effectively for only a limited period. The physiological reason is:

• A Active transport restores cerebrospinal fluid and brain extracellular bicarbonate over several hours ✓
• B Arachnoid villi increase their absorption rate within hours
• C Choroid plexus secretion increases to compensate for reduced flow
• D Autoregulatory vasoconstriction becomes refractory to carbon dioxide

Explanation

Hyperventilation raises extracellular pH and constricts cerebral arterioles. Within about 6 hours, active transport across the blood-brain barrier returns CSF and brain interstitial bicarbonate toward baseline, so pH normalizes despite continued hypocapnia and the vasoconstrictive effect wanes. This is why hyperventilation is recommended only as a short-term measure for impending herniation. Arachnoid villus function and choroid plexus secretion rates are not meaningfully altered, and the adaptation is chemical rather than vascular refractoriness.

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

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