In a head-injured patient with signs of acute transtentorial herniation, the neurosurgeon orders controlled hyperventilation. Lowering PaCO2 from 40 to 25 mmHg lowers intracranial pressure because:

• A Cerebrospinal fluid production falls by approximately half
• B Alkalosis constricts cerebral arterioles, reducing cerebral blood volume ✓
• C Hypocapnia increases cerebral oxygen extraction and shrinks neurons osmotically
• D Respiratory alkalosis accelerates reabsorption of CSF at the arachnoid villi

Explanation

Extracellular pH, determined largely by PaCO2, is the strongest chemical controller of cerebral vascular tone. Reducing PaCO2 to 25 mmHg raises interstitial pH, producing arteriolar constriction and cutting cerebral blood flow roughly 30 to 40 percent, which reduces intracranial blood volume and thus ICP. The effect is temporary as CSF bicarbonate readjusts over hours. Hypocapnia does not meaningfully alter choroid plexus secretion rate or arachnoid villi reabsorption.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.