A head-injured patient on a ventilator has PaCO2 reduced from 40 to 30 mmHg by mild hyperventilation. The expected change in cerebral blood flow is approximately:

• A No change, since PaCO2 affects only ventilation
• B An increase of about 10 percent
• C A decrease of about 70 percent
• D A decrease of about 30 percent ✓

Explanation

Cerebral blood flow changes by roughly 2 to 4 percent per mmHg change in PaCO2 within the physiological range of about 20 to 80 mmHg. A fall of 10 mmHg therefore reduces flow by roughly 30 percent, which is why brief moderate hyperventilation lowers intracranial pressure in acute neurological deterioration. The response depends on pH changes in extracellular fluid around arterioles and is lost in brain death, making option C far too large.

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

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Written and medically reviewed by the StethoPrep medical team.