Monroe-Kellie doctrine governs cerebral perfusion pressure (CPP) management in head injury. In a patient with CPP = MAP − ICP, with MAP 80 mmHg and ICP 35 mmHg, the CPP is 45 mmHg — below the target threshold. Which intervention most directly raises CPP while simultaneously reducing ICP?
- A Hyperventilate to PaCO2 25 mmHg to cause cerebral vasoconstriction
- B Bolus normal saline to raise MAP, accepting secondary ICP rise
- C Administer dexamethasone 10 mg IV to reduce cerebral oedema
- D Administer 20% mannitol 1 g/kg IV bolus — osmotic diuresis reduces cerebral oedema and ICP while maintaining MAP ✓
Explanation
Mannitol (20%, 0.25-1 g/kg IV) reduces ICP through two mechanisms: immediate plasma expansion (rheological effect, improves CBF within minutes) and delayed osmotic dehydration of brain parenchyma (peak effect at 20-60 minutes) by creating an osmotic gradient across the blood-brain barrier. This reduces ICP and consequently raises CPP. The target CPP is 60-70 mmHg per Brain Trauma Foundation guidelines. Hyperventilation to <35 mmHg PaCO2 causes cerebral vasoconstriction that may worsen ischaemia and is only for acute herniation control. Dexamethasone is not indicated in TBI (no benefit, potential harm per CRASH trial).
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.