Which drug reduces ICP by expanding plasma osmotic pressure to draw fluid from brain tissue into the vasculature?

• A. Oxygen administration
• B. Pentobarbital
• C. Mannitol (Osmitrol) 25%
• D. Dexamethasone

Answer: (C)

The key idea is osmotic therapy to lower intracranial pressure by creating a higher osmolality in the blood so water moves out of the brain into the vascular system. Mannitol serves this purpose. When given IV as an osmotic diuretic, it raises plasma osmolality, drawing water from brain tissue into the blood vessels. That movement decreases brain edema and reduces ICP. Mannitol typically works quickly, but it requires careful monitoring because it also increases urine output and can shift electrolytes; if the blood-brain barrier is damaged, mannitol can enter brain tissue and potentially worsen edema or cause rebound ICP, so ongoing assessment is important. Other options don’t rely on this osmotic gradient mechanism: oxygen helps with hypoxia but not by drawing intracellular water into the vasculature; pentobarbital lowers ICP by reducing cerebral metabolic demand and blood flow; dexamethasone reduces some edema in specific conditions but not through osmotic gradient.