The nurse receives an order to administer mannitol to a patient with meningitis. The nurse knows this medication will help lower:

Question

BlackTom AI · Accepted Answer

Mannitol is an osmotic diuretic that reduces intracranial pressure by creating an osmotic gradient that pulls water out of brain tissue into the intravascular compartment.

1. Relevant knowledge points and concepts
- Mannitol is an osmotic agent that increases plasma osmolality.
- The blood–brain barrier allows water movement in response to osmotic gradients.
- Cerebral edema (common in meningitis) increases intracranial volume and pressure (ICP).
- Mannitol does not treat infection, inflammation directly, or alter white blood cell count or ammonia metabolism.

2. Step-by-step reasoning process
- Step 1: IV mannitol raises plasma osmolality immediately after administration.
- Step 2: Higher plasma osmolality creates an osmotic gradient across the blood–brain barrier.
- Step 3: Water moves from the relatively hypo-osmolar brain parenchyma and interstitial spaces into the hyperosmolar vascular compartment.
- Step 4: This transfer reduces brain tissue volume and cerebral edema, thereby lowering intracranial pressure.
- Step 5: The fluid shifted into the vascular space is then filtered and excreted by the kidneys, producing a diuretic effect and preventing reaccumulation.

3. Intermediate physiological steps (no numerical calculation needed)
- Increase plasma osmolality → osmotic gradient across BBB → net movement of water out of brain → decreased brain water content → lowered ICP.

4. Why the correct answer is correct
- The mechanism of mannitol directly targets fluid balance and edema in the brain; thus it specifically reduces intracranial pressure. This is why it is used emergently for elevated ICP from cerebral edema.

5. Why the other choices are incorrect
- White blood cell count: mannitol does not affect leukocyte production or counts; antibiotics and the immune response govern WBC changes.
- Inflammation in the brain: mannitol does not have anti-inflammatory action; it reduces edema but does not treat the inflammatory process itself.
- Ammonia levels: hyperammonemia is treated with lactulose or rifaximin; mannitol does not lower ammonia.

6. Conclusion
- Mannitol lowers intracranial pressure; therefore the correct answer is intracranial pressure, which matches the provided answer.

The nurse receives an order to administer mannitol to a patient with meningitis. The nurse knows this medication will help lower: Single choice

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