Mannitol is being considered to reduce intracranial pressure in a head injury patient. Review of his chart shows urine output of less than 100 mL over the past 12 hours with rising creatinine. Why should mannitol be withheld?
- A It will precipitate in the renal tubules and cause crystal nephropathy
- B It crosses the blood brain barrier freely and would cause rebound cerebral edema immediately
- C Without adequate urine flow it cannot be excreted and accumulates, expanding extracellular volume and risking pulmonary edema and heart failure ✓
- D It causes profound hypokalemia that precipitates arrhythmia in oliguric patients
Correct answer: C. Without adequate urine flow it cannot be excreted and accumulates, expanding extracellular volume and risking pulmonary edema and heart failure
Explanation
Mannitol is filtered at the glomerulus and acts as an osmotic diuretic only if it can be excreted. In established anuria or severe oliguria it is retained in the circulation, raising plasma osmolality and drawing water from cells into the extracellular compartment, which expands intravascular volume and can precipitate pulmonary edema and cardiac decompensation. Rebound elevation of intracranial pressure occurs later when the blood brain barrier is disrupted, not because mannitol freely crosses an intact barrier.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.