osmotic diuretic
Summary
Osmotic diuretics are substances that increase urine output by creating an osmotic gradient in the renal tubules, preventing water reabsorption. Mannitol is the prototype drug, primarily working in the proximal tubule and descending limb of the loop of Henle. They are used to reduce intracranial pressure, treat cerebral edema, and force diuresis in certain toxin ingestions.
Detail
Osmotic diuretics are pharmacologically inert substances that are freely filtered at the glomerulus but poorly reabsorbed by renal tubules. When present in high concentrations in the tubular lumen, they create an osmotic force that opposes water reabsorption, leading to increased urine volume. Mannitol, the most clinically important osmotic diuretic, acts primarily in the proximal tubule and descending limb of the loop of Henle where it prevents the normal reabsorption of water. This mechanism differs from other diuretics that block specific transporters. Clinical applications include reducing elevated intracranial pressure (ICP) in neurosurgical patients, treating cerebral edema, and enhancing elimination of certain toxins. Mannitol can also be used prophylactically to prevent acute kidney injury during certain surgical procedures. Side effects include initial plasma volume expansion (which can worsen heart failure), electrolyte imbalances, and potential precipitation in renal tubules if given in very high doses. The drug must be given intravenously and is contraindicated in patients with severe heart failure or pulmonary edema due to its initial volume-expanding effect.
Sources
- Katzung's Basic & Clinical Pharmacology
- Goodman & Gilman's The Pharmacological Basis of Therapeutics
- First Aid for the USMLE Step 1
- Lippincott Illustrated Reviews: Pharmacology
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