Mannitol (Osmitrol) is a six-carbon sugar that does not undergo appreciable metabolic degradation. It is not absorbed from the gastrointestinal tract and there fore must be given intravenously. Humans do not reabsorb it in the proximal tubules.
Mannitol is particularly useful in clinical conditions characterized by hypotension and decreased glomerular filtration. These symptoms are usually the result of some physical trauma or surgical procedure. Mannitol is useful in maintaining kidney function in these conditions, since even at reduced rates of filtration, a sufficient amount of the sugar may enter the tubular fluid to exert an osmotic effect and thus continue urine formation. However, if circulatory failure is profound and glomerular filtration is severely compromised or absent, not enough mannitol may reach the tubules to be effective. The ability to maintain urine flow when renal shutdown might otherwise be expected aids in preventing kidney tubular damage. In addition, mannitol has been used to reduce cerebral edema during neurosurgery, to reduce intraocular pressure before surgery for glaucoma, and to promote the elimination of ingested toxic substances.
The major characteristics of the renal response to mannitol diuresis include a fall in urine osmolality and a decrease in the osmolality of the interstitial fluid of the renal medulla. The quantity of urine formation and Na+ excretion is generally proportional to the amount of mannitol excreted. Although there is a significant inhibition of proximal water reabsorption, the effects of man-nitol on proximal Na+ reabsorption are not marked.
The major adverse reactions associated with manni-tol administration are headache, nausea, vomiting, chest pain, and hyponatremia. Too rapid an administration of large amounts may cause an excessive shift of fluid from the intracellular to the extracellular compartment and result in congestive heart failure.
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