Osmotic diuretics cause water to
be retained within the proximal tubule
and descending limb of loop of Henle
(freely permeable to water)
Mannitol is an
example of osmotic diuretic.
Clinical Use: mainly used
to reduce increased intracranial
pressure;
Osmotic diuretics: properties
Mannitol:
not metabolized, freely filtered at the
glomerular
Usually
administered by IV; oral administration
results in an osmotic diarrhea-- perhaps
useful to promote elimination of toxic
substances from the GI tract (in
conjunction with activated charcoal)
Urine volume increases
with mannitol excretion due to direct
osmotic effects
Sodium reabsorption is reduced because
of increased urine flow rates
{decreased contact time between
urine and tubular epithelial
cells}
Clinical Uses:
To decrease intracranial or
intraocular pressure:
Mannitol
extract water from intracellular
compartments, reducing total body
water
Following
IV administration, intracranial
pressure falls within 60-90
minutes.
To increase urine volume:
May be used to prevent anuria if the kidney due to
hemolysis or rhabdomyolysis is
presented with a large pigmented
load.
When renal hemodynamics
are compromised
Toxicity:
Volume expansion effects --
increased extra cellular fluid volume and
hyponatremia may cause pulmonary edema,
complicating congestive heart failure
Headache, nausea, vomiting --
commonly observed
Dehydration and hypernatremia:
Fluid loss leads to
significant dehydration and in
the absence of adequate fluid
replacement leads to hypernatremia.
Ives, H.E., Diuretic Agents, in:
Basic and Clinical Pharmacology, (Katzung, B. G., ed)
Appleton-Lange, 1998, pp 242-259.
