Osmotic diuretics cause diuresis through osmosis,
moving fluid into the extracellular spaces. They include mannitol and urea.
Administered I.V. for rapid distribution, osmotic
diuretics are freely filtered by the glomeruli of the kidney—except for
manni-tol, which is only slightly metabolized. Osmotic diuretics are ex-creted
primarily in urine.
Osmotic diuretics receive their name because they
increase the osmotic pressure of the glomerular filtrate, which inhibits the
re-absorption of sodium and water. They create an osmotic gradient in the
glomerular filtrate and the blood. In the glomerular filtrate, the gradient
prevents sodium and water reabsorption. In the blood, the gradient allows fluid
to be drawn from the intracellular into the intravascular spaces.
Osmotic diuretics are used to treat acute renal
failure and cere-bral edema and to reduce intracranial and intraocular
pressure. Mannitol is used to promote diuresis in acute renal failure and to
promote urinary excretion of toxic substances.
Taking osmotic diuretics with lithium may
increase renal excre-tion of lithium, which in turn decreases the effectiveness
of lithi-um. Patients taking both drugs require lithium level monitoring. (See Adverse reactions to osmotic diuretics.)