Osmotic diuretic

Osmotic diuretic are drugs that increase the excretion of water (diuresis). These compounds are excreted into primary urine, but they are not reabsorbed into the nephron. Importantly, these compound are osmotically active, therefore, they "attract" water (from proximal tubule, which is permisible for water). As a result, these compounds increase the diuresis.

Examples
The main example of osmotic diuretic is mannitol, usually in 10 to 20% IV solutions.

Mannitol is not metabolized and is excreted via glomelural filtration (within 30 to 60 min past administration) without any major active tubular secretion or resoption. However, mannitol is only poorly absorbed from GIT, therefore, its per os administration is usually avoided (as it would cause major diarrhoea) - mannitol is usually administered intravenously. The increase rate of urine formation leads to decreased Na+ resorption (leading to natriuremia), but this natriuremia is usually only minor.

Indications
Osmotic diuretic is used when increased diuresis is required, but loss of Na+ needs to be minimalized. For examples, it can be indicated in patients with insufficient renal hemodynamics or in patients with oligouria (to increase the volume or produced urine). Osmotic diuretics can also be prescribed to decrease the intracranial or intraoccular pressure (administration of mannitol decreases the rate of CSF and aqueous humour production). Finally, mannitol can be used along with loop diuretic to induce forced diuresis in intoxication (to decrease the concentration of toxin in blood).

Contraindications
Osmotic diuretic should be avoided in patients with anuria (because se diuretics will not increase urine production, but they will only effect other organs) or in patients with heart failure.

Side effects

 * Mannitol is quickly distributed into extracellular fluid, which results indilution hyponatremia and expansion of extracellular volume and intravascular volume of fluirs.
 * Common side effects include headache, nausea and vomiting.
 * Concurrent heart failure may result in accute lung oedema.
 * High doses of mannitol may result in dehydration and hyponatremia.

Links

 * Diuretics
 * Renin-angiotenzin-aldosterone system
 * Hypertension
 * Hypertension crisis