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| Osmotic therapy can be defined as the induction of renal water excretion
by increasing the osmolality in the extracellular space with non-ionic
substances.
A proven method is the infusion of hyperosmolar mannitol solutions ( e.g.
Osmofundin® 15%). These are rapidly filtered in the glomerulus
and not reabsorbed in the tubules. As a result of the osmotic gradients
thereby achieved in the renal tubules, the reabsorption of water is inhibited,
and urine excretion increased. Although electrolytes are also lost, the
loss of water is much higher. This makes osmotic diuresis with mannitol
especially useful in situations of overhydration that are accompanied
by
hyponatraemia, e.g. in TURP-syndrome. On the other hand, care must be taken
not to induce hypernatraemia in patients with normal values of sodium
in
serum.
Osmotic diuresis works only if the renal tubules are not damaged and
there is no hypovolaemia. It is therefore appropriate to treat dehydration
first. Water and electrolyte balance must be monitored and imbalances
corrected during this treatment. Similarly, pulse, blood pressure, central
venous pressure and urine production should be monitored. Osmotic therapy
is used in:
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- Prophylaxis of acute renal failure
- Distinction between functional and organic acute renal failure
- Forced diuresis in poisonings
- Reduction of intracranial pressure in cerebral oedema
- Reduction of intraocular pressure in acute glaucoma and before
cataract operations
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| Prophylaxis of acute renal failure The appropriately timed
administration of mannitol solutions is an effective prophylaxis of acute
renal failure in the context of adequate oxygenation, stabilisation of
the circulation and correction of disorders of water, electrolyte, and
acid-base balances. The target is a urinary flow of 50-100 ml/hr.
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Dose:
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Up to 10 ml/kg body weight
per day Osmofundin® 15%. |
| Infusion rate: |
Up to 1.0 ml/kg body weight
per hour Osmofundin® 15%. |
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| Distinction between functional and organic acute renal failure
If there is ongoing oliguria or anuria in the absence of hypovolaemia
or mechanical hindrance to urine flow, a rapid infusion of mannitol solutions
will result in an increased diuresis in the case of a functional disturbance,
whereas in the case of an organic disorder the increase is small or absent.
The Osmofundin® test consists of an infusion of 150 ml
of Osmofundin® 15% within 3-5 min. If the urine flow rises
to more than 40 ml/hr, there is a functional renal failure, and the Osmofundin®
15% infusion rate is adjusted to produce the desired urinary flow rate,
generally 50-100 ml/hr. If the urinary flow remains less than 30-40 ml/hr,
there is probably an organic renal failure present, and the infusion is
stopped. The administration of too large a quantity of mannitol in the
presence of persistent oliguria or anuria can produce an acute hypervolaemia,
with the danger of pulmonary or cerebral oedema.
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| Forced diuresis in poisonings
A forced diuresis is often carried out in the case of poisoning with
substances which can be excreted in urine, using water and electrolyte
administration. This can be supported by the additional infusion of Osmofundin®
15%, the dosage being determined by the desired urinary flow rate.
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| Reduction of intracranial pressure in case of cerebral oedema
Osmofundin® 15% may be used to lower the intracranial
pressure in the presence of cerebral oedema. The dosage is dependent on
the result obtained, i.e. the disappearance of symptoms of raised pressure
or the pressure measurement itself. 1.5-2.0 g/kg body weight mannitol
is given over 30-60 min. To avoid a rebound effect, the treatment
should be repeated every 6-8 hr. Caution is necessary if there has
been cerebral bleeding because the relief of pressure may produce further
bleeding.
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| Reduction of intraocular pressure in acute glaucoma and before
cataract operations
Osmofundin® 15% reduces the intraocular volume, particularly
of the vitreous body, and can therefore be used in cases of acute glaucoma
and before cataract operations. In general, one gives 750 ml Osmofundin®
15% in a 60 min. period. The maximum reduction in pressure is at
the end of the infusion, or around 15-20 min.
later.
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