Polyuria
Polyuria is the excretion of too much water for a given physiological state.
When assessing polyuria consider:
- Urine volume
- Osmole excretion
- Urine osmolality
Causes of polyuria:
Look at urine osmolality:
- Hyperosmolar urine
If a large volume of hyperosmolar urine is excreted there must be the same number of osmoles being taken in. Normally adults excrete approx. 900 mOsm/day. If amounts greater than this are being excreted, an osmotic diuretic such as urea or glucose must be present.
During an osmotic diuresis, Na (50 mmol/l) and K+ (25-50 mmol/l) will also be found in the urine. This can lead to a depletion of these ions and ECF contraction.
- Isosmolar urine
These patients are characterised by a loss of medullary hypertonicity. The main cause is renal damage secondary to infection, hypoxic injury, obstructive uropathy or drug-induced. The use of loop diuretics will produce a similar temporary picture.
There is no significant increase in osmolality following administration of ADH.
- Hypo-osmolar urine
Most of these patients with very dilute urine will have central diabetes insipidus.
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