Postobstructive diuresis
Relief of urinary tract obstruction can lead to a postobstructive diuresis, which is defined as a diuresis that persists after decompression of the bladder. A postobstructive diuresis is primarily a problem with chronic, not acute, urinary retention and usually represents an appropriate attempt to excrete excess fluid retained during the period of obstruction.
Any patient with urinary retention can develop postobstructive diuresis. Many patients can manage the increase in urine output by increasing oral fluid intake. In patients who are unable to do so or have severe postobstructive diuresis, IV fluid replacement is indicated.
Causative factors:
Physiologic factors:
- Excess sodium and water retention
- Accumulation of urea and other non-reabsorbable solutes resulting in an osmotic diuresis.
Pathologic factors:
- Decreased tubular reabsorption of sodium secondary to altered expression of proximal and distal sodium transporters.
- Inability to maximally concentrate urine, secondary to a decreased medullary concentrating gradient, leading to decreased response to antidiuretic hormone (ADH).
- Increased tubular transit flow time reducing equilibration time for absorption of sodium and water.
Patients who develop post obstructive diuresis need to be monitored closely. Especially if the patient is unable to eat and drink on their own. Once the accumulated excess of sodium and water has been excreted, severe volume contraction and hypokalemia can occur. It is important to monitor urine output closely in this setting. Once the patient has diuresed to the point of euvolemia, fluid replacement should be administered as needed to prevent volume contraction. Persistent polyuria beyond a week is often due to overzealous volume repletion.