Bilateral Urethral Obstruction (BUO)

Postobstructive diuresis is defined as diuresis of more than 200 mL/h for at least 2 hours. Patients with severe diuresis should receive intravenous fluid replacement in the form of half normal saline at 80% of the hourly urine volume for the first 24 hours, then 50%. Postobstructive diuresis usually lasts 24-72 hours. The development of a brisk diuresis (>200 mL/hr) following the relief of obstruction constitutes POD and warrants appropriate monitoring, investigation and treatment.

There are several physiologic and pathologic factors that lead to the development of this condition.

Most cases represent physiologic POD and, as such, the diuresis is allowed to proceed until the euvolemic state is reached as determined by clinical parameters such as orthostatic vital signs, breath sounds, jugular venous distention, and peripheral edema. During this time, serum electrolytes should be evaluated every 6 to 12 hours because electrolyte imbalance, particularly hypokalemia and hypomagnesemia, may develop

Three types of diuresis:

1. Urea diuresis- It is the most common diuresis and self limiting. It last 24-48 hrs and needs fluid and electrolyte monitoring.
2. Sodium Diuresis- It is 2nd most common. Usually, it is self limiting, but may last > 72hrs. Also needs aggressive monitoring of fluids and electrolytes.
3. Water diuresis - It is rare and self limiting. It may lead to temporary nephrogenic diabetes insipidus.

Physiologic factors

1. Excess sodium and water retention
2. Accumulation of urea and other non-reabsorbable solutes resulting in an osmotic diuresis.

Pathologic factors

1. Decreased tubular reabsorption of sodium secondary to altered expression of proximal and distal sodium transporters.
2. Inability to maximally concentrate urine, secondary to a decreased medullary concentrating gradient, leading to decreased response to ADH
3. Increased tubular transit flow time reducing equilibration time for absorption of sodium and water.
4. Increased production of prostaglandins immediately following relief of obstruction.

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. This is done by replacing 75% of the urine losses with 0.45% NS. This condition is usually self-limiting and resolves over several days to a week. Persistent polyuria beyond a week is often due to overzealous volume repletion.

Vesicoureteral reflux (VUR)

common anatomical cause of recurrent UTI in children

due to incompetent vesicoureteral valve