Post-obstructive diuresis • LITFL • FFS

Post-obstructive diuresis is a polyuric state in which large amounts of sodium and water are eliminated after the relief of a urinary tract obstruction.

It is defined as the production of:

≥ 200 mL of urine for at least two consecutive hours
OR
≥ 3 liters (i.e. 3000 mL) of urine over 24 hours immediately following the relief of urinary retention.

It is underrecognized and potentially (or at least theoretically) lethal. Patients should be monitored in an SSU (or similar) following catheterization, especially after draining large volumes.

History

Traditional teaching once held that large bladder volumes should be drained slowly or intermittently clamped to prevent hematuria, vagal responses, diuresis, and hypotension.

However, immediate, complete drainage with an unrestricted Foley catheter has proven safe and effective with no increase in complications.

Epidemiology

Estimates of post-obstructive diuresis range from 0.5% to 52% — though this wide range is based on limited and outdated data. The true incidence of pathological post-obstructive diuresis is unknown but appears uncommon.

Pathophysiology

Causes:

Foley catheterization for bladder obstruction
Percutaneous nephrostomy
Double-J stenting in bilateral ureteric obstruction
Unilateral obstruction in a solitary kidney

Acquired tubular resistance:
Normal diuresis eliminates retained solutes/volume and self-resolves in ≤24 hours. Pathological diuresis continues beyond that due to tubular resistance to:

ADH → leading to water loss
Aldosterone → leading to sodium loss

Consequences if unrecognized:

Hypovolemia (potentially shock)
Electrolyte disturbances:
- Hyponatremia / Hypernatremia
- Hypokalemia
- Hypomagnesemia

Risk factors:

Obstruction >750–1000 mL
Prolonged obstruction
Heart failure
Elderly patients

Clinical Features

Defined as:

≥ 200 mL urine/hour for two consecutive hours
OR
≥ 3 L over 24 hours
immediately post-relief of obstruction.

Pathological diuresis usually lasts >48 hours.

Investigations

Bloods (8–12 hourly):

Electrolytes: Na+, K+, Mg++, Phosphate
Urea & Creatinine

Urinary sodium:

Na⁺ > 40 mmol/L suggests tubular injury (pathological)

Urine specific gravity:

1.010 = iso-osmotic = physiological, self-limiting
1.020 = concentrated = resolving
1.000 = dilute = pathological salt-wasting, requires close monitoring

Management

Monitoring and fluid replacement:

Aim for negative fluid balance
Replace ~75% of previous hour’s urinary output
(This is an arbitrary but practical approach)
Oral rehydration for milder cases
IV fluids (e.g., Hartmann’s or saline) as needed in more significant or symptomatic cases, depending on electrolytes

Refractory diuresis = persistent beyond 48 hours
→ Consider renal/ICU referral

Disposition

Patients who void large retained volumes should be observed for at least 24 hours. Refractory cases despite supportive treatment require renal/ICU referral — particularly if renal replacement therapy is anticipated.