Postobstructive Diuresis

Postobstructive diuresis can occur following relief of urinary obstruction. It isn’t rare and can be potentially lethal.

CASE

A 65 year old male presents to the emergency department with a one day history of dysuria. He is diagnosed with a urine infection,  commenced on oral antibiotics and has an appointment made the next day for an outpatient ultrasound. He presents the next day with worsening abdominal pain and left flank pain and an inability to pass urine.

The ultrasound shows peri-nephric stranding of the left kidney and a large bladder. A urinary catheter is passed and drains just over 900mL of urine.

His blood tests show:

Hb 122, WCC 15.5, Plt 190

Na 126, K 4.3, Urea 6.5, Creatinine 119, Serum osmolality 265.

He is admitted to the observation unit for medical review and admission for possible pyelonephritis. When the medical team review and diagnose a urine infection and recommend trial of void in am and discharge home ie., not for admission. What is missed is the massive diuresis. Over the past 8 hours the patient has produced a total of 4.5L of urine and his blood pressure starts to drop.

I re-refer the patient to the inpatient registrar for review and admission. The response is not favourable.  “It should be OK. Let’s take the catheter out in the morning for a trial of void and I’ll get my boss to see him then”. Hmmmmm.

Should I worry about this patient? Would you?

Absolutely. He has post obstructive diuresis, which can be lethal, unless we treat.

Definition

Postobstructive diuresis is a clinical diagnosis and involves the loss of salt and water in the urine following correction of a urinary obstruction. The volume produced is more than 200mL of urine production  per hour for 2 consecutive hours or more than 3 L of urine is produced in 24 hours (1).

It tends to occur following relief of bladder outlet obstruction, when the volume drained approaches 1.5L. However this is not always the case. It is part of a normal physiological process of elimination of  excess volume and solutes that have accumulated during the obstruction. However it becomes pathological, when loss of water and salt continue after homeostasis is achieved. It can occur in up to 50% of patients who have relief of obstruction(2) and can lead to severe dehydration, leading to hypovolaemic shock as well as electrolyte abnormalities and can result in mortality.

Approach

In cases where large volumes have been produced, inputs and outputs must be closely monitored.

Measure electrolytes every 12-24 hours, especially Na, K, Mg and Ph. A urine sample looking for Na, K and osmolality should also be done. Salt loss diuresis can convert to post obstructive diuresis. A urinary Na greater than 40mmol/L suggests renal tubular injury.

When replacing fluid aim to run the patient at a negative fluid balance ie replace 70-80% of what is lost in the urine. Running positive or in a euvolaemic state can prolong the diuresis. The type of fluid to use is usually normal saline, but this will depend on the the electrolytes measured.

The Outcome

The fluid is replaced with normal saline and the patient placed on strict fluid balance with the aim of replacing 75-80% of urine output. The patient continues to produce large volumes of urine, 300ml in the next hour, then 270ml/hr and then 340 ml/hr.

Bloods are repeated some 8 hours from the initial set:

Na 140, K 4.9, Serum Osm 298, Urine osmolality 290 Urinary Na 86.

We see that the excess fluid is disappearing, however there is significant salt-wasting.

The patient was admitted for careful fluid monitoring

References

1. Maher JF, Schreiner GE, Waters TJ. Osmotic diuresis due to retained urea after release of obstructive uropathy. N Engl J Med. 1963;268:1099–104.
2. Nyman MA, Schwenk NM, Silverstein MD. Management of urinary retention: rapid versus gradual decompression and risk of complications. Mayo Clin Proc. 1997;72(10):951–6.

Associate Professor Peter Kas MBBS MArch BArch FACEM