Viva Practice for the FRCS(Urol) and Postgraduate Urology Examinations, 2nd ed.

Acute Urinary Retention

Q. A 65-year-old man is referred to you by his GP with moderate LUTS. He is primarily concerned that he may develop a ‘complete stoppage’. How would you assess and counsel him?

A. There are a number of risk factors which have been identified for acute urinary retention (AUR) in both community-based observational studies and in RCTs of drug treatment for BPH. These include age, reduced flow rate, prostate volume, PSA and a previous episode of urinary retention (Table 12.3).

Table 12.3 The important risk factors with respect to acute urinary retention (AUR) and their respective relative risk

Q. A 78-year-old man presents to accident and emergency with a painfully distended bladder and an inability to pass urine. How would you manage this patient?

A. I would take a concise history, examine the patient and then insert a urinary catheter. Having relieved his discomfort I would then arrange further investigations including urinalysis, FBC, U&E and ultrasound scan.

Q. Your investigations reveal a serum creatinine of 427 ^mol/L and ultrasound demonstrates bilateral hydronephrosis. What is your further management of this patient?

A. I would arrange for the patient to be admitted. I would ask the nurses to record hourly monitoring of pulse, blood pressure and urine output. I would ask to be informed if the patient produces more than 200 mL of urine per hour for 2 consecutive hours. I would ensure that his admission weight is recorded and request daily weighing to monitor for gross fluid shifts.

If there is evidence of post-obstructive diuresis then I would give replacement intravenous normal saline equivalent to 90% of the patient’s previous hour’s urine output (the amount of fluid replacement is variable - some would only replace 50% of the previous hour’s urine output). I would recheck the serum U&E to ensure that the creatinine is falling and that potassium levels remain within range. I would expect to continue intravenous fluid support for 24-48 hours.

Q. What is the mechanism behind post-obstructive diuresis?

A. There is both a physiological and pathological component to the diuresis. A physiological diuresis occurs because of the accumulation of fluid, electrolytes and waste products in the preceding period of renal failure. Relief of obstruction allows elimination of excess amounts of these substances to occur. Pathological diuresis occurs because of a number of factors which lead to tubular dysfunction and inappropriate salt and water handling by the kidney. These include the following:

1. Defective generation of medullary solute gradient secondary to j re-absorption of NaCl by thick ascending LOH j reabsorption of urea by collecting tubule

2. Inability to maintain medullary solute gradient secondary to t medullary blood flow (solute washout)

3. t endogenous production of ANP (+ other natriuretic peptides)

4. Poor response of collecting duct to ADH

Q. Draw a graph to show the recovery of glomerular filtration rate (GFR) following the relief of obstruction.

A. This is illustrated in Figure 12.4. As the GFR continues to recover for up to 3 months thus plasma creatinine will continue to decrease over this period.

Figure 12.4 Changes in creatinine clearance and ^99mTc DTPA clearance following relief of obstruction. Both are estimates of glomerular filtration rate.

Q. If creatinine clearance and ^99mTc DTPA (diethylene-triamine-penta-acetic acid) clearance are both indicators of glomerular filtration rate, then why does creatinine clearance improve more quickly?

A. Creatinine is excreted by the tubules of the kidney as well as through glomerular filtration. Following relief of bilateral ureteric obstruction in this case tubular function recovers in the first 14 days but full recovery of glomerular function may take up to 3 months. The return of tubular function increases creatinine clearance and allows serum creatinine to fall but this does not reflect a real change in GFR.

Note: Creatinine clearance estimates the GFR and can be measured over 12-24 hours by the equation UV/P where U is the urinary concentration of creatinine, V is volume of urine produced over a timed interval and P is the plasma concentration of creatinine. About 80% of creatinine clearance is due to GFR and 20% is due to renal tubular secretion. Thus creatinine clearance is an overestimate of actual GFR (becomes important in very poor renal function when tubular secretion may be responsible for a relatively greater proportion of the creatinine clearance).

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