■ Excessive salt and water, laid down during the period of retention, is appropriately excreted by the kidney.
Usually the patient comes to no harm from this diuresis, even when several litres of urine are excreted per 24 hours. However, occasionally the intravascular volume may fall and postural hypotension may develop. One good way of anticipating this is to record lying and standing blood pressure. If there is a large discrepancy between the two, consider intravenous fluid replacement with normal saline.
WHAT TO DO NEXT FOR THE MAN WITH ACUTE RETENTION
Precipitated retention often does not recur. Spontaneous retention often does.
Precipiated urinary retention should be managed by a trial of catheter removal. In spontaneous retention, many urologists will try to avoid proceeding straight to TURP after just one episode of retention, instead recommending a trial of catheter removal, with or without an alpha blocker, in the hope that the patient will void spontaneously and avoid the need for operation. A trial without catheter is clearly not appropriate in cases where there is back pressure on the kidneys—high-pressure retention. About a quarter of men with acute retention will void successfully after a trial without catheter (Djavan et al. 1997, Hastie et al. 1990). Of those who pass urine successfully after an initial episode of retention, about 50% will go back into retention within a week, 60% within a month, and 70% after a year. This means that after 1 year, only about one in 5 to 10 men originally presenting with urinary retention will not have gone back into retention. Recurrent retention is more likely in those with a flow rate <5 mL/s or average voided volumes of <150mL. An alpha blocker started 24 hours before a trial of catheter removal increases the chances of voiding successfully (30% taking placebo voiding successfully, and 50% taking an alpha doing so; McNeill et al. 1999). However,
2. LOWER URINARY TRACT EMERGENCIES 15
whether continued use of an alpha blocker after an episode of acute retention reduces the risk of a further episode of retention (McNeill et al. 2001) isn't yet known.