Sometimes the patient with high-pressure chronic retention is suddenly unable to pass urine, and in this situation so-called acute-on-chronic high-pressure retention of urine has developed.
2. LOWER URINARY TRACT EMERGENCIES 13
On catheterisation, a large volume of urine is drained from the bladder (often in the order of 1 to 2 L and sometimes much greater) The serum creatinine will be elevated and an ultrasound will show hydronephrosis (Fig. 2.1) with a grossly distended bladder.
Recording the volume of urine obtained following catheteri-sation can help define two groups of patients, those with acute retention of urine (retention volume <800mls) and those with acute-on-chronic retention (retention volume >800mls). Prior to catheterisation, if the patient reports recent bedwetting you may suspect that you are dealing with a case of high-pressure acute-on-chronic retention. The retention volume will confirm the diagnosis.
Where the patient has a high retention volume (more than a couple of litres), the serum creatinine is elevated, and a renal ultrasound shows hydronephrosis, anticipate that a post-obstructive diuresis is going to occur. This can be very marked and is due to a number of factors:
■ Reduction in urine flow through the loop of Henle removes the 'driving force' behind development of the corticomedullary concentration gradient. In addition, continued perfusion of the kidney effectively also 'washes out' this gradient, which is
FIGURE 2.1. Hydronephrosis in a case of high-pressure chronic retention.
14 J. REYNARD
essential for allowing the kidney to concentrate urine. Once normal flow through the nephron has recommenced following emptying of the bladder and removal of the back pressure on the kidney, it takes a few days for this corticomedullary concentration gradient to be re-established. During this period, the kidney cannot concentrate the urine and a diuresis occurs until the corticomedullary concentration gradient is re-established.
■ The elevated serum urea acts as an osmotic diuretic.