Diuresis After Urethral Obstruction

Cats in this study had been hospitalized due to atraumatic urethral obstruction. Laboratory blood analyses were examined, and cats underwent urinary catheterization and measurement of urine output. Venous blood pH, glucose, bicarbonate, and creatinine concentrations were recorded at admission. Blood pH values were classified as acidotic if measured at < 7.35 and hyperglycemia was defined at glucose values > 168 mg/dL. Bicarbonate concentrations were defined as decreased if measured at < 14 mEq/L; serum creatinine values measuring > 2 mg/dL suggested decreased renal function. An indwelling urinary catheter connected to a sterile closed collection system was maintained in all cats. Cats were concurrently treated with intravenous fluid therapy at rates to match urine output rates. Postobstruction diuresis (urine output of > 2 mL/kg/H) developed in 46% of cats during the first 6 hours of urine collection. The likelihood of developing diuresis was significantly higher during the 18 to 24 H, 30 to 36 H, 42 to 48 H, and > 48 H postobstruction time periods as compared with the 6 to 12 H postobstruction time period. Cats with urethral obstruction were 5 times more likely to demonstrate postobstructive diuresis if they had concurrent acidemia. Hyperglycemia, hypobicarbonatemia, azotemia, or elevated calculated osmolality (defined as > 316 mOsm/kg) were not associated with diuresis. Urine production, hydration, and body weight should all be monitored in cats with naturally occurring urethral obstruction in order to individualize replacement fluid therapy.

Commentary: Adequate and aggressive fluid therapy is an integral component in the treatment of urethral obstruction. Fluid therapy corrects azotemia, promotes toxin elimination, and encourages elimination of bladder debris. Increased urine output can be improperly attributed to overhydration; however, it is actually the consequence of postobstructive diuresis. It can also result in renal injury, treatment failure, and prolonged hospitalization. This study has provided plausible evidence that postobstructive diuresis is a real problem in many cats following correction of urethral obstruction, and calculation of fluid infusion rates should include frequent quantification of urine output.

Retrospective study to characterize post-obstructive diuresis in cats with urethral obstruction. Francis BJ, Wells RJ, Rao S, Hackett TB. J Fel Med Surg 12:606-609, 2010.