How should post-obstructive diuresis be managed in cats following urethral obstruction relief?

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Post-obstructive diuresis (POD) is common and potentially life-threatening following urethral obstruction relief in cats — urine output must be monitored at frequent intervals and intravenous fluid rates adjusted to match output. POD occurs in 46–87.7% of cats following urethral deobstruction. Urine output exceeding 2 mL/kg/h within the first 4–6 hours of catheterization defines POD.13 Severe POD, defined as urine output ≥ 5 mL/kg/h, occurs in 35% of cats.2 Cats with severe POD are hospitalized a median of 1 day longer than those without POD.2 Urine output should be monitored at 1- to 4-hour intervals following catheterization, with intravenous fluid rates adjusted based on measured output at each interval.5 A significant correlation exists between fluid rate administered at a given time point and urine output at the subsequent time point, except for the fluid rate at time zero and urine output at 4 hours — meaning the initial fluid rate does not reliably predict early diuresis magnitude.3 True POD independent of fluid administration — defined as urine output exceeding the administered intravenous fluid volume on at least two consecutive time points — occurs in 36.8% of cats.3 Cats at highest risk for POD and severe POD can be identified at presentation. Admission venous pH < 7.35 is associated with a statistically greater likelihood of developing POD.1 Acidemia, azotemia, hyperphosphatemia, hyperkalemia, hyponatremia, hypochloremia, hypocalcemia, hypermagnesemia, and hypoalbuminemia on admission are all associated with POD development.2 Lower body weight and hypovolemia on presentation are associated specifically with severe POD.2 For cats with ureteral obstruction managed surgically, higher preoperative serum creatinine, potassium, phosphorus, and blood urea nitrogen concentrations positively correlate with both POD duration and maximum severity, and cats with anuria before surgery have longer POD than those without.8 Fluid therapy must prevent both dehydration an