Feline Urethral Obstruction with Severe Hyperkalemia
A 6-year-old male neutered domestic shorthair cat is presented at 7:40 a.m. after 36 hours of straining in the litter box and no urine production. He is dull, 7% dehydrated, and has a heart rate of 92 bpm with weak femoral pulses. Rectal temperature is 36.4°C. The bladder is firm and painful, about 4 cm. The practice has an in-house analyzer, ECG, IV fluids, and a cart for anesthesia, but no continuous blood pressure monitor. Initial venous blood gas/electrolytes: potassium 8.9 mmol/L, sodium 151 mmol/L, ionized calcium 1.02 mmol/L, BUN 84 mg/dL, creatinine 6.9 mg/dL. ECG shows a wide QRS complex, absent P waves, and a sine-wave pattern. The veterinarian asks you to prepare for emergency stabilization and unblocking. The cat is breathing spontaneously but is hypothermic.
1. List your first three actions in order and explain the physiologic reason for each.
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- Give calcium gluconate 10% IV slowly while watching the ECG. This does not lower the serum potassium; it antagonizes potassium's effect on the cardiac membrane and can restore a more normal rhythm within minutes. With absent P waves and a sine-wave pattern, the heart is at immediate risk of fibrillation or arrest.
- Start 0.9% sodium chloride. Volume expansion improves renal perfusion and urine production, dilutes extracellular potassium, and avoids adding more potassium. Rehydration must be balanced against the obstructed bladder, so the veterinarian will set the rate and reassess frequently.
- Prepare insulin with dextrose and continue ECG monitoring. Regular insulin shifts potassium intracellularly; dextrose is given concurrently to prevent hypoglycemia. Calcium buys time, fluids help remove potassium, and insulin/dextrose lowers it. Unblocking is definitive but should not delay stabilization of a life-threatening rhythm.
2. The veterinarian orders 0.9% sodium chloride at 10 mL/kg/hr. The cat weighs 4.6 kg. Calculate the pump rate in mL/hr and explain why lactated Ringer's is not preferred at this moment.
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4.6 kg × 10 mL/kg/hr = 46 mL/hr. Lactated Ringer's contains approximately 4 mEq/L potassium; in severe hyperkalemia, adding even a small potassium load is undesirable when a potassium-free isotonic crystalloid is available. 0.9% sodium chloride also provides the volume expansion needed to improve renal potassium excretion. The fluid rate is a starting point, not a fixed order: reassess hydration, respiratory rate, urine output, and ECG frequently, especially once the obstruction is relieved.
3. Twenty minutes after unblocking, the cat produces 18 mL of urine in 10 minutes and the ECG normalizes. What complication do you monitor for and which parameters do you track?
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The main concern is post-obstructive diuresis, which can cause profound hypovolemia and electrolyte losses, especially hypokalemia and hyponatremia. Track urine output with a closed collection system, body weight, PCV/TS, mentation, heart rate, hydration, and serial electrolytes. Compare urine output with fluid input; if output is high, the veterinarian may need to replace losses with a balanced isotonic fluid rather than continue a fixed maintenance rate. Also monitor for re-obstruction, pain, and cardiac rhythm changes. Do not assume the cat is stable because the ECG improved; the diuresis phase can be delayed.