Feline Urethral Obstruction with Severe Hyperkalemia at 11 pm
Tommy is a 4-year-old, 5.2 kg, castrated male domestic shorthair presented at 11 pm after being seen straining in the litter box since that morning. He is obtunded and approximately 8% dehydrated. HR 170 bpm with weak, irregular pulses; RR 36; T 36.1°C; mucous membranes pale. The bladder is large, firm, and painful. An ECG shows a heart rate of 60 bpm with slow, wide QRS complexes, absent P waves, and tall tented T waves. In-house bloodwork: K 8.9 mmol/L, BUN 132 mg/dL, creatinine 9.1 mg/dL, glucose 268 mg/dL, PCV 42%, TS 8.6 g/dL. The veterinarian is placing an IV catheter. The practice stocks 0.9% NaCl, lactated Ringer's solution, 10% calcium gluconate, 50% dextrose, regular insulin, and sodium bicarbonate.
1. Rank the first four interventions for this cat and justify the order. Include the calculated calcium gluconate dose from the 10% solution.
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The ECG changes (bradycardia, absent P waves, wide QRS, tented T waves) mean the heart is about to fail - that outranks the bladder.
- IV access, oxygen, and continuous ECG - everything else depends on a route and a monitor.
- 10% calcium gluconate 0.5-1 mL/kg IV slowly over 10-15 minutes → 0.5 × 5.2 = 2.6 mL to 1 × 5.2 = 5.2 mL. Calcium antagonizes potassium's effect on the myocardium immediately; it does not lower serum K, so it buys time while the next steps work. Watch the ECG during infusion and stop if bradycardia or a new arrhythmia develops. Flush the line afterward - perivascular calcium causes tissue necrosis.
- 0.9% NaCl bolus 10-15 mL/kg = 52-78 mL over 15-20 minutes, then reassess and continue at roughly 10-20 mL/kg/hr. Volume expansion improves renal perfusion and dilutes potassium, and sodium shifts potassium intracellularly. This is the workhorse of hyperkalemia therapy.
- Dextrose with or without regular insulin if the ECG does not improve after calcium and fluids: 50% dextrose 1 mL/kg = 5.2 mL diluted 1:4 in saline, over 15-30 minutes, ± regular insulin 0.1-0.2 U/kg. This drives potassium into cells. Monitor glucose every 30-60 minutes for hypoglycemia.
Only after cardiac stabilization do you relieve the obstruction (catheter or, if that fails, decompressive cystocentesis). Bicarbonate is reserved for severe documented acidosis and is not a first-line drug here.
2. The overnight relief technician asks why you are not using lactated Ringer's solution, saying 'it has potassium in it and that cat's potassium is 8.9.' Calculate how much potassium 60 mL/hr of LRS would actually deliver over 2 hours, and explain how that number should inform the choice.
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LRS contains 4 mEq/L of potassium. At 60 mL/hr for 2 hours, the cat receives 120 mL = 0.12 L.
0.12 L × 4 mEq/L = 0.48 mEq of potassium delivered over 2 hours - about 0.09 mEq/kg over 2 hours, which is trivial compared with a serum potassium of 8.9 mmol/L.
The reasoning matters more than the reflex. 0.9% NaCl is still the better first fluid, for three reasons: it is potassium-free, it is the strongest volume expander of the isotonic crystalloids, and it does not depend on hepatic lactate metabolism, which may be impaired in a cat with azotemia and poor perfusion. But the honest answer to the technician is that the 'LRS is dangerous in hyperkalemia' rule is about direction of effect, not about a dangerous potassium load - the 0.48 mEq is not what harms the cat. If LRS were the only fluid available and the cat were profoundly hypovolemic, giving it would be better than giving nothing, with ECG monitoring. What is genuinely harmful is giving a potassium-containing fluid as a bolus or continuing it after the potassium has normalized, which is why the fluid plan needs rechecking, not just a one-time choice.
3. Twelve hours after successful unblocking, the cat is producing 4 mL/kg/hr of urine and the potassium is 3.0 mmol/L. Explain what is happening, what complications you are watching for, and what you will do about the fluid plan.
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This is post-obstructive diuresis - the tubules have lost their ability to concentrate after prolonged obstruction and pressure, and the cat is now dumping solute and water. Four mL/kg/hr = about 21 mL/hr in a 5.2 kg cat, which is roughly four times normal maintenance.
Complications to watch:
- Hypovolemia and hypotension - losses are being replaced inadequately unless matched; check weight, PCV/TS, mentation, and blood pressure every few hours.
- Hypokalemia - 3.0 mmol/L and falling; potassium is being lost in the urine, which can cause weakness and ileus.
- Re-obstruction - the catheter may be removed and the cat may block again; measure urine output and watch for straining or a firm bladder.
- Ongoing azotemia and uremic risk - BUN and creatinine should be rechecked daily; some cats remain azotemic for days.
Fluid plan: measure urine output with a closed collection system and replace losses milliliter for milliliter with an isotonic crystalloid, on top of maintenance (roughly 2-3 mL/kg/hr here). Once urine flow is established and the potassium is low, add KCl 20-30 mEq/L to the fluids and recheck potassium every 4-6 hours, since the previous hyperkalemia can flip quickly. Document hourly output - the trend is the treatment decision.