Blocked Cat at 2 a.m.: Hyperkalemia, Bradycardia, and the Order of Interventions
Simba, a 5-year-old male castrated domestic shorthair weighing 5.1 kg, is presented at 2 a.m. after his owner noticed 20 hours of straining with only drops of urine. On examination he is dull and poorly responsive, heart rate 180 but femoral pulses are weak and irregular, respiratory rate 34, temperature 36.4 degrees C, and the bladder is large, firm, and painful. A urinary catheter is placed and 40 mL of dark red urine with gritty material is drained. Blood results: potassium 8.0 mEq/L, BUN 92 mg/dL, creatinine 9.2 mg/dL, sodium 152 mEq/L, glucose 210 mg/dL, PCV 48%, total solids 8.4 g/dL. The ECG shows a slow idioventricular rhythm with absent P waves and a widened QRS complex. The overnight veterinarian is in surgery and asks you to begin treatment and prepare a plan.
1. Sequence your immediate interventions and explain why each comes when it does.
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The ECG is the emergency, not the potassium number. Absent P waves with a widened QRS mean hyperkalemia is already threatening the myocardium, so membrane stabilization comes first: calcium gluconate 10% at 0.5-1 mL/kg IV slowly over 10-15 minutes with continuous ECG monitoring. Its effect is immediate but lasts only 20-30 minutes, and it does not lower potassium - it buys time. Watch for bradycardia during administration and stop if the rate drops.
Next, shift potassium intracellularly. Regular insulin 0.1-0.2 units/kg IV, paired with dextrose 1-2 g per unit as a bolus followed by a dextrose-containing CRI, drives potassium into cells within 15-30 minutes and lasts hours; monitor blood glucose every 30-60 minutes because hypoglycemia is the predictable complication. Nebulized albuterol is an additive option. Do not start a potassium-containing fluid - lactated Ringer's and Plasma-Lyte both contain potassium - so use 0.9% sodium chloride for resuscitation.
Relieving the obstruction is definitive, but decompress gradually rather than emptying the bladder in one go, because rapid decompression can cause mucosal hemorrhage and hypotension. Recheck the ECG after calcium and again after insulin; a conversion to tachycardia or premature ventricular complexes signals reperfusion and needs to be reported to the veterinarian immediately, not managed silently by the technician.
2. Justify the fluid plan for the first two hours, including the choice of fluid and how you would set the rate.
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Three problems drive the fluid choice: hyperkalemia, hypernatremia with a free water deficit from 20 hours of no intake, and obstructive post-renal azotemia. 0.9% sodium chloride addresses all three. It contains no potassium, so it does not add to the load being managed; it is the most alkalinizing of the common isotonic crystalloids relative to this patient's likely acidosis; and its sodium concentration is closest to his measured 152 mEq/L, which matters when correcting hypernatremia.
Start with a resuscitation bolus of 10-15 mL/kg (about 50-75 mL) over 20-30 minutes, reassess perfusion - heart rate, pulse quality, mentation, mucous membrane color, and blood pressure if available - and repeat once if he is still hypovolemic. Do not stack unlimited boluses: once the obstruction is relieved and diuresis begins, urine output can exceed 3-5 mL/kg/hr, and the goal shifts from pushing volume to matching losses.
After resuscitation, set the rate as maintenance (roughly 2-3 mL/kg/hr for a cat, so 10-15 mL/hr) plus measured urine output. That means a closed collection system and hourly measurement rather than estimating from the litter box. Record every rate change with a time and reason, and recheck sodium and potassium every 4-6 hours, since both will fall quickly once diuresis starts.
3. What complications would you specifically watch for in the 12 hours after the obstruction is relieved, and what findings would trigger an urgent call to the veterinarian?
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Post-obstructive diuresis is expected and is the main thing to quantify. If urine output climbs above roughly 3-5 mL/kg/hr for several hours and the fluid rate is not adjusted, the cat becomes hypovolemic and hyponatremic; a falling sodium that is corrected too fast (more than about 10-12 mEq/L per day) risks neurologic injury. Hypokalemia is the mirror image of the presentation: potassium that was 8.0 can drop below 3.5 within hours of diuresis and insulin therapy, producing weakness and ileus. Hourly urine volume, serial weights, and electrolytes every 4-6 hours are the tools.
Reperfusion after decompression can unmask arrhythmias, so watch the ECG trace and the pulse quality rather than trusting the heart rate alone; the conversion from bradycardia to tachycardia, or new premature beats, is an urgent call. Also watch temperature (he was already 36.4 degrees C and will not rewarm without active support), respiratory rate and effort for fluid overload or pulmonary contusion from a prolonged obstruction, and the urine itself - continued gross hematuria, clot retention, or a bladder that becomes large and firm again after catheter removal signals re-obstruction. Poor mentation that does not improve with perfusion, or a falling blood pressure, should not wait for morning rounds.