Blocked Cat with Hyperkalaemia: Order of Operations
A 5-year-old male neutered domestic shorthair cat, 4.6 kg, arrives at 22:00. The owner reports 24 hours of repeated litter-box visits with only drops of urine, and no urine at all in the last 8 hours. The cat is obtunded and hypothermic at 36.6 °C, with a heart rate of 88 beats/min, weak femoral pulses and a respiratory rate of 34. The bladder is large, firm and painful; a gentle attempt to express it yields nothing. An ECG shows tall, peaked T waves, a widened QRS complex and no visible P waves. Point-of-care bloodwork: potassium 8.6 mmol/L, sodium 130 mmol/L, ionised calcium 0.92 mmol/L, PCV 46%, total solids 8.0 g/dL. The veterinarian is still closing an emergency caesarean and asks you over the intercom to start stabilising and to get the urinary catheter in as soon as you can.
1. Rank this cat's problems by how quickly they will kill it, and justify the order in which you would address them during the next 15 minutes.
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Ranking:
- Hyperkalaemic cardiotoxicity - K 8.6 with absent P waves is a pre-arrest ECG. Ventricular fibrillation or asystole can occur at any moment; this is measured in minutes.
- Poor perfusion and hypothermia - obtunded, weak pulses, 36.6 °C. Hypothermia worsens bradycardia and arrhythmia risk.
- Bladder obstruction - the cause of everything above, but relieving it is not risk-free (see below).
- Pain and stress - real, but not immediately lethal.
First 15 minutes: get the veterinarian's attention - this cat needs a doctor at the table now, and 'as soon as you can' is not an acceptable instruction for a K of 8.6 with no P waves. Place or confirm IV access, start oxygen, begin active but gentle warming, set up continuous ECG, and draw up 10% calcium gluconate at 0.5–1 mL/kg (2.3–4.6 mL) for slow IV administration by the veterinarian over 10–15 minutes with ECG running. Calcium does not lower potassium; it stabilises the myocardium and buys time. Meanwhile prepare 0.9% sodium chloride and the catheter kit.
What you do not do is catheterise first. Decompressing the bladder before the myocardium is protected can precipitate arrhythmia.
2. Choose the resuscitation fluid, calculate a bolus for this cat, and explain what monitoring tells you it is working.
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Fluid: 0.9% sodium chloride. It contains no potassium and no calcium. Lactated Ringer's is not ideal here: it supplies 4 mmol/L potassium, which is avoidable, and its calcium content is incompatible in the same line as calcium gluconate - calcium salts precipitate with lactated solutions, so the line must be flushed between them regardless of which fluid is chosen.
Bolus: 10 mL/kg = 46 mL over 15–20 minutes, then reassess and repeat in aliquots up to a total of about 20–30 mL/kg if the cat remains hypoperfused. Judge each aliquot on heart rate, pulse quality, mucous membrane colour, capillary refill time, mentation and respiratory effort - cats with obstruction are prone to volume overload once the obstruction is relieved, so do not blindly stack boluses.
What 'working' looks like: heart rate rising out of the 80s, P waves reappearing and QRS narrowing on the ECG, stronger and more synchronous femoral pulses, improving mentation. Recheck potassium 30–60 minutes after starting - the number should fall and the ECG change usually comes first. A potassium that is falling but an ECG that is unchanged still warrants the veterinarian's eyes. Report any bradycardia below 60 beats/min, new arrhythmia, or a change in respiratory pattern immediately.
3. The veterinarian unblocks the cat and a large volume of urine is drained. What do you monitor for over the next 12–24 hours, and what would make you escalate?
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During decompression: slow drainage rather than emptying the bladder in one go, watch the ECG for vagally mediated bradycardia and for reperfusion arrhythmias as potassium and acid load shift back into circulation, and keep the line for calcium gluconate accessible.
Post-obstructive diuresis: output can exceed 100 mL/hr. Measure it - catheter, closed collection system, hourly volumes recorded - because you cannot replace what you do not measure. Replace losses with crystalloids, generally matching millilitres out with millilitres in after maintenance is accounted for, and expect the veterinarian to switch to a potassium-containing fluid once the potassium has normalised; ongoing diuresis can produce hypokalaemia within hours.
Escalate for: urine output falling to near zero (re-obstruction or a displaced catheter), gross haematuria with a deteriorating cat, heart rate below 60 or any new arrhythmia, hypothermia that will not correct, or a cat that becomes duller rather than brighter. Recheck electrolytes and renal values on the veterinarian's schedule, keep the cat warm and quiet, and provide analgesia - an unblocked cat is not a comfortable cat.