Emergency and critical careFeline urethral obstruction - the emergency approach
The first thirty minutes in a blocked cat, treating hyperkalaemia before anything else, catheterisation technique, and the post-obstructive diuresis that is easy to miss.
When a male cat arrives that the owner says cannot pass urine, you have minutes rather than hours. Urethral obstruction is one of the genuine emergencies in small animal practice: fatal within twenty-four to forty-eight hours if untreated, yet most patients treated in time recover completely.
This article sets out the order to follow from the moment the cat comes through the door. The order matters, and the commonest mistake is going straight for the obstruction.
The first thirty minutes
The patient before the bladder
Instinct says to relieve the distended bladder immediately. The correct order is the reverse. A blocked cat does not die because its bladder is full; it dies because hyperkalaemia stops the heart. Relieving the obstruction requires anaesthesia, and anaesthetising a hyperkalaemic, acidotic, hypothermic patient is the easiest way to lose one.
On arrival:
- Place an intravenous catheter and take blood — potassium, urea, creatinine, blood gas, PCV/TP
- Attach an ECG
- Start warming; these patients are frequently hypothermic
- Provide analgesia
While you wait for the biochemistry, the ECG already tells you what you need.
What hyperkalaemia looks like on the ECG
As potassium rises, the ECG changes in sequence: tall spiked T waves, then a diminishing and finally absent P wave, then a widening QRS, then a sinusoidal rhythm and asystole. A bradycardic cat with no visible P wave has advanced hyperkalaemia, and you do not wait for the laboratory.
Treatment does three separate jobs, and they should not be confused with one another:
- Calcium gluconate does not lower potassium; it protects the myocardium from its effect. It acts within minutes and lasts twenty to thirty minutes. It is the first drug in a patient with ECG changes, and the ECG is monitored while it is given.
- Dextrose, with insulin if needed, drives potassium into cells. Slower to act, longer lasting. If insulin is given, the patient needs monitoring for hypoglycaemia for several hours.
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