The Obstructed Cat With Peaked T Waves at 02:00
A 4-year-old, 6.1 kg male castrated domestic shorthair is brought in at 02:00. He has not produced urine since the previous morning, roughly 18 hours. He is dull and has a firm, distended bladder the size of a baseball, and is estimated 5% dehydrated. Triage: heart rate 128/min (bradycardic for a cat), respiratory rate 36/min, temperature 36.4 °C, mucous membranes pale, capillary refill 3 seconds. You place an IV catheter and run a baseline panel: potassium 7.9 mmol/L, sodium 148 mmol/L, BUN 92 mg/dL, creatinine 5.8 mg/dL, glucose 210 mg/dL, venous pH 7.22. The ECG shows tall, narrow, peaked T waves, a widened QRS complex, and no visible P waves. The veterinarian asks you to set up for emergency treatment and says she will pass a urinary catheter "as soon as we've stabilised him." The clinic has 0.9% NaCl, lactated Ringer's solution, 10% calcium gluconate, regular insulin, 50% dextrose, and nebulised albuterol available.
1. Explain what the ECG is showing and why hyperkalemia produces these changes. Then list the treatment priorities in order.
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What the ECG means
Hyperkalemia raises the resting membrane potential toward threshold. Mild elevation increases excitability - classically tall, narrow, peaked T waves. As potassium climbs, the voltage-gated sodium channels begin to inactivate, slowing conduction: PR prolongation, then loss of P waves, then QRS widening. At the stage in this cat (K+ 7.9, wide QRS, no P waves, heart rate 128) the myocardium is close to a sine-wave rhythm and asystole. This is a rhythm arrest waiting to happen, and it is a medical emergency.
Priorities, in order
- Protect the myocardium - IV calcium gluconate immediately. It does not lower potassium; it restores the threshold difference across the cardiac membrane and buys 30–60 minutes.
- Shift potassium intracellularly - regular insulin with dextrose (0.1–0.2 U/kg IV, with 1–2 g dextrose per unit), ± nebulised albuterol as an adjunct.
- Remove potassium and restore excretion - 0.9% NaCl to expand extracellular volume and drive renal excretion, then relieve the obstruction.
- Relieve the obstruction once the heart is protected; the bladder is the source and will keep generating potassium until it is drained.
- Treat the consequences - hypothermia, dehydration, acidosis, and post-obstructive diuresis.
Continuous ECG during all of it. The technician's role is to have calcium drawn up and labelled, the dextrose and insulin ready, the ECG running, and a defibrillator or crash cart within reach before the first syringe is given.
2. Detail the administration of 10% calcium gluconate in this cat: dose, rate, what you watch for, and the incompatibility you must avoid.
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Dose and route
- 10% calcium gluconate 0.5–1 mL/kg IV slowly over 10–20 minutes, i.e. roughly 3–6 mL for this 6.1 kg cat, given through a patent, well-secured IV catheter. Some clinicians use a slightly higher dose; the veterinarian's order governs.
- Calcium gluconate is preferred over calcium chloride in cats because the chloride salt is far more damaging if it extravasates - it causes severe tissue necrosis. Gluconate is also less likely to cause profound bradycardia.
What you monitor
- ECG continuously during and after administration. Watch for bradycardia, new arrhythmias, or a shortening QT. Stop the injection and alert the veterinarian if the heart rate drops sharply or the rhythm deteriorates.
- The catheter site: swelling, pain, or resistance means stop immediately. Extravasation of calcium is a soft-tissue emergency.
- Onset is 1–5 minutes and the effect lasts roughly 30–60 minutes, so the ECG must be rechecked before that window closes; the potassium is still 7.9 and will reassert itself.
What you must not do
- Do not mix calcium with sodium bicarbonate or phosphate-containing solutions in the same line or syringe - calcium precipitates as insoluble salt. Flush the line between drugs and use separate ports.
- Do not give it as a rapid push: a fast bolus can cause bradyarrhythmias and hypotension.
- Do not treat it as definitive therapy. It is a bridge, and if you stop there the cat will arrest.
3. The veterinarian asks whether to start lactated Ringer's solution or 0.9% NaCl. Answer, and explain what you will monitor once the catheter is passed.
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Fluid choice: 0.9% NaCl
- Lactated Ringer's contains approximately 4 mmol/L potassium. That is a small amount, and the dog-and-cat literature does not show harm from it in most hyperkalemic patients - but in an anuric cat that cannot excrete a potassium load, and whose heart is already showing conduction block, there is no reason to add any. Sodium chloride contains no potassium, no calcium, and no lactate, and it expands extracellular volume, which improves renal perfusion and potassium excretion.
- Reassess once potassium is trending down and urine output is established; at that point a balanced crystalloid is often preferable for ongoing maintenance.
- Never give potassium-containing fluids before the potassium is normal and urine is flowing.
After decompression, watch for
- Post-obstructive diuresis. Expect large volumes of urine - potentially several times maintenance - for 24–72 hours. Measure urine output and weigh the cat every 6–12 hours; replace losses cautiously or the cat will become hypovolemic.
- Rebound hypokalemia. Unblocking removes the potassium source and the diuresis flushes more out; a cat that was at 7.9 can drop fast. Recheck electrolytes every 4–6 hours initially.
- Bladder rupture or urethral trauma. The bladder was tense and overdistended; decompress gradually. If urine output falls, the abdomen is painful, or the bladder cannot be palpated, stop and reassess.
- Re-obstruction after the catheter is removed or the patient is discharged - teach the owner the signs and the importance of returning.
- Hypothermia and stress, an ECG, blood pressure, and the pain score.
- Renal recovery: creatinine and BUN should fall over 24–48 hours; if they do not, the prognosis conversation changes and the veterinarian needs the numbers early rather than late.