The Blocked Cat with a Potassium of 8.9
A 5-year-old male neutered domestic shorthair (5.2 kg) presents with a 36-hour history of straining to urinate. He is depressed, has a tense painful bladder, and is producing only drops of urine. Bloodwork: potassium 8.9 mmol/L, BUN 38 mmol/L, creatinine 640 µmol/L, sodium 148 mmol/L, glucose 7.2 mmol/L. ECG shows bradycardia at 90 bpm with tall peaked T waves and a widened QRS. The clinician is scrubbing into an emergency surgery and asks you to "get him stabilized and start fluids." The practice has LRS, 0.9% NaCl, calcium gluconate 10%, regular insulin, 50% dextrose, and sodium bicarbonate.
1. Prioritize the stabilization steps for this hyperkalemic cat and justify your fluid choice.
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This is a life-threatening hyperkalemia (K 8.9) with ECG changes - a medical emergency that outranks the obstruction itself for the first few minutes.
Order of actions:
- Cardioprotection first: calcium gluconate 10% at 0.5–1 mL/kg IV slowly over 10–15 min with ECG monitoring (≈2.6–5.2 mL). This does not lower K but stabilizes the myocardium.
- Shift K intracellularly: regular insulin 0.1–0.2 U/kg IV with dextrose (0.5–1 g/kg) to prevent hypoglycemia; monitor glucose every 15–30 min. This is the fastest way to lower serum K.
- Fluid choice: 0.9% NaCl is preferred over LRS here because LRS contains potassium (~4–5 mEq/L) and is slightly less effective at dilutional reduction; normal saline also corrects the volume deficit driving the azotemia. Start at a rate appropriate for rehydration (e.g., 10–15 mL/kg bolus then reassess).
- Definitive treatment: relieve the obstruction - catheterize and decompress the bladder as soon as the clinician is available; this is the only cure.
- Bicarbonate only if acidotic and after calcium/insulin; it is adjunctive, not first-line.
Judgement: do not delay decompression for labs, but do not skip cardioprotection to get to the catheter. Both happen in parallel.
2. Thirty minutes after treatment, the ECG normalizes and K is 6.1. The clinician asks whether you can proceed to unblock the cat. What do you check before saying yes, and what post-obstructive risk do you monitor for?
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Before proceeding, confirm: (1) ECG rhythm stable and rate improved; (2) glucose not hypoglycemic after insulin (check now - this is the most common iatrogenic complication); (3) calcium given and no bradycardia from it; (4) IV access patent and fluids running; (5) the cat is stable enough for handling.
Post-obstructive risks to monitor:
- Post-obstructive diuresis - large urine output after relief can cause rapid dehydration and electrolyte loss; measure urine output and match losses.
- Re-lowering of potassium - K can fall quickly after relief; recheck in 2–4 hours.
- Hypoglycemia from insulin - continue glucose checks.
- Re-obstruction - ensure the catheter is secure and the bladder is not over-distended again.
- Uroabdomen - if the bladder was very tense, verify no leak after catheterization.
The judgement point: "stable enough" is a checklist, not a feeling, and the post-relief period is where the next crisis hides.