A patient with a 6-hour history of bowel obstruction requires rapid sequence intubation. Suxamethonium (1.5 mg/kg) is administered. At what serum potassium level would this dose be absolutely contraindicated due to risk of fatal hyperkalaemia?

• A Potassium >5.0 mEq/L in a patient with chronic renal failure
• B Potassium of 4.8 mEq/L in a healthy adult with no neuromuscular disease
• C Potassium of 5.5 mEq/L in a patient with acute renal failure of 12-hour duration
• D Normal baseline potassium in a patient with 72-hour old burns covering 40% BSA ✓

Explanation

Suxamethonium causes a normal potassium rise of 0.5–1.0 mEq/L by depolarising the neuromuscular junction. In patients with upregulation of extrajunctional acetylcholine receptors (burns >24–48 hours post-injury, prolonged immobilisation, denervation injuries, upper motor neuron lesions, severe infection/sepsis), massively exaggerated hyperkalaemia (potassium may rise to 10–12 mEq/L) can cause ventricular fibrillation and cardiac arrest. Burned patients become susceptible from approximately 24–48 hours post-burn and remain at risk for 1–2 years. The risk is highest at 10 days to 2 months. A mild pre-existing hyperkalaemia of 5.5 mEq/L alone in acute renal failure is a relative contraindication, not absolute, unless upregulation is also present.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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