A 6-year-old with acute kidney injury has serum potassium 7.2 mEq/L with widened QRS complexes on ECG. Calcium gluconate is administered. What is the primary mechanism by which calcium protects the myocardium in hyperkalemia?
- A Lowers serum potassium by renal excretion
- B Shifts potassium intracellularly via Na-K ATPase
- C Stabilizes cardiac cell membrane threshold potential ✓
- D Antagonizes potassium at the neuromuscular junction
Correct answer: C. Stabilizes cardiac cell membrane threshold potential
Explanation
Calcium raises the threshold potential of cardiac myocytes, counteracting the depolarizing effect of hyperkalemia on the resting membrane potential. This stabilizes the cardiac membrane and reduces arrhythmia risk within 1-3 minutes. It does not lower potassium levels. Intracellular shifting is achieved by insulin-glucose or beta-agonists. Sodium bicarbonate also shifts K+ intracellularly.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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