Which electrocardiographic change is characteristically the earliest to appear as serum potassium rises above 6.5 mEq/L?
- A Peaked, narrow-based T waves with shortened QT interval ✓
- B Loss of P waves with a sine-wave pattern
- C Prolongation of the PR interval with widened QRS
- D ST segment depression with U waves
Correct answer: A. Peaked, narrow-based T waves with shortened QT interval
Explanation
Hyperkalemia produces a sequential ECG evolution: tall peaked narrow-based T waves with a shortened QT appear first, followed by PR prolongation and QRS widening, then loss of P waves, and finally a sine-wave morphology preceding ventricular fibrillation or asystole. ST depression with prominent U waves is the signature of hypokalemia, not hyperkalemia, which kills that distractor immediately.
Reference: Goldberger's Clinical Electrocardiography, 9th ed.
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