A patient on spironolactone has serum potassium of 7.2 mEq/L. The earliest electrocardiographic abnormality expected is:

• A Flattened T waves with prominent U waves
• B Shortened PR interval with delta waves
• C Tall, narrow-based, peaked T waves ✓
• D Deep S waves with tall R waves in precordial leads

Explanation

Hyperkalemia steepens the transmembrane repolarization gradient, accelerating phase 3 repolarization and narrowing the action potential duration in ventricular myocytes. This produces tall, peaked, narrow-based T waves, the earliest ECG sign, followed later by PR prolongation, QRS widening, and sine-wave morphology. Flattened T waves with U waves characterize hypokalemia, the opposite electrolyte state, and delta waves reflect accessory pathway conduction unrelated to potassium concentration.

Reference: Ganong Review of Medical Physiology, 26th ed.

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