A 58-year-old man with chronic heart failure is started on a loop diuretic. After 2 weeks he develops weakness and his ECG shows flattened T-waves with prominent U-waves. Which physiological mechanism best explains the ECG changes?

• A Hypokalaemia raising the resting membrane potential closer to threshold
• B Hypokalaemia hyperpolarising ventricular myocytes and prolonging repolarisation ✓
• C Hyponatraemia reducing phase-0 upstroke velocity
• D Metabolic alkalosis shortening the action potential duration

Explanation

Loop diuretics cause renal potassium wasting, producing hypokalaemia. Low extracellular K+ hyperpolarises the resting membrane potential and prolongs phase-3 repolarisation of ventricular myocytes, manifesting as T-wave flattening and prominent U-waves on ECG. Hyponatraemia affects phase-0 (Na+ channels) but is not the primary mechanism here, and alkalosis shortens rather than explains U-wave prominence.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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