Which electrolyte abnormality is most commonly associated with a prolonged QT interval?

• A. Hypokalemia (low potassium)
• B. Hyperkalemia
• C. Hypercalcemia
• D. Hypocalcemia

Answer: (A)

Low potassium levels prolong ventricular repolarization, lengthening the QT interval. Potassium helps drive the outward currents that bring the cardiac cell back toward resting potential during the latter part of the action potential. When extracellular potassium is low, these outward currents diminish, the repolarization phase slows, and the QT interval lengthens. This pattern is a classic cause of a prolonged QT and increases the risk of torsades de pointes. Hyperkalemia tends to shorten the QT, and while hypocalcemia can also lengthen the QT by extending the plateau phase, hypokalemia is the most commonly associated electrolyte abnormality with a prolonged QT.