Hypocalcemia is associated with which ECG change?

• A. Prolonged QT interval
• B. Short QT interval
• C. Prolonged PR interval
• D. Wide QRS

Answer: (A)

Calcium mainly influences the ventricular plateau phase during repolarization. In hypocalcemia, reduced calcium entry slows this plateau, delaying repolarization and lengthening the QT interval on the ECG. The QT interval covers from the start of depolarization to the end of repolarization, so a slower repolarization shows up as a longer QT. Hypercalcemia does the opposite, shortening the QT. The PR interval and QRS width are not primarily driven by calcium in this way, so they aren’t the classic changes seen with low calcium. This is why hypocalcemia is associated with a prolonged QT interval and, clinically, can increase the risk of torsades de pointes.