Which interval is used to measure the total time for ventricular depolarization and repolarization?

• A. PR interval
• B. QT interval
• C. PP interval
• D. QRS complex

Answer: (B)

Understanding ECG intervals: the QT interval represents the total time from the beginning of ventricular depolarization to the end of ventricular repolarization. Ventricular depolarization corresponds to the QRS complex, while ventricular repolarization corresponds to the T wave. So, the span from the start of the Q wave to the end of the T wave captures the full duration of ventricular electrical activity. In contrast, the PR interval tracks conduction from the atria to the ventricles, the QRS complex reflects depolarization only, and the P-P interval reflects atrial rhythm. Therefore, the QT interval is the one that sums both depolarization and repolarization. (Clinically, QT is often corrected for heart rate, giving QTc.)