Which interval reflects the total duration of ventricular depolarization and repolarization and varies with heart rate?

Which interval reflects the total duration of ventricular depolarization and repolarization and varies with heart rate?

• A. QT Interval
• B. PR Interval
• C. QRS Duration
• D. ST Segment

Answer: (A)

The interval that captures the whole time the ventricles are electrically active—from the start of their depolarization to the end of their repolarization—and that changes with heart rate is the QT interval. It spans from the beginning of the Q wave to the end of the T wave, thus including ventricular depolarization (the QRS complex) plus ventricular repolarization (the ST segment and the T wave). Because the duration of ventricular repolarization varies with heart rate, the QT interval lengthens at slower rates and shortens at faster rates, which is why clinicians use a corrected QT (QTc) to compare across heart rates. The other intervals reflect different phases: the PR interval covers atrial depolarization and conduction to the ventricles; the QRS duration reflects only ventricular depolarization; and the ST segment represents a portion of ventricular repolarization, not its total duration.