Which ECG interval represents the duration of ventricular depolarization?

Which ECG interval represents the duration of ventricular depolarization?

• A. ST segment
• B. QRS duration
• C. PR interval
• D. QT interval

Answer: (B)

The main concept here is that the QRS duration represents how long the ventricles take to depolarize. It is the interval from the start of the Q wave to the end of the S wave on the ECG, reflecting the rapid spread of electrical activity through the ventricles via the His-Purkinje system. In a healthy adult, this duration is usually under about 120 milliseconds; a longer duration can indicate slowed intraventricular conduction, such as a bundle branch block. The other intervals don’t measure ventricular depolarization duration. The ST segment appears after depolarization is complete and reflects a period of ventricular plateau in repolarization, not the depolarization itself. The PR interval tracks conduction from the atria through the AV node to the ventricles, so it measures atrioventricular conduction time, not how long the ventricles take to depolarize. The QT interval spans from the start of ventricular depolarization to the end of ventricular repolarization, encompassing both depolarization and the subsequent repolarization, so it isn’t specific to the depolarization duration alone.

The main concept here is that the QRS duration represents how long the ventricles take to depolarize. It is the interval from the start of the Q wave to the end of the S wave on the ECG, reflecting the rapid spread of electrical activity through the ventricles via the His-Purkinje system. In a healthy adult, this duration is usually under about 120 milliseconds; a longer duration can indicate slowed intraventricular conduction, such as a bundle branch block.

The other intervals don’t measure ventricular depolarization duration. The ST segment appears after depolarization is complete and reflects a period of ventricular plateau in repolarization, not the depolarization itself. The PR interval tracks conduction from the atria through the AV node to the ventricles, so it measures atrioventricular conduction time, not how long the ventricles take to depolarize. The QT interval spans from the start of ventricular depolarization to the end of ventricular repolarization, encompassing both depolarization and the subsequent repolarization, so it isn’t specific to the depolarization duration alone.