Which ECG interval is normally less than 0.10 seconds?

• A. QRS complex
• B. P wave
• C. QT interval
• D. PP interval

Answer: (A)

The interval being tested is the one that represents ventricular depolarization, and it is the briefest part of the ECG. The QRS complex reflects how quickly the ventricles activate, and this process happens through the fast Purkinje system, so the duration is very short—typically about 0.06 to 0.10 seconds in a healthy heart. That makes it normally less than 0.10 seconds. If this interval were longer, it would point to slowed ventricular conduction, such as a bundle branch block or other intraventricular delay. In contrast, the QT interval covers both depolarization and repolarization of the ventricles and is much longer, while the PP interval simply tracks the time between atrial activations and varies with heart rate. The P wave duration is also short, but the defining short interval most classically cited for normal ECG timing is the QRS duration.