Which events are associated with the QRS complex on an ECG?

• A. 1 and 3 only
• B. 2 and 4 only
• C. 1, 2, and 3 only
• D. 2, 3, and 4 only

Answer: (B)

The QRS complex signals ventricular depolarization—the rapid spread of electrical activation through the ventricles via the His-Purkinje system, which triggers the start of ventricular contraction. This portion of the ECG is the timing when the ventricles are activated after atrial depolarization (the P wave) and just before the ventricles begin to repolarize (the T wave). The large, rapid deflections reflect the large mass of ventricular muscle and its fast conduction, typically lasting about 0.06 to 0.12 seconds. Therefore, the events associated with the QRS are the activation of the ventricles and the onset of their contraction; atrial depolarization and ventricular repolarization occur outside of the QRS.