The QRS Complex on an ECG primarily reflects the electrical activity of which event?

• A. The contraction of the ventricles
• B. The relaxation of the atria
• C. The atrial depolarization
• D. The AV node delay

Answer: (A)

The main idea is that different parts of the ECG correspond to specific electrical events in the heart. The QRS complex represents ventricular depolarization—the rapid spread of electrical activity through the ventricles via the His-Purkinje system. This depolarization triggers the ventricles to contract (ventricular systole). The reason it’s so prominent is the ventricles’ large muscle mass, which produces a large, sharp deflection.

Atrial depolarization, which triggers atrial contraction, is represented by the P wave, not the QRS complex. The AV node delay, creating the PR interval, occurs before ventricular activation and is not depicted by the QRS. Atrial repolarization does occur during the QRS interval but is masked by the much larger ventricular depolarization. So the QRS complex most directly reflects the electrical activation leading to ventricular contraction.