QRS complex on an ECG indicates which electrical events?

• A. Atrial depolarization
• B. Ventricular repolarization
• C. Atrial repolarization
• D. Ventricular depolarization and atrial repolarization

Answer: (D)

The main idea is that the QRS complex marks the ventricles becoming activated and ready to contract. This rapid spread of depolarization through the ventricular muscle produces the sharp, tall waves we see as the QRS. At the same moment, the atria are repolarizing, but that signal is tiny and hidden within the much larger ventricular depolarization, so you don’t see a separate wave for atrial repolarization on the ECG. So the QRS complex mainly shows ventricular depolarization, with atrial repolarization occurring concurrently but not being visible.