Which part of the ECG corresponds to ventricular depolarization?

• A. QRS complex
• B. P wave
• C. T wave
• D. U wave

Answer: (A)

The main idea here is how the ECG maps electrical events to the heartbeat. Ventricular depolarization is the rapid activation of the ventricles that triggers their contraction, and on an ECG this is seen as the QRS complex. The impulse travels quickly through the His-Purkinje system and the ventricular myocardium, producing a sharp, large-amplitude deflection that lasts a short time—usually under 120 milliseconds. This rapid, sizable complex reflects a massive amount of tissue depolarizing in a short interval, which is why it stands out on the tracing.

In contrast, the P wave represents atrial depolarization, the T wave marks ventricular repolarization (resetting the ventricles for the next beat), and a U wave, when present, is an additional small deflection that is not the primary sign of ventricular depolarization. This is why the QRS complex is the best indicator of ventricular depolarization.