What do the P, QRS complex, and T wave represent on an ECG?

• A. P wave = atrial depolarization; QRS complex = ventricular depolarization; T wave = ventricular repolarization.
• B. P wave = atrial repolarization; QRS complex = ventricular depolarization; T wave = ventricular repolarization.
• C. P wave = atrial depolarization; QRS complex = atrial depolarization; T wave = ventricular repolarization.
• D. P wave = atrial depolarization; QRS complex = ventricular repolarization; T wave = ventricular depolarization.

Answer: (D)

The main idea is that each part of the ECG trace corresponds to a specific electrical event in the heart’s rhythm. The P wave shows atrial depolarization, the wave of electrical activation that triggers atrial contraction. Right after, the ventricles depolarize, which is seen as the QRS complex; this is large and rapid because the ventricular muscle is extensive and the impulse travels quickly through the His-Purkinje system. During this same interval, atrial repolarization occurs but is hidden within the QRS complex, so you don’t see a separate wave for it. Finally, the ventricles repolarize, returning to their resting state, which appears as the T wave. So the sequence is atrial depolarization (P wave), ventricular depolarization (QRS complex), and ventricular repolarization (T wave).