Which statement best describes the P wave on an ECG?

• A. The ventricles are contracting
• B. The atria are contracting
• C. The ventricles are repolarizing
• D. The AV node fires

Answer: (B)

The P wave shows atrial depolarization—the electrical activation of the atria as the impulse travels through the atrial muscle. This electrical event triggers atrial contraction, so the P wave is the graphical representation of atrial excitation that leads to contraction. The other parts of the ECG correspond to different events: the QRS complex reflects ventricular depolarization (ventricles contracting), the T wave represents ventricular repolarization (ventricles relaxing), and the AV node is involved in conducting the impulse (creating a brief pause that shapes the PR interval) rather than being depicted by the P wave itself.