Which waveform represents atrial activity?

• A. QRS complexes
• B. T waves
• C. P waves
• D. U waves

Answer: (C)

Atrial activity on an ECG is shown by the P wave. It’s the small, rounded deflection that appears before the larger ventricular activity, and it represents atrial depolarization—the atria contracting to push blood into the ventricles. The big, sharp deflections that come next are the QRS complexes, which reflect ventricular depolarization. After that, the T wave represents ventricular repolarization, the ventricles resetting for the next beat. U waves, when present, are smaller waves that can follow the T wave and aren’t always seen. Atrial repolarization does occur, but it happens during the QRS complex and is usually hidden by the much larger ventricular depolarization. So the waveform that specifically indicates atrial activity is the P wave.