Which waveform is primarily associated with atrial activity on an ECG?

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

Answer: (B)

On an ECG, atrial activity is shown by the waveform that marks atrial depolarization, which is what initiates atrial contraction. This wave appears first in the sequence, coming before the ventricular-focused events of the QRS complex and the T wave. The P wave is typically smooth and rounded and lasts up to about 0.12 seconds, reflecting the atria’s electrical activation as they prepare to push blood into the ventricles. In contrast, the QRS complex represents ventricular depolarization, the T wave represents ventricular repolarization, and the U wave is an extra, less consistent wave that is not tied to atrial activity. So the waveform most closely tied to atrial activity is the P wave.