The P-Wave on an electrocardiogram represents which event?

• A. Atrial depolarization
• B. Ventricular depolarization
• C. Atrial repolarization
• D. Ventricular repolarization

Answer: (A)

The P-wave shows atrial depolarization—the electrical activation of the atrial muscle that triggers atrial contraction as the impulse moves from the SA node through the atria. This depolarization precedes the atrial contraction and is seen as a small, rounded wave on the ECG. The ventricular depolarization is represented by the QRS complex, which is a much larger deflection corresponding to the ventricles turning on and contracting. The T wave reflects ventricular repolarization, the ventricles recovering after contraction. Atrial repolarization does occur, but it is masked by the larger QRS complex, so you don’t see it as a separate wave on a normal tracing.