What does the P wave represent on an ECG?

The P wave on an electrocardiogram (ECG) specifically represents the depolarization of the atria. During this phase, the electrical impulse that originates in the sinoatrial (SA) node spreads through the atria, initiating contraction and allowing blood to flow from the atria into the ventricles. This depolarization is crucial as it sets the stage for the subsequent electrical events in the heart cycle, leading to ventricular depolarization and contraction.

Understanding the importance of the P wave is critical for interpreting an ECG, as variations in this waveform can indicate atrial enlargement or other atrial conditions. The physiological significance of accurate readings of atrial activity underscores the P wave's role in assessing cardiac health and diagnosing various heart conditions.

The other options refer to different phases of the cardiac cycle. Depolarization of the ventricles occurs during the QRS complex, repolarization of the atria does not have its own distinct wave on the ECG and is usually obscured by the QRS complex, and relaxation of the ventricles happens during diastole, but it is not represented by a specific wave on the ECG, instead being inferred from the absence of activity during certain periods.