What does the P wave on an ECG signify?

The P wave on an ECG is an essential component that represents the contraction of the atria. When the heart's electrical impulse is generated in the sinoatrial (SA) node, it spreads throughout the atria, causing them to depolarize and contract. This electrical activity is reflected on the ECG as the P wave.

The significance of the P wave lies in its role in initiating the cardiac cycle, signifying that blood is being pushed from the atria into the ventricles. Understanding the characteristics of the P wave is crucial for interpreting cardiac rhythms and diagnosing various types of heart conditions. The other choices relate to different electrical or mechanical events in the heart that do not pertain to the P wave specifically. For instance, relaxation of the ventricles is associated with the T wave, while ventricular repolarization is also depicted by the T wave, and the electrical activity of the nodes involves multiple components of the cardiac cycle not solely represented by the P wave.