What does the P-wave in an ECG represent?

The P-wave in an electrocardiogram (ECG) represents atrial depolarization, which occurs as a result of the sinoatrial (SA) node triggering. When the SA node, known as the heart's natural pacemaker, fires, it generates an electrical impulse that spreads through the atria, leading to their depolarization. This process is critical because it signals the atria to contract and push blood into the ventricles, thus initiating the cardiac cycle.

Understanding the significance of the P-wave is essential for interpreting ECGs. It confirms that the electrical activity originating from the SA node is functioning properly, which indicates normal atrial activity. This wave is typically the first waveform seen on the ECG cycle, preceding the complex that represents ventricular activity.

The other options relate to different aspects of the cardiac cycle or specific electrical activities not associated with the P-wave. Ventricular depolarization, ST segment changes, and ventricular repolarization correspond to separate components of the ECG, such as the QRS complex and T-wave, which reflect activities associated with the ventricles rather than the atrial activity indicated by the P-wave.