Which waveform on an electrocardiogram (ECG) represents atrial depolarization?

The P Wave on an electrocardiogram (ECG) represents atrial depolarization, which is the electrical activity that triggers the contraction of the atria. During this phase, the sinoatrial (SA) node fires, generating an impulse that spreads through the atrial muscle, leading to its depolarization and subsequent contraction. This waveform is typically the first deflection observed in a normal ECG cycle, indicating the start of the heart's electrical cycle.

In contrast, the QRS complex reflects the depolarization of the ventricles, which occurs following atrial depolarization. The T Wave corresponds to ventricular repolarization, while the U Wave is not always visible but is thought to represent repolarization of the Purkinje fibers or other aspects of cardiac repolarization. Understanding the role of each waveform helps in deciphering the electrical activity of the heart and diagnosing various cardiac conditions.