What primarily represents atrial depolarization in a sinus rhythm?

In a sinus rhythm, atrial depolarization is primarily represented by the P wave. The P wave is the first deflection seen on an electrocardiogram (ECG) and corresponds to the electrical activity that occurs as the atria contracts and depolarizes. This contraction allows blood to flow from the atria into the ventricles, and the P wave's shape and size can give insight into the health and functioning of the atria.

The QRS complex, while important, represents the depolarization of the ventricles rather than the atria. The PR interval signifies the time between the end of atrial depolarization and the beginning of ventricular depolarization, encompassing both the P wave and the interval before the QRS complex, but it does not represent atrial depolarization alone. Lastly, the T wave represents ventricular repolarization, which occurs after the ventricles have contracted, and does not relate to atrial depolarization in any way. Therefore, the P wave is the definitive marker for atrial depolarization in the context of a sinus rhythm.