The P wave indicates atrial depolarization. The P wave occurs when the sinus node, also known as the sinoatrial node, creates an action potential that depolarizes the atria.

The P wave should be upright in lead II if the action potential is originating from the SA node. In this setting, the ECG is said to demonstrate a normal sinus rhythm, or NSR. As long as the atrial depolarization is able to spread through the atrioventricular, or AV, node to the ventricles, each P wave should be followed by a QRS complex.