The P wave occurs when the SA 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. If this is so, the ECG is said to demonstrate a "normal sinus rhythm" abbreviated "NSR". As long as the atrial depolarization is able to spread through the AV node to the ventricles, each P wave should be followed by a QRS complex.
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