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.
Multiple abnormalities of the P wave are discussed in detail in ECG Reviews and Criteria. Atrial enlargements can widen the P wave or increase the P wave amplitude. Ectopic atrial rhythms can alter the normal morphology of the P waves. There are many heart rhythms in which the P waves are not able to be identified, including atrial fibrillation and sometimes junctional rhythms. At times, the P waves can be buried at the end of the QRS complex, causing a “short RP” scenario, as seen in atrioventricular reentrant tachycardia, or AVNRT.