The P wave represents in medical terms the change in potential of the atrial muscle depolarization, which can be imagined as a single contraction of the atrial muscle. The morphology of the P wave is generally obtuse and circular in most leads, and sometimes there may be a mild tangent. I, II, AVL, and V4-V6 are positive, AVR leads are negative, and the rest of the leads are either bidirectional inverted or low average available. Normal P wave time is generally less than 0.12s, amplitude is generally less than 0.25mV in limb leads and less than 0.2mV in thoracic leads. if the P wave is sharp and elevated, amplitude is greater than or equal to 0.2mV, and is prominent in II, III, AVF leads, it is called pulmonary P wave and is generally seen in patients with lung disease. If the P wave is broadened and its time limit is greater than or equal to 0.12s, the P wave is often bimodal, and the distance between the peaks is generally greater than 0.04s, and it is obvious in the Ⅰ, Ⅱ and AVL leads, it is called mitral P wave.