A normal P wave is positive in the standard second lead and negative in the aVR lead, usually referred to as sinus rhythm. If there is an inverted P-wave, there are usually three conditions: 1. The inverted P-R interval is greater than 0.12 seconds and may originate from an ectopic rhythm at the coronary sinus orifice, which is not a sinus rhythm. 2. The resulting P-R interval is less than 110 milliseconds, i.e., less than 0.11 seconds. It is positionally related to the QRS wave group, i.e., there is no conduction relationship, and this heart rate is considered a junctional rhythm, which is also an ectopic rhythm. 3. During a premature ventricular beat, a retrograde transmission of the ventricular beat occurs, and an inverted P wave appears. This inverted P wave is called an atrial echo, which is a prerequisite for the emergence of retrograde tachycardia, and is evidence of a double pathway in conduction. There are two other cases of attention, one is the P wave in the second lead is not clear, looks like a low flat or bidirectional, the direction of positive and negative direction is not very clear. Then we must look at the standard I lead, if the P wave in the standard I lead is still positive, it may be caused by the leftward deviation of the T-wave electrical axis, or sinus rhythm. There is also a situation where the voltage can be doubled, and when it is doubled, the lower flat, or shorter p-wave can be seen clearly, helping to identify it carefully.