The morphology of the P wave will not be similar to the sinus P wave, which is normally upright in lead II and biphasic in lead V1. Often, the P wave is inverted in lead II, if it can be seen at all.

Therefore, the criteria for diagnosing LAE on a 12-lead ECG is as follows: The length of the P wave in lead II is greater than 120 milliseconds OR The downward deflection of the P wave in lead V1 ...