A sinus morphology is an upright P wave in lead II and biphasic (up and down) P wave in lead V1. The first ECG strip below shows a P wave with sinus morphology, thus normal sinus rhythm.

The ECG criteria for diagnosing right atrial enlargement (RAE) are as follows: The P wave amplitude in lead II > 2.5 mm, or The upward deflection of the P wave in lead V1 > 1.5 mm in amplitude.