The P wave is the first waveform on the ECG and is usually upright. It indicates SA nodal activity and atrial depolarization. Because the atria are thin-walled structures, the deflection is small, and not normally more than 2.5 mm in height.

P wave variables may occur. They can be inverted in Lead V1 in sinus rhythm and are usually upright in Lead V1 in junctional rhythm although inverted in Lead II. Whatever the direction of the P wave in Lead II, that is the rhythm.

Right and left atrial hypertrophy will also cause P wave variables. In Lead II, notched, wide, or tall P waves occur. In Lead V1, biphasic (upright and then inverted) or deeply inverted occur. Atrial hypertrophy will be interpreted on the 12-Lead ECG and will be part of the overread. When using a standard telemetry monitor, “funky” P waves will be seen.