The p wave represents atrial depolarization, which starts in the sinus node. The right atrium (RA) is depolarized towards the AV node. A separate signal travels through Bachmann’s bundle to depolarize the left atrium (LA).

Notice how both the RA and LA are depolarized towards the bottom left in a sequential manner (not simultaneously).

Sinus vs non-sinus p wave

A sinus p wave is a p wave representing depolarization of the atria initiated by the sinus node. A non-sinus p wave represents atrial depolarization initiated by a non-sinus focus. (A focus is a term used to describe non-sinus cells in the heart that exhibit automaticity, or the tendency to generate electrical impulses.)

Polarity of a sinus p wave

Morphology of a sinus p wave

The sinus p wave is round and symmetric in appearance in lead II. But why? Let’s think about RA and LA depolarization separately. Both atria are activated from top-to-bottom, and right-to-left. Therefore, both RA and LA waves would show up as positive in lead II, albeit with a delay between the two.

If we superimpose the deflections created by RA and LA depolarization, we get the overall sinus p wave. Notice it’s a round bump that is roughly symmetric in lead II. 

The normal dimensions of the p wave in lead II are <2.5 mm tall, and <3 mm wide.

When looking at the heart from the transverse plane, the RA and LA depolarization waves tend to contribute positive deflections to V1. Therefore, the overall sinus p wave in V1 tends to be positive.

There is a normal variation in heart anatomy from person to person. Lead placement may also vary between measurements. Therefore, a biphasic p wave in V1, with a small negative deflection at the end may be a normal variant. Notice how the LA depolarization waves move away from V1 in the diagram below. 

Note: the initial positive deflection must be greater than the terminal negative deflection in the V1 p wave for it to be a normal variant. 

The p-p interval

The p-p interval is the period of time elapsed between two adjacent p waves. You can measure this from the start of one p wave to the start of the next. In normal sinus rhythm, the p-p wave generally shouldn’t vary from beat to beat. This represents a regular firing frequency of the sinus node.

Summary of the sinus p wave

Armed with an understanding of what a normal p wave is supposed to look like, you may be able to appreciate why some of the p waves below are abnormal.