P waves from the same focus look the same in a given lead ( i.e. when looked at in the same lead). If P waves have different shapes in the same exact lead, they are certainly coming from different foci. The same lead is like one camera positioned at a specific angle, pointing to the heart. The camera doesn’t move and should take the same exact picture if the electrical activity is coming from the same focus. A change in the shape of the p wave means a change in focus. *Also, each focus always acts the same way. It doesn’t change.
P Wave Morphology in a Tachyarrhythmia
What is the P wave morphology and what is the P wave’s relationship to the QRS complex?
In a patient with a tachyarrhythmia, you should closely study the p-wave (atrial activity) that you see on the ECG. Study it for morphology and axis. “These properties can indicate the regional origin of the atrial activity. Negative p waves in the inferior leads (II, II, aVF) suggest an atrial origin near the AV node. A negative P wave in lead V1 can indicate an origin in the left atrium.
P waves that have morphology and axis similar to sinus beats (i.e. upright in I, II, aVF; biphasic in V1) suggest an origin near or within the sinus node complex. Alternatively, one may see the classic “sawtooth appearance of atrial flutter waves in the inferior leads confirming the presence of typical, counterclockwise atrial flutter.
A “P:QRS” relation > 1:1 excludes AVRT and most forms of AVNRT. In this case, EAT or atrial flutter is more likely. ”
Daniel Cooper and Mitchell Faddis, in The Washington Manual of Medical Therapeutics, 32nd edition.
See this great article from LITFL on the P wave.