A Premature Atrial Contraction (PAC) occurs when a focus in the atrium (not the SA node), generates an action potential before the next scheduled SA node action potential. There are four main characteristics of PACs:
They are premature. That is they occur earlier than you would expect if you were to measure the previous P to P intervals.
They are ectopic. Meaning originating outside of the SA node. Thus the P wave morphology would be different than the normal sinus P wave.
They are narrow complexes. Since they come from the atrium, they will eventually travel through the AV node and use the normal conduction system to spread to the ventricles. Unlike a premature ventricular contraction, which is wide-complexed since it does not use the normal ventricular conduction system.
There is a compensatory pause after the PAC. The extra atrial action potential causes the SA node to become refractory to generating its next scheduled beat. Thus it must "skip a beat" and it will resume exactly 2 P to P intervals after the last normal sinus beat.
When every other QRS complex is a PAC, then the rhythm is referred to as "atrial bigeminy".
Premature Atrial Contractions (PACs) (Example 1)
Premature Atrial Contractions (PACs) (Example 2)
Atrial Bigeminy (Example 1)