A premature atrial complex (PAC). (In this example the atrial p-wave is difficult to discern).
A premature atrial complex (PAC) with evident negative p-wave
This ladder diagram shows the three possible faits of an atrial premature complex. First two normal sinus beats are shown. The first atrial extrasystole is conducted normally. The second atrial extrasystole follows the previous sinus beat a bit earlier and is conducted with RBBB
aberration. The third atrial extrasystole is still a bit earlier after the previous sinus beat and is blocked in the AV node and therefore not-conducted (a P wave, but no QRS).
Premature atrial complexes origin from an ectopic pacing region in the atria. They are an example of ectopic beats. The result is a p-wave with often a different morphology from the preceding ones.
A premature atrial complex is usually followed by a noncompensatory pause caused by the fact that atrial depolarization enters the sinus node and resets the sinus rhythm.
Premature atrial complexes are common and usually benign.