What characterizes unifocal PVCs?

Unifocal PVCs, or unifocal premature ventricular contractions, are characterized by having only one shape or morphology. This means that all the PVCs arise from a single ectopic focus in the ventricle, resulting in a consistent waveform pattern on the ECG for each PVC. Because they originate from the same point, they display the same configuration, which allows for easy identification of their occurrence within a rhythm strip.

In contrast, when multiple shapes of PVCs appear, it indicates that the ectopic impulses are coming from different foci in the ventricles, which is referred to as multifocal PVCs, not unifocal. The occurrence of PVCs every third beat describes a specific pattern known as trigeminy, which can involve more than one focus. Similarly, the presence of pairs of PVCs grouped together is indicative of a pattern known as couplets or paired PVCs, which also suggests a different underlying mechanism and is not representative of unifocal PVCs. Thus, the defining characteristic of unifocal PVCs is their consistent and singular shape.