In patients with multiple premature wide complex beats, the wave form may be uniform or multiform. When the morphology of the premature QRS complexes is uniform, a single ectopic focus is assumed. When the premature QRS complexes are variable or multiform, there are three possibilities. The first is that the premature beats arise from more than a single focus, i.e. are multifocal. The second is that they arise from the same ectopic focus i.e., are unifocal, but are conducted differently (or aberrantly). The third is that they are a fusion between a unifocal premature beat and the sinus beat.
The upper of the two ECG strips shown here demonstrates ventricular premature beats in a bigeminal pattern with fixed coupling intervals and uniform morphology. The lower strip shows premature QRS complexes of different morphologies (they are multiform). The coupling intervals are only slightly variable and there are no fusion beats. Thus, although parasystole can be excluded, it is impossible to state with certainty whether these multiform beats are multifocal or unifocal but conducted via different pathways.