QRS #3 is the "giveaway"

Figure 1

This atrial fibrillation as seen by the fibrillatory waves. After QRS 1-3 are wide QRS complexes at regular intervals with a rate of about 60 bpm. Are these aberrant QRS complexes or ventricular ectopics (acceleated idioventricular rhythm or AIVR)?

QRS #3 is the "giveaway" in this case. This QRS complex  is intermediate in morphology between the sinus beat and the ectopic ventricular beat. This is because the ventricles are depolarized both by the ectopic ventricular impulse and a supraventricular impulse.  QRS#3 is a "fusion" beat. Thus, this wide QRS rhythm is AIVR.