Premature ventricular complexes (PVC) usually occurs in post-myocardial infarction patients. On ECG they have very wide QRS complex, (> 120 msec, which is 0.12 seconds). Remember one small box is 0.04 seconds and one large box is 0.20 seconds. QRS complexes should normally be 0.08-0.12 seconds.
On ECG, PVC will also show a bizarre morphology, and a compensatory pause. The prognosis is usually bad, but no treatment is necessary unless the patient is symptomatic. With symptomatic patients DO NOT treat with antiarrythmic medications, because it will make worse. Instead, treat patients with beta-blockers, which are the first like therapy for symptomatic patients.
The picture on the side should help differentiate PVC from other abnormal ventricular contractions. The video below explains PVC very well.
Image Source: 5MinuteConsult