Figure 18.

Premature ventricular complexes. Most cycles contain narrow QRS complexes and could represent any of the supraventricular rhythms described in groups A or B. But occasionally one sees a wide QRS complex interposed between the cardiac cycles. Therefore, the primary rhythm may be sinus- or supraventricular-paced, but occasionally an extra impulse is fired from an ectopic pacemaker within the ventricle and creates a wide QRS complex. These complexes are called premature ventricular complexes and may accompany any of the supraventricular rhythms described thus far. If the complexes on a tracing all resemble one another in shape, a single irritable focus is the culprit and is described as unifocal. If the premature ventricular complexes have variable shapes, multiple foci are implicated and the rhythm is described as multifocal.