Indeed, when the same patient is in sinus rhythm (lower panel), similar wide complex beats are present and these are clearly ventricular premature beats. This strongly suggests that the similar beat occurring during atrial fibrillation (upper panel) weAS also ventricular in origin and not an aberrantly conducted supraventricular beat. The decision as to whether wide complex beats are supraventricular and conducted aberrantly or are ventricular beats is of major importance in patients with wide complex tachyarrhythmias and will be discussed in greater detail in chapter 7.