Wide complex beats and rhythms may also occur when impulses arising in the atria or the AV node, encounter a portion of the conducting system or the ventricles that has not fully recovered its excitability. This is referred to as "aberrant ventricular conduction" and occurs most frequently when there is a short RR interval preceded by a long RR interval, as with early atrial premature beats (the upper panel shown here), atrial fibrillation (the lower panel) and less commonly, atrial flutter. These features were discussed in chapter 3 and two of the examples shown earlier in this chapter (page 7.3.16) are shown again here.