This tracing is from the same patient, recorded during an episode of light-headedness. The rhythm is atrial fibrillation and the impulses reach the ventricules via the bypass tract. As a result, they are not slowed or blocked within the AV node and the ventricular rate is vary rapid (between 250 and 310/minute). The QRS complexes are wide and have the same general configuration as the pre-excited beats during sinus rhythm, shown on the left of the figure. This wide QRS-complex tachyarrhythmia, which may be mistaken for ventricular tachycardia, is life threatening and is discussed in greater detail in Chapter 7.