Wide, bizarre QRS complexes may accompany isolated supraventricular impulses or sustained supraventricular rhythms. Thus, ventricular complexes which display prolonged, abnormal configuration indicate that conduction through the ventricle is abnormal; they do not necessarily indicate that the impulse originated in the ventricles. In the preceding issues, the importance of demonstrating fusion and capture complexes in order to diagnose ventricular tachycardia or accelerated ventricular rhythm with a reasonable degree of certainty was emphasized. It was also indicated that all of the electrocardiographic manifestations of ventricular tachycardia including the presence or absence of AV dissociation, and complexes which appear to represent capture or fusion beats, may be mimicked, under certain circumstances, by supraventricular arrhythmias.1 In the next few issues, we shall discuss the problem of abnormal intraventricular conduction during supraventricular rhythms and review the criteria which, when present, may allow differentiation of supraventricular from ventricular rhythms.
Identification of Atrial