Since all of the electrocardiographic manifestations of ventricular tachycardia may be mimicked, under certain circumstances, by supraventricular tachycardia, many exceptions must exist to the criteria that help to differentiate supraventricular arrhythmias with abnormal QRS complex from ventricular arrhythmias.1-4 For example, premature ventricular complexes that arise in the left ventricle (and, therefore, have a right bundle-branch block pattern) and conform to the rule of bigeminy may be difficult to differentiate from aberrantly conducted supraventricular complexes following a long-short cycle pattern2; functional bundle-branch block may occur within the left bundle-branch system2,4; widened QRS complexes apparently introduced by a P wave may be due to coincident ventricular activity occurring independently (Fig 1). Often, a given arrhythmia with wide QRS complexes lacks distinct features that may characterize it as supraventricular or ventricular, or it may share features of both supraventricular and ventricular arrhythmias.
In this final article of the series on