With the advent of surgical and ablation therapy for tachycardias, it becomes imperative to discern the correct type and site of origin of the arrhythmia. A regular tachycardia of 120-200 beats/min and a QRS duration of 0.12 seconds may be either ventricular or supraventricular. Unfortunately, definitive diagnosis may require invasive electrophysiologic studies. Wide complexes appear with supraventricular tachycardia if there is preexisting bundle branch block, anterograde conduction through bypass tracts, or aberrant ventricular conduction. Ventricular tachycardia is more likely if there is AV dissociation, right bundle branch block with QRS duration 0.14 seconds or left bundle branch block with QRS duration 0.16 seconds, QRS axis in right upper quadrant (-90° to +180°), positive QRS deflections in all precordial leads (V[-V6), captive or fusion beats, or a QRS pattern identical to that of premature ventricular beats during sinus rhythm.