Value of preexisting bundle branch block in the electrocardiographic differentiation of supraventricular from ventricular origin of wide QRS tachycardia.

The relation between the morphologic configuration of QRS complexes during wide QRS tachycardia induced during electrophysiologic studies and sinus rhythm was examined in 18 patients who had preexisting left or right bundle branch block. Representative QRS complexes during sinus rhythm and during tachycardia were isolated from each patient and juxtaposed for comparison. The QRS complexes that constituted each pair were judged by 4 observers as being identical, different or, if the decision was equivocal, similar. Nine patients had supraventricular tachycardia (SVT). In 8 of the 9 patients, all 4 observers found the QRS complexes during sinus rhythm and SVT identical in morphologic configuration. In the other patient, 2 observers found the QRS complexes identical and 2 found them similar. In 12 patients ventricular tachycardia (VT) was induced. In 11 of these 12, all 4 observers found the QRS complexes during VT different from their respective sinus beats. In the other patient, 3 observers found the QRS complexes different, whereas the fourth found them similar. During SVT, the QRS duration was unchanged from the corresponding value during sinus rhythm, whereas in patients with VT, QRS width increased by a mean of 56 +/- 20 ms (p less than 0.001). The results of our study suggest that the electrocardiographic differentiation of wide QRS tachycardia in patients with preexisting bundle branch block can be accomplished easily and accurately by comparing the QRS complexes during tachycardia with those during sinus rhythm: If the complexes are identical, the tachycardia is supraventricular, but if they are different, the arrhythmia is ventricular in origin.