Pulignano G, Patruno N, Urbani P, Greco C, Critelli G. Univ of Rome. Pacing Clin Electrophysiol 1990;13(2):144-50  To investigate the electrophysiological significance of QRS alternans during narrow QRS tachycardia, transesophageal atrial pacing and recording was performed in 24 patients with a history of paroxysmal supraventricular tachycardia. Standard electrocardiograms showed ventricular preexcitation in 15 patients and normal QRS pattern in nine patients. The ventriculoatrial interval during tachycardia, as defined by means of transesophageal electrogram, allowed tentative diagnosis of the tachycardia mechanism. A 12-lead ECG was recorded either during spontaneous or induced tachycardia, as well as during transesophageal atrial pacing at increasing rates. Electrical alternans occurred spontaneously in eight patients (33%, group A): five with accessory pathway reentry (mean VA: 136 +/- 43 msec), and three with AV nodal reentry (mean VA: 48.3 +/- 12 msec). Tachycardia rate ranged between 170 and 230 beats/min (mean 200.7 +/- 16). In two patients, alternation of the QRS occurred only in the presence of a heart rate exceeding 180 and 190 beats/min, respectively. The amplitude of QRS remained stable during tachycardia in 16 patients (67%, group B): 14 had accessory pathway reentry (mean VA: 137.5 +/- 32 msec), and two had AV nodal reentry (mean VA: 45 +/- 7 msec). In this group, the tachycardia rate ranged from 150 to 210 beats/min (mean 175 +/- 12). Incremental transesophageal atrial pacing up to rates equal to that of tachycardia was performed in five patients from group A and in five patients from group B. Electrical alternans could not be induced in both groups with pacing at progressively increasing rates.