ECG indicators of accessory pathway involvement in paroxysmal supraventricular tachycardia.

Paroxysmal supraventricular tachycardia results from circular movement of the depolarization wave, with or without the involvement of an accessory pathway. Two ECG signs have been proposed as indicators of such accessory pathway involvement in the retrograde limb of the circular movement: a negative retrograde atrial activity in lead I, and a paradoxical tachycardia slowdown with the onset of a functional bundle-branch block ipsilateral to the accessory pathway. Electrophysiological studies were carried out on 17 patients--12 with manifest and five with concealed preexcitation--and the data were analyzed with respect to the two above-mentioned signs. In the 14 patients with a left accessory pathway, there was a negative P wave in lead I during tachycardia. Six of them had functional left bundle-branch block; in four of these six, the tachycardia slowed down with the onset of the block and in one, the rate of the tachycardia did not change. In the sixth patient, the left bundle-branch block pattern appeared in one atrial echo beat and its effect on the rate of tachycardia could not be determined. In 6 of the 14 patients with a left accessory pathway, there was no change in the rate of tachycardia with the onset of a right bundle-branch block. It was concluded that the two ECG signs are reliable indicators of the involvement of an accessory pathway in paroxysmal supraventricular tachycardia.