A possible mechanism of reciprocating tachycardia initiated by critical shortening of the atrial cycle length in concealed Wolff-Parkinson-White syndrome. Report of a case.

Reciprocating tachycardia in a patient with concealed septal accessory pathway is described. His arrhythmia is characterized by incessant form of tachycardia which is not initiated by a triggering extrasystole but by critical shortening of P-P interval. Electrophysiological studies showed that the occurrence of the reciprocating tachycardia was dependent on atrial pacing rate. By comparing effective refractory period of the atrium, retrograde effective refractory period of the accessory pathway, the conduction time over the reentrant circuit and the difference of conduction time via normal conduction system and via the accessory pathway to the ventricular end of the accessory pathway, it was found that an atrial echo was elicited only at atrial pacing rates at which the difference of conduction times to the ventricular end of the accessory pathway was greater than the effective refractory period of retrograde accessory pathway. These observations suggest that the initiation of the tachycardia in this patient is related to a critical antegrade conduction delay necessary for complete recovery of the ventricular end of the accessory pathway from the preceding antegrade depolarization.