An Unusual Mechanism of Initiation of Circus Movement Tachycardia in a Patient with Concealed Wolff‐Parkinson‐White Syndrome

Infranodal conduction delay is an uncommon mechanism of initiation of circus movement tachycardia in the Wolff-Parkinson-White syndrome. A patient with a concealed left free wall atrioventricular accessory pathway who underwent electrophysiological study is presented. During atrial extrastimulus testing, a pattern of alternating bundle branch block was observed and was most marked after the administration of intravenous procainamide. Conversion from right bundle branch block to left bundle branch block was reproducibly associated with the initiation of circus movement tachycardia utilizing the normal conducting system antegradely and the accessory pathway retrogradely. Initiation of tachycardia was critically related to the increase in the HV interval associated with left bundle branch block. Of interest, conduction in the right bundle branch became progressively slower as the prematurity of the atrial extrastimulus increased, whereas conduction time through the left bundle branch remained fixed prior to block. Implications of these findings are discussed. To our knowledge, this is the first report of alternating bundle branch block during atrial extrastimulus testing as a mode of initiation of circus movement tachycardia in the Wolff-Parkinson-White syndrome.