DETECTION OF A LOCAL SLOW POTENTIAL PRECEDING THE SURFACE QRS COMPLEXDURING NON-PREEXCITED IMPULSE PROPAGATION - A PHENOMENON REFLECTING ANTEROGRADE CONCEALED CONDUCTION THROUGH THE ACCESSORY PATHWAY

A case of Wolff-Parkinson-White (WPW) syndrome with several interestin g electrophysiologic findings is presented. Although manifest preexcit ation had not been documented in clinical routine check-ups for the 2 years before the ablation session, an intermittent preexcitation emerg ed after an initial unsuccessful radiofrequency current delivery direc ted at the subvalvular mitral annulus 1 cm distal from the subsequent successful ablation site. During intermittent manifestation of preexci tation, the following observations were made: (1) during manifest pree xcitation, a possible Kent potential was recorded at the successful ab lation site; (2) during non-preexcited impulse propagation, a local sl ow potential preceding the QRS complex (pre-QRS potential) was clearly observed at the same site; (3) the pre-QRS potential disappeared duri ng orthodromic atrioventricular reciprocating tachycardia, spontaneous atrial premature contraction and after the subsequent successful abla tion; and (4) when the pre-QRS potential was obvious, a small change i n QRS morphology of the body-surface ECG was appreciable, compared wit h that during beats of negative pre-QRS potential, A comparable preced ing component was also detected in a signal-averaged ECG. It is consid ered that the pre-QRS potential might be related to the anterograde co ncealed conduction through the accessory pathway.