["Slowing" right bundle-branch block in reciprocating tachycardia from a latent right lateral Kent bundle]

The authors report the case of a 56 year old man with paroxysmal reciprocating tachycardia. The participation of a right lateral Kent bundle, latent in sinus rhythm and with retrograde atrioventricular conduction during tachycardia was proved by : 1) the slowing of the tachycardia rhythm and lengthening of the ventriculo-atrial conduction time by 50 ms during right bundle branch block ; 2) atrial mapping during tachycardia showing right lateral atrial pre excitation ; 3) the spontaneous termination of some attacks after a blocked Hisian depolarisation. Analysis of the mechanisms of spontaneous termination of tachycardia showed a block in the accessory pathway in 80% of cases, leading to the successful use of Amiodarone. The particular electrophysiological mechanism of functional bundle branch block makes it the most reliable positive diagnostic criterion in reciprocating tachycardia. A review of previously reported series shows participation of right lateral and septal accessory pathways to be uncommon during reciprocating tachycardia. Functional bundle branch block does not necessarily lengthen the ventriculo-atrial interval with septal accessory pathways. Left lateral Kent bundles are much more common. These points are analysed together with the mechanism of functional bundle branch block in the discussion.