Electrophysiologic mechanisms of functional bundle branch block at onset of induced orthodromic tachycardia in the Wolff-Parkinson-White syndrome. Role of stimulation method.

The mechanisms of aberrant conduction at the onset of induced orthodromic tachycardia in the Wolff-Parkinson-White syndrome were analyzed in 20 consecutive patients in whom this tachycardia was initiated by the atrial (A2) and/or right ventricular (V2) extrastimulus techniques. Of 13 patients in whom orthodromic tachycardia was induced by the A2 method, functional right bundle branch block occurred at tachycardia onset in four (31%) and left bundle aberrancy in two (15%), one of whom also manifested right bundle aberrancy. The occurrence of bundle branch block at the onset of tachycardia was linked to aberrant conduction of the initiating A2 impulse which, in turn, was associated with attainment of relatively short His1His2 intervals within the tachycardia initiation zone. Aberrant conduction of A2 was also more common in patients without manifest preexcitation. In contrast, of 14 patients in whom orthodromic tachycardia was induced by the V2 method, left bundle aberrancy occurred at the onset of tachycardia in 11 (79%), one of whom manifested right bundle branch block as well. Left bundle aberrancy was more likely to occur when the interval from the initiating V2 (or macro-reentrant V3) impulse to the first anterograde His deflection was less than 300 ms. This suggests that left bundle aberrancy at the onset of orthodromic tachycardia induced by the V2 method results from concealed retrograde penetration of the His-Purkinje system, with the left bundle being last to recover. Our findings provide the conceptual basis for a physiologic approach to the deliberate induction of specific types of aberrant conduction at onset of orthodromic tachycardia in patients with Wolff-Parkinson-White syndrome.