The effect of fascicular block on ventriculoatrial conduction during AV reentrant tachycardia.

Two patients with Wolff-Parkinson-White syndrome and surgically mapped anterior left free wall atrioventricular bypass tracts had orthodromic atrioventricular reentry tachycardia conducted with complete left bundle branch block (CLBBB), complete right bundle block (CRBBB), left anterior fascicular block (LAFB), and a narrow QRS. Ventriculoatrial conduction increased by 35 and 85 ms with CLBBB compatible with the left free wall location of the bypass tracts. In one patient, resolution of CLBBB occurred in two stages. Initially, left posterior fascicular block (LPFB) resolved, decreasing VA conduction by 20 ms. With resolution of the remaining LAFB, there was a further 15 ms decrease in VA conduction. In the other patient, the isolated occurrence of LAFB increased ventriculoatrial conduction by 30 ms. These changes confirmed the location of the bypass tracts in the anterior portion of the left ventricular free wall. Changes in VA conduction with fascicular block can help localize the ventricular insertion of atrioventricular bypass tracts.