Figure 94-17

Possible 4:3 Wenckebach period in the right bundle branch (V1 lead). Because minor degrees of conduction delay through the right bundle branch produce a decrease in the depth of the S wave in leadV1, it is conceivable for the first (and last)P waves to have been conducted normally through the right bundle branch. The second P wave could have been conducted with some delay, the third with a greater delay, whereas the fourth wave was completely blocked in the right bundle branch, thus ending the 4:3 Wencke bach sequence.

(A. Castellanos and others:Recent advances in the diagnosis of fascicular blocks.Cardiol Clin 5:469-488.1987)