Wenckebach Phenomenon in the His-Purkinje System.

The author discusses Wenckebach phenomenon (WP) in the His-Purkinje system (HPS). Subtle PR changes may be interpreted as no change, often seen in the HPS-WP. Changes in the QRS axis help to localize the site of delay and block along fascicles of the left bundle branch. Marked PR changes may occur during HPS-WP. All the QRS complexes that are conducted depict right bundle branch and left anterior superior fascicular block, so the visible changes in the HV interval suggest WP mostly occurring in the left posterior inferior fascicle.