Sites of entrance block and impulse formation in intermittent atrioventricular junctional parasystole.

The Wenckebach phenomenon of entrance block in intermittent atrioventricular junctional parasystole is reported for the first time in a 40-year-old woman. In this case the presence of dual pathways in the atrioventricular junction is demonstrated. One of them is a pathway without conduction disturbance. The other is a pathway containing both the ectopic focus and the site of second-degree entrance block. This site is located a considerably long distance above the focus. Entrance block occurs also at some site below the ectopic focus, which is a part of the pathway containing the focus. It is suggested that entrance block in this lower site might exist during the whole ectopic cycle.