A generalized description of Wenckebach behavior with analysis of determinants of ventricular cycle-length variation during ambulatory electrocardiography.

Although variation in ventricular cycle length during Wenckebach-type second-degree atrioventricular block traditionally has been explained by the direction of incremental change in PR lengthening preceding the blocked complex, changing PP intervals can also affect Wenckebach periodicity. A generalized algebraic solution was derived to define changing ventricular cycle length as a function of both changing PP and changing incremental PR interval behavior in Wenckebach block. Based on this solution, the determinants of cycle-length variation were examined for 65 episodes of Wenckebach block detected by ambulatory electrocardiography in 51 patients. As previously demonstrated, only 20% (13 of 65) of Wenckebach episodes were characterized by the "classic" shortening of RR intervals; in contrast, ventricular cycle length increased in 57% (37 of 65) and remained constant in 23% (15 of 65) of cases. Algebraic analysis of these episodes revealed that the direction of ventricular cycle-length change preceding the blocked complex was primarily determined by the direction of change of incremental PR intervals in only 35% (23 of 65) of Wenckebach episodes; RR change was governed by the direction of change of preceding PP intervals in 34% (22 of 65) and by equal change of PP and incremental PR intervals in 31% (20 of 65) of these episodes. Both inverse and concordant relationships between changing RP and PR intervals were primarily determined by the direction of PP variation during in vivo Wenckebach block. These data confirm that classic Wenckebach block is less common than "atypical" Wenckebach periodicity and demonstrate that RR variation in Wenckebach block is governed by the changing PP interval as often as by the changing incremental PR interval.