The nature of Wenckebach cycles

Abstract

Various patterns of unstable second-degree AV block were observed in the dog following selective injection of acetylstrophanthidin into the AV node artery. Analysis of the changes in RR and PR intervals during slow transitions between 1:1 rhythm and 2:1 block showed that well-organized sequences of periodic arrhythmias occurred during these transitions, thus verifying a previous hypothesis made on the basis of model experiments. Wenckebach cycles of variable duration are seen near 1:1 rhythm, whereas cyclic patterns of partial 2:1 block (also called "reverse Wenckebach" cycles) appear near 2:1 block; these two modes are separated by 3:2 block characterized by alternation between long and short ventricular intervals. The close correspondence between experimental observations and the results of a simulation study with a relaxation-oscillator model of the AV junction raises the possibility that these periodic arrhythmias may be explained solely in terms of the properties of threshold and refractoriness of the AV junction.