Three unusual cases of parasystole

Three cases of parasystole with simple interference of two rhythms showing unusual features are described. In the first case the ectopic center was situated in the auriculoventricular bundle above the bifurcation. Combination beats were absent. Both rhythms were occasionally united when the protection of the ectopic center towards normal sinus stimuli disappeared. This phenomenon is explained by a supernormal phase of excitability of the ectopic center. The two other observations concern “intermittent parasystole” in which an ectopic center periodically appears and disappears. The first ectopic beat of each series is coupled to the preceding sinus beat and apparently elicited by the latter. Case 2 shows a parallel variation of the sinus rate and the rate of the ectopic center. Case 3 shows an interesting intraventricular disturbance of conduction following interpolated beats. It is emphasized that the term “protective block” is not applicable because we are not dealing with a zone of unidirectional block. The protection of the ectopic center against conducted stimuli is fully explained by its diminished excitability. Periodic changes of this excitability may cause the appearance and disappearance of parasystole. All three observations show temporary disappearance of the protection of a parasystolic center. This leads to hitherto unknown disturbances of rhythm and permits a better understanding of the mechanism of parasystole.