Mechanism of ventricular extrasystoles with fixed coupling. A theoretical model derived from the concept of longitudinal dissociation in the reentrant pathway of extrasystoles.

An electrocardiogram taken from a 29-year-old man with old myocardial infarction is presented as an exemplary case of ventricular extrasystoles with fixed coupling. To explain the mechanism of ventricular extrasystoles with fixed coupling, a theoretical model is derived from the concept of longitudinal dissociation in the reentrant pathway. In the model, functional longitudinal dissociation divides the reentrant pathway into dual pathways F and S. When a sinus impulse is blocked in pathway F and passes only through pathway S, it becomes a manifest reentrant extrasystole because of marked conduction delay in pathway S. When the sinus rate does not exceed a certain value, such an impulse always becomes a manifest extrasystole with fixed coupling. Part of the impulse passing through pathway S enters pathway. F retrogradely. In some cases, thereafter, it reenters pathway S and initiates ventricular reentrant tachycardia. When, on the other hand, a sinus impulse passes through both of pathways F and S, it becomes a concealed reentrant extrasystole because of insufficient conduction delay in the pathways.