Electrotonic metabolism of pacemaker activity. Further biological and mathematical observations on the behavior of modulated parasystole.

An in vitro biologic model of parasystole and a mathematical model of parasystole based on the phase-response relationships derived from the biologic model were used in tandem to further develop our understanding of the patterns of ectopic activity that might arise as a consequence of the interaction of two pacemakers across a zone of block. Superfusion of the central segment of a dog Purkinje fiber with an ion-free isotonic sucrose solution provided a narrow region of block. The modulation of pacemaker activity by electronic potentials transmitted across the area of block was shown to be importantly influenced by the position of the "ectopic" pacemaker relative to the site of block. Effects of repetitive electrotonic influences on a single pacemaker cycle, the degree of entrance block and capture of the pacemaker during a phase of supernormal excitability were also studied in both the biologic and mathematical models. Our results indicate that marked shifts in the incidence and pattern of manifest ectopic activity can occur as a result of slight changes in heart rate, ectopic pacemaker rate, level of block and the position of the parasystolic pacemaker relative to the block border.