A biologic model of parasystole.

The electrotonic interactions of a parasystolic pacemaker with ventricular responses to the normal pacemaker across an area of depressed excitability were simulated in a model consisting of strands of canine Purkinje fibers mounted in a sucrose gap preparation. Experiments were conducted to study the patterns of ectopic activity that result from entrainment of the "ectopic" pacemaker (EP) on one side of the sucrose gap by evoked responses (sn) on the other side of the gap. When one-way conduction ("entrance block") was established, manipulations of the SN frequency and of the impedance between the two outer chambers resulted in periods of silence, concealed or manifest bigeminy, trigeminy and quadrigeminy, and periods of more complex patterns of group beating as the entrainment ratios changed. The results confirm the predictions of the previously described mathematical model that these patterns depend on the magnitude of the electrotonic influence of SN on the EP cycle length and also on the ratio of the intrinsic frequencies. These studies should help to distinguish between reentrant and parasystolic mechanisms in clinical arrhythmias.