Modulation of propagation from an ectopic focus by electrical load and by extracellular potassium concentration

Cardiac arrhythmias may be due to spontaneous automatic activity from an ectopic focus. We have developed a technique in which a computer model of an ectopic focus (represented by an SA Node cell model), running in real time, can be coupled by a variable conductance, G_c, to a real ventricular cell. We used this technique to investigate the effects of G_c, cell size, and elevated potassium on the ability of an ectopic focus to successfully drive the ventricular cell. For the hybrid cell pair there are three possible outcomes in the steady state: 1) pacing of the SAN model cell but not driving of the ventricular cell, 2) cessation of pacing, or 3) successful pacing of the SAN model cell and driving of the ventricular cell. Elevation of potassium concentration increases both the lower and upper bound of the values of G_c which define the successful pacing and driving range.