Interactions between spontaneously pacing and quiescent but excitable heart cells

The authors previously developed a technique for studying a mathematical model cell with spontaneous activity, namely, a 'real time' simulation of a rabbit sinoatrial node (SAN) model cell that is simultaneously electrically coupled via a 'coupling clamp' circuit to a real, isolated ventricular myocyte. This technique was applied to investigate the effects of coupling conductance (Gc), cell size and modulation of membrane potential by elevated extracellular potassium ion concentrations on the ability of an ectopic focus, represented by the SAN model cell, to successfully drive a ventricular cell. Values of Gc and the relative sizes of the two cells define three possible outcomes: spontaneous pacing of the SAN model cell but not driving of the ventricular cell; cessation of spontaneous pacing; or pacing of the SAN model cell and driving of the ventricular cell. Below a critical size of the SAN model cell, only the first two outcomes are possible. Above this critical size, there is a range of Gc that allows successful operation of the system as an ectopic focus. Elevation of extracellular potassium ion concentrations from 4 to 8 mM increases both the lower and upper boundaries of Gc for this range. Elevation of extracellular potassium ion concentrations, commonly observed in myocardial ischemia, may affect either inhibition or release of inhibition of an ectopic focus.