Experimental model for an ectopic focus coupled to ventricular cells

Background We used a mathematical model of a sinoatrial nodal cell (SAN model) electrically coupled to real ventricular cells (VCs) to investigate action potential conduction from an automatic focus. Methods and Results Since input resistance of a VC is less than that of an SAN cell, coupling of the SAN model, with a size factor of 1, to a VC produced either (1) spontaneous pacing at the slower rate of the SAN model but without driving (activation) of the VC for lower values of coupling conductance (G(i)) or (2) inhibition of pacing of the SAN model by electrical coupling to the VC for higher values of G(j). When the SAN model was adjusted in size to be 3 to 5 times larger than a sinoatrial nodal cell, thus making effective SAN model capacitance 3 to 5 times larger and input resistance 3 to 5 times smaller, the SAN model propagated activity to the coupled VC for G(j) above a critical value. When the VC was paced at 1 Hz, the coupled cell pair demonstrated a stable rhythm of alternating cycle lengths and alternating conduction directions. By increasing pacing frequency to 2 Hz, we converted this rhythm to a regular 2-Hz frequency in which each action potential originated in the VC. More complex periodic interactions were observed at intermediate cycle lengths and lower or higher values of G(j). Conclusions The phenomena we observed demonstrate the critical role of the size of an automatic focus as well as the coupling in the propagation of activity from the focus into surrounding myocardium.