  A two-component model is developed that consists of a discrete loop of
cardiac cells that circulates action potentials together with a cardiac pacing
mechanism. Physiological properties of cells such as restitutions of
refractoriness and of conduction velocity are given via experimentally measured
functions. The dynamics of circulating pulses and their interactions with the
pacer are regulated by two threshold relations. Patterns of spontaneous
initiations and terminations of reentry (SITR) generated by this system are
studied through numerical simulations and analytical observations. These
patterns can be regular or irregular; causes of irregularities are identified
as the threshold bistability of reentrant circulation (T-bistability) and in
some cases, also phase-resetting interactions with the pacer.
