In cardiac tissue, beat-to-beat alternation of action potential duration (APD) is a warning sign of life-threatening danger.  When APD alternans is detected, it is desirable to guide the tissue back to a normal rhythm in which APD has little beat-to-beat variation.  Mathematically, this can be accomplished by applying feedback control to stabilize an unstable equilibrium near a periodic (or chaotic) orbit.  Clinically, it is accomplished by applying well-timed electrical stimuli via a medical device such as a pacemaker.  Such device intervention can be implemented in several ways, two of which are point stimulation and far-field pacing (FFP).  In point stimulation, the device applies spatially localized stimuli through the tip of an electrode, whereas in FFP, large plate electrodes apply electric field pulses across the entire heart.  FFP creates "virtual" electrodes within the tissue by depolarizing or hyperpolarizing cells near the boundaries of non-conducting obstacles (e.g., dead tissue) and, if the field strength is strong enough, propagating action potentials can emanate from these obstacles.