An anodal stimulation method and apparatus for the prevention or treatment of tachyarrhythmias using anodal stimulation (AS) energy for effecting hyperpolarization of myocardial cells of a heart chamber to enhance the relaxation thereof in the diastolic phase and to enhance cardiac function, reverse or inhibit cell activation, and thereby treat or prevent tachyarrhythmias. In a preemptive mode with a recognizable ventricular rhythm, the AS pulse is optimally timed to be delivered in an AS delivery interval following an AS delay interval timed from a preceding ventricular depolarization to effect maximal cardiac relaxation and suppress aberrant electrical activity. In a reactive mode responsive to a detected tachyarrhythmia requiring delivery of an anti-tachyarrhythmia therapy, e.g. a cardioversion shock therapy, the AS pulse is delivered during charging of high voltage output capacitors providing the cardioversion shock energy. The sub-threshold AS pulse or train of pulses is increased in energy (amplitude) and/or decreased in energy to and from a peak energy level gradually rather than abruptly. The AS pulses are delivered through a plurality of discrete AS electrodes distributed about the heart chamber or through or large surface area epicardial patch or endocardial AS electrodes.