Embodiments of the present invention relate to implantable systems, and methods for use therewith, for assessing a patients' myocardial electrical stability. Implanted electrodes are used to obtain an electrogram (EGM) signal, which is used to identify periods when the patient experiences T-wave alternans. Additionally, the EGM signal is used to determine whether premature ventricular contractions (PVCs) cause phase reversals of the T-wave alternans. The patient's myocardial electrical stability is assessed based on whether, and in a specific embodiment the extent to which, PVCs cause phase reversals of the T-wave alternans. This abstract is not intended to be a complete description of, or limit the scope of, the invention.