1. A system for evaluating progression of heart failure within a patient using an implantable medical device, comprising: a ventricular end-diastolic volume (EDV) detection unit for detecting values representative of ventricular EDV; wherein the system is coupled to at least two electrodes configured for implant within the patient's ventricles and wherein the EDV detection unit is adapted to: (i.) identify a baseline point in time within each of a plurality of cardiac cycles for detecting the values representative of ventricular EDV, wherein the baseline point in time within each of the plurality of cardiac cycles is identified by tracking a pre-ejection interval and then selecting a point in time within the pre-ejection interval; (ii.) detect a signal representative of an impedance between the two ventricular electrodes at each baseline point in time; and (iii.) determine a baseline ventricular EDV value based on the impedance signal detected at each baseline point in time; and wherein the system further comprises a ventricular EDV-based heart failure evaluation unit operative to detect progression of heart failure within the patient based on detecting changes, if any, over time in the baseline ventricular EDV values.