Implantable systems, and methods for use therewith, are provided for monitoring a patient's diastolic function and/or heart failure (HF) condition. A signal indicative of changes in arterial blood volume and a signal indicative of electrical activity of the patient's heart are obtained. Beginnings of diastolic periods can be detected based on a feature of the signal indicative of changes in arterial blood volume. Ends of the diastolic periods can be detected based on a feature of the signal indicative of electrical activity of the patient's heart, or on the signal indicative of changes in arterial blood volume. Diastolic periods (DPs), isovolumic relaxation times (IVRTs) and/or diastolic filling times (DiFTs) can be estimated based on the detected beginnings of the diastolic periods and detected ends of the diastolic periods. The patient's diastolic function and/or HF condition (and/or changes therein) can be monitored based on the estimates of DP, IVRT and/or DiFT.