An implantable cardiac device is used to measure one or more parameters relating to cardiac activity of a patient's heart, from which diastolic heart failure ("DHF") may be monitored and/or detected. These parameters are used to calculate ventricular isovolumetric relaxation time or a related time value. Heart conditions possibly having an influence on the ventricular isovolumetric relaxation time, other than heart conditions due to reduced compliance, may be detected and used to prevent an incorrect calculation of the ventricular isovolumetric relaxation time. The parameters may be measured and the relaxation time calculated multiple times over a period of time, which enables monitoring of the progression of change in the relaxation time. The relaxation time and the progression of change therein are indicators of DHF.