An implantable medical apparatus for detecting diastolic heart failure, DHF, has a DHF determining device for determining at least one DHF parameter for detecting a DHF state of the heart of a patient. The DHF includes circuitry for determining, as the DHF parameter, the time duration of a predetermined phase of diastole. A pacemaker has such an apparatus and a control unit that optimizes pacing therapy and pacemaker settings depending on the determined time duration. A corresponding method of detecting diastolic heart failure, DHF, includes determining at least one DHF parameter for detecting a DHF state of the heart of a patient. As the DHF parameter, the time duration of a predetermined phase of diastole is determined.