An implantable medical device for use in delivering therapy to a patient's heart comprising: a sensing module configured to monitor an impedance between at least two electrodes (40, 42, 48, 50, 56) proximate the patient's heart to provide an impedance signal; a therapy delivery module configured to deliver cardiac therapy to the patient's heart; and a control module coupled to the sensing module and the therapy delivery module and configured to: determine an E-wave portion representative of an early diastolic filling phase during a cardiac cycle based on the impedance signal, determine an A-wave portion representative of a late diastolic filling phase during the cardiac cycle based on the impedance signal, deliver cardiac therapy to the patient based upon one or more characteristics associated with at least one of the E-wave portion and the A-wave portion; and determine a maximum peak impedance for the cardiac cycle, and characterised in that the control module is configured to determine an earliest trough of the impedance signal following the maximum peak impedance as a start of the E-wave portion; and to execute at least one of the following: adjust the cardiac therapy delivered to the patient by delaying atrial electrical stimulation by a first selected time period after a first selected fiducial point within the E-wave portion, and adjust the cardiac therapy delivered to the patient by delaying ventricular electrical stimulation by a second selected time period after a second selected fiducial point within the A-wave portion. a sensing module configured to monitor an impedance between at least two electrodes (40, 42, 48, 50, 56) proximate the patient's heart to provide an impedance signal; a therapy delivery module configured to deliver cardiac therapy to the patient's heart; and a control module coupled to the sensing module and the therapy delivery module and configured to: determine an E-wave portion representative of an early diastolic filling phase during a cardiac cycle based on the impedance signal, determine an A-wave portion representative of a late diastolic filling phase during the cardiac cycle based on the impedance signal, deliver cardiac therapy to the patient based upon one or more characteristics associated with at least one of the E-wave portion and the A-wave portion; and determine an E-wave portion representative of an early diastolic filling phase during a cardiac cycle based on the impedance signal, determine an A-wave portion representative of a late diastolic filling phase during the cardiac cycle based on the impedance signal, deliver cardiac therapy to the patient based upon one or more characteristics associated with at least one of the E-wave portion and the A-wave portion; and determine a maximum peak impedance for the cardiac cycle, and characterised in that the control module is configured to determine an earliest trough of the impedance signal following the maximum peak impedance as a start of the E-wave portion; and to execute at least one of the following: adjust the cardiac therapy delivered to the patient by delaying atrial electrical stimulation by a first selected time period after a first selected fiducial point within the E-wave portion, and adjust the cardiac therapy delivered to the patient by delaying ventricular electrical stimulation by a second selected time period after a second selected fiducial point within the A-wave portion.