In an implantable medical device and a method and computer-readable medium for operating the medical device to detect a condition of a heart of a patient, activity level of a patient and acoustic energy in a patient are sensed, and acoustic signals are generated that are indicative of heart sounds of the patient over predetermined periods of a cardiac cycle during successive cardiac cycles. A signal corresponding to a first sound is extracted from the sensed acoustic signal of a cardiac cycle. A relation is calculated between a first signal corresponding to the first heart sound in a first activity range, and a second signal corresponding to the first heart sound in a second predetermined activity level range. The calculated relation is compared with at least one reference value to detect the condition or a change of the condition.