Abstract: Implantable medical device configured to detect an atrial electric signal of a heart, and a ventricular electric signal of the same heart. Atrial events are evaluated in the atrial electric signal detected by a first detection unit and/or ventricular events are evaluated in the ventricular electric signal detected by a second detection unit for recognizing a condition of the device in which atrial electric signals are insufficiently detected. Evaluation is done by applying at least one of: morphology of the detected atrial electric signals; lacking stability of atrial events; absence of atrial events over a period of time; an amplitude of the detected atrial electric signal being lower than a predefined threshold value; absence of atrial events during detection of ventricular electric signals simultaneously; comparison of atrial events sensed with first and second sensing profiles, the second being more sensitive than the first.