The present disclosure relates to an implantable medical device, in particular a cardiac defibrillator, comprising an implantable defibrillator configured to generate an electrical defibrillation signal, an implantable electrode connected with the implantable defibrillator by a lead and configured to deliver the electrical defibrillation signal to a patient, an implantable sensor configured to detect mechanical vibrations by the heart of the patient and to provide a detection signal based on the detected mechanical vibrations, and a controller configured to analyze the detection signal to determine at least one parameter characterizing the mechanical vibrations and to initiate a defibrillation operation of the implantable defibrillator based on the determined parameter characterizing the mechanical vibrations.