The invention relates to leadless cardiac pacemakers (LBS), and elements and methods by which they affix to the heart. The invention relates particularly to a secondary fixation of leadless pacemakers which also include a primary fixation. Secondary fixation elements for LBS's may passively engage structures within the heart. Some passive secondary fixation elements entangle or engage within intraventricular structure such as trabeculae carneae. Other passive secondary fixation elements may engage or snag heart structures at sites upstream from the chamber where the LBS is primarily affixed. Still other embodiments of passive secondary fixation elements may include expandable structures.
