An implantable medical device (402) includes an attachment member (424) having an aperture (428) with a first diameter. A delivery device (400) includes a catheter shaft (406), a tube (450′；) within the shaft, and a tether (422′；) within the tube. The tube has a main portion (455′；) with a second diameter and an expandable distal end (457′；). The tether has a body (423′；) and a tether member (426′；) having a third diameter greater than the second diameter and smaller than the first diameter. The tether is slideable relative to the tube from a released condition in which the tether member is positioned at least partially distal to the distal end of the tube, and a locked condition in which the tether member is at least partially surrounded by the distal end of the tube. The distal end of the tube can pass through the aperture only in the released condition.
