In some examples, a system includes an implantable medical device configured for implantation in a chamber of the heart, an extension attached to the implantable medical device, the extension comprising a housing comprising at least one electrode, the housing defining a hole, and a tether comprising a first tether portion and a second tether portion and configured to be threaded through the hole. When the tether is threaded through the hole, the first tether portion and the second tether portion are on opposite sides of the hole. The tether may be used to implant the extension in a different chamber of the heart of the patient than the implantable medical device.