An implantable medical device system, including an implantable medical device and an associated implant tool. The device has a hermetic housing containing a power source and electronic circuitry. One or more tines are mounted to the housing movable from a first position extending away from the housing to a second position adjacent the housing. The device is provided with a rotational fixation mechanism. The Implant tool includes an elongated sheath sized to receive the device and provided with internal grooves sized to engage with the tines when the tines are located in their second position. The implant tool may further include a push tool located with the sheath and movable within the sheath to advance the device distally out of the sheath. The sheath may be provided with a closed distal end openable by passage of the device therethrough.