An implantable medical device is provided for sensing cardiac activity within a patient's body. The device comprises a canister, control circuitry disposed within the canister, and a lead assembly extending from the canister. At least one sense electrode is coupled to the circuitry and disposed on the lead assembly for sensing cardiac activity. At least one spacer is coupled between the lead assembly and the canister for maintaining a minimum predetermined distance between the at least one sense electrode and the canister.