The leading professional group for doctors who implant defibrillators said it was cooperating in a Justice Department investigation into proper use of the devices. The Heart Rhythm Society said in a statement on Friday that it was providing expertise to officials conducting a civil investigation into implanted defibrillators, which correct dangerous heart rhythms using electrical shocks. A spokeswoman for the group declined to give details. The announcement comes two weeks after a medical journal article suggested one in five heart defibrillators may be implanted for questionable reasons.