WordNet sense 1 (a leading instance in its field):

pacesetter

WordNet sense 2 (a specialized bit of heart tissue that controls the heartbeat):

cardiac pacemaker, SA node, sinoatrial node

WordNet sense 3 (an implanted electronic device that takes over the function of the natural cardiac pacemaker):

artificial pacemaker

WordNet sense 4 (a horse used to set the pace in racing):

pacer