Because of an inherited condition, MariAnne LaFleur's heart could stop beating at any time. Her life depends upon an implanted device designed to restore her heart's normal rhythm. "If it failed, I'd die," said the Wheaton schoolteacher. Once a day, the defibrillator in LaFleur's body wirelessly communicates information about her health -- and its own -- to her physician. Any indication of a short circuit or failing battery would cause LaFleur's caregivers to replace the defibrillator.