As St. Jude Medical defends itself against reports of deaths and injuries linked to problems with an implanted heart device, it finds itself in familiar territory. Since 2005, two competitors, Medtronic and Guidant, have faced similar scrutiny about critical flaws in their products. But St. Jude is distinguishing itself with what doctors and others say is a rare response—a scorched-earth defense of its policies and products. The company's chief executive, Daniel J. Starks, has led that charge, accusing Medtronic of trying to undercut St. Jude to gain business. He and other company executives have also gone after a prominent researcher.