1. From the Cardiac Unit (M.E.B.) and the Pathology Department (J.F.S.), Massachusetts General Hospital and Harvard Medical School, Boston; the Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics (G.J.T., B.E.B., J.A.I., M.R.H., J.G.F.), Massachusetts Institute of Technology, Cambridge; and Lincoln Laboratory (E.A.S.), Massachusetts Institute of Technology, Lexington.